Astrobites Archives - Page 34 of 36

Astrobites RSS

24 January 2017

The Curious Case of Planetary Nebulae in Globular Clusters

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Masses of the Planetary Nebula Central Stars in the Galactic Globular Cluster System from HST Imaging and Spectroscopy
Authors: George H. Jacoby, Orsola De Marco, James Davies, I. Lotarevich, Howard E. Bond, J. Patrick Harrington, Thierry Lanz
First Author’s Institution: Lowell Observatory
Status: Accepted to ApJ, open access

Have you ever looked at something and wondered, “How did that get there?!” Has that something ever been a planetary nebula? Astronomers are scratching their heads over four planetary nebulae that have turned up in the unlikeliest of locations: globular clusters.

We know of thousands of planetary nebulae in the Milky Way and can even study planetary nebulae in other galaxies. So the first question is: why shouldn’t we see them in globular clusters? To understand this, we need to know a little bit more about how planetary nebulae are formed.

Planetary nebulae (PNe, singular PN) form during a fleeting phase in the life cycle of low- and intermediate-mass stars (a good range to remember is 0.8 to 8 times the mass of the Sun; our Sun is likely to become a PN in ~5 billion years). PNe consist of a central white dwarf wreathed in shells of hot diffuse gas ejected during the asymptotic giant branch phase late in the star’s evolution. The gas is ionized by ultraviolet photons from the central star, giving rise to some of the most colorful and beautiful objects in the Universe (see Figure 1). The term “planetary nebula” is an unfortunate misnomer; an overzealous William Herschel, fresh from his discovery of Uranus in 1781, thought the fuzzy greenish object he spied through a telescope bore a striking resemblance to his recent planetary find.

Figure 1. M57, otherwise known as the Ring Nebula, is an elliptical planetary nebula that can be observed even with a small telescope. [NASA/HST]

In order to create a visible PN, the central star must become hot enough to produce ultraviolet photons to ionize the nebula before it drifts away from the central star and dissipates. Stars with masses less than ~0.8 solar masses are thought not to form visible PNe because by the time the central star becomes hot enough to ionize the nebula, the nebular material has already dispersed. Given the age of the globular clusters investigated in the work (11.6 – 13.2 billion years) and stellar evolution models, stars in these clusters with masses of ~0.8 solar masses should be departing the main sequence. In other words, the turn-off mass is approximately equal to the lower limit for PN formation. If the stars departing the main sequence in globular clusters aren’t massive enough to form PNe, how did the PNe we observe come to be?

The leading theories for how we might be able to see PNe in globular clusters involve interactions between two stars. First, the observed PNe might be the descendants of blue stragglers — main sequence stars with masses higher than the cluster turn-off mass which are thought to be the result of two stars merging. The resultant PN would appear to be the evolutionary product of a single massive star, but the PN central star would be more massive than a typical white dwarf in the cluster. Alternatively, the PNe could result from post-common-envelope binaries. (You can learn more about post-common-envelope binaries and how they relate to PNe here.) The common envelope accelerates the star’s transition between asymptotic giant branch star and white dwarf. As a result, the PN central star can have a mass equal to or less than the typical cluster white dwarf mass.

In this paper, the authors analyzed Hubble Space Telescope observations of the four known globular cluster PNe in order to determine the most likely formation scenario for these objects. Optical images of the four target objects can be seen in Figure 2.

Figure 2. The four PNe investigated in this paper, as seen by the Hubble Space Telescope. From left to right: Ps 1, IRAS 18333, JaFu 1, JaFu 2. [Jacoby et al. 2017]

The authors used stellar evolutionary tracks — models of how the temperature and luminosity evolve after a star leaves the main sequence — to determine the masses of the central stars. Combining the derived central star masses with secondary information such as the morphology of the individual nebulae, they conclude:

Two PNe (first and third in Figure 2) most likely resulted from a merger or mass transfer. However, the masses cannot yet be determined precisely enough to distinguish between the formation scenarios described above.
One PN (far right in Figure 2) shows only weak evidence for a binary interaction. If PNe in globular clusters arise from single stars, it would require a re-evaluation of established evolutionary timescales.
The last object (second from the left in Figure 2) is so bizarre that the authors questioned its membership in the PN class altogether!

With only half of the known four globular cluster PNe requiring some form of binary interaction, we can’t yet invoke binaries as the cause of globular cluster PNe. Despite this, it’s still important to understand the role that binary and multiple star systems play in PN formation because so many stars in the Galaxy fall into the PN progenitor mass range. Do binary systems help shape many PN? Or can single stars put on a spectacular end-of-the-line display without assistance from a companion? Earthlings, five billion years from now, will be waiting to find out.

About the author, Kerrin Hensley:

I am a second year graduate student at Boston University, where I study the upper atmospheres and ionospheres of Venus and Mars. I’m especially interested in how the ionospheres of these planets change as the Sun proceeds through its solar activity cycle and what this can tell us about the ionospheres of planets around other stars. Outside of grad school, you can find me rock climbing, drawing, or exploring Boston.

17 January 2017

A Too-Hot Pulsar Speeding Through the Galaxy

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Hubble Space Telescope detection of the millisecond pulsar J2124–3358 and its far-ultraviolet bow shock nebula
Authors: B. Rangelov, G. G. Pavlov, O. Kargaltsev, A. Reisenegger, S. Guillot, M. van Kerkwijk & C. Reyes
First Author’s Institution: George Washington University
Status: Accepted to ApJ, open access

Pulsars — the rapidly rotating, highly magnetized neutron stars that beam radiation from their magnetic axes — are as mysterious as they are exotic. They’re most often observed at radio frequencies using single-dish telescopes, and they’re sometimes glimpsed in X-ray and gamma-ray bands. Far rarer are pulsar observations at “in-between” frequencies, such as ultraviolet (UV), optical, and infrared (IR) (collectively, UVOIR); in fact, only about a dozen pulsars have been detected this way. However, their study in this frequency range has proved enlightening, as we will see in today’s post.

A pulsar too hot to handle

While one would expect a neutron star to cool with age if an internal heating mechanism does not operate throughout its lifetime, observations of the millisecond pulsar J0437–4715 (an interesting object in its own right) yielded surprising results. In a 2016 study, far-UV observations revealed the 7-billion-year-old pulsar to have a surface temperature of about 2 × 10⁵ K — about 35 times the temperature of the Sun’s photosphere. This finding inspired Rangelov et al. to observe another millisecond pulsar, J2124–3358 (a 3.8-billion-year-old pulsar with a spin period of 4.93 ms), in the far-UV and optical bands using the Hubble Space Telescope (HST).

Because so few pulsars have been studied in these frequency ranges, their spectral energy distributions (SEDs) in this regime are poorly understood. Generally speaking, the spectra of normal, rotation-powered pulsars reveal a nonthermal (not dependent on temperature) component in optical and X-rays caused by electrons and positrons in the pulsar magnetosphere. In the far-UV, some pulsars show a thermal (blackbody) component in their spectra, thought to come from the surface of the cooling object. Analysis of the team’s HST images revealed an SED that is best modeled by a combined nonthermal and thermal spectral fit, with nonthermal emission dominating at optical wavelengths and thermal emission appearing in the far-UV (see Figure 1). If their interpretation is correct, this implies a surface temperature for J2124–3358 that is between 0.5 × 10⁵ and 2.1 × 10⁵K, which is very much in line with the temperature of J0437–4715. If this proves to be the case, these two measurements will strongly suggest the presence of a heating mechanism in millisecond pulsars. However, various fits using only nonthermal components in the far-UV are still valid, so it is impossible to make an absolute determination of the correct fit.

There are quite a few heating mechanisms that could be invoked to explain these objects’ high temperatures, ranging from the release of stored strain energy from the pulsar’s crust to dark matter annihilation in the pulsar’s interior. More spectral coverage of J2124–3358 is necessary to both check the validity of the nonthermal and thermal combined fit and to get closer to determining more specifically the heating mechanism in play.

Figure 1: thermal and nonthermal combined fit to HST far-UV/optical data for J2124

Figure 1: Thermal (red dashed) and nonthermal (blue dashed) combined spectral fit to HST far-UV/optical data for J2124–3358. The black line signifies the sum of both components. Because there is uncertainty about the nature of the nonthermal component, two possible spectral slopes are shown. [Rangelov et al. 2016]

A (bow) shocking find in the far-UV

Images of J2124–3358 also show the presence of a bow shock, which is an arc-shaped shock that occurs when an object is moving faster than the interstellar medium (ISM) sound speed. J2124–3358 was known before this study to be accompanied by such a shock in H-alpha (Hydrogen transition from n=3 to n=2) filters, for which plenty of neutral hydrogen is required. As a result of the HST observations, J2124–3358 was found to have an (albeit fainter) far-UV shock coincident with the H-alpha shock (see Figure 2). This is only the second such object (after J0437–4715) to show a far-UV bow shock. It is absolutely possible that many pulsars cause bow shocks that don’t emit in H-alpha, but do in other wavelength regimes. Studying these more carefully will yield information about the nature of the ISM.

In order to learn more about the heating mechanisms operating in these objects as well as the bow shocks that sometimes accompany them, many more pulsars will need to be studied using various optical, UV, and IR filters. Studies in the far-UV are only possible with Hubble, so it will be a long time before a sufficient number of objects will be studied at these frequencies in order to make solid conclusions about the nature of such interesting phenomena.

Figure 2: New observations of J2124–3358 from this study using the HST at three different wavelengths are shown in the top (left and right) and bottom left images. The shock is clearly visible in the far-UV using the F125LP filter. The bottom right image shows a previous H-alpha observation of the same pulsar. [Rangelov et al. 2016]

About the author, Thankful Cromartie:

I am a graduate student at the University of Virginia and completed my B.S. in Physics at UNC-Chapel Hill. As a member of the NANOGrav collaboration, my research focuses on millisecond pulsars and how we can use them as precise tools for detecting nanohertz-frequency gravitational waves. Additionally, I use the world’s largest radio telescopes to search for new millisecond pulsars. Outside of research, I enjoy video games, exploring the mountains, traveling to music festivals, and yoga.

10 January 2017

A Counterpart for Fast Radio Bursts

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Discovery of a Transient Gamma-Ray Counterpart to FRB 131104
Authors: J. J. DeLaunay, D. B. Fox, K. Murase, P. Mézáros, A. Keivani, C. Messick, M. A. Mostafa, F. Oikonomou, G. Tešić, and C. F. Turley
First Author’s Institution: Pennsylvania State University
Status: Published in ApJL, open access

The Parkes radio telescope in Australia. [CSIRO]

It’s a mysterious case worthy of Sherlock Holmes: seemingly random bursts of radio emission generated from somewhere outside the Milky Way, with no obvious source. These emissions, known as Fast Radio Bursts (FRB), have plagued astronomers over the last several years. They were initially discovered in data archives of the Parkes radio telescope in Australia, popping up as relatively strong radio bursts that lasted only 5 milliseconds. The usual radio bursts we detect are usually repeating, emanating from rapidly spinning neutron stars known as pulsars. This radio signal was all alone.

Even stranger, the signal was dispersed, which means the higher frequency portion of the burst was detected before the lower frequencies. Dispersion is a result of the lower frequency signal being slowed preferentially compared to the high frequencies by the electron clouds between us and the source. Measuring the delay between low and high frequency gives us the distance the signal has traveled. This dispersion indicated that the burst must have originated from very far away — at least 1 Gigaparsec distant. That’s a span of over 3 billion light-years!

Since the archival discovery of FRBs, these sneaky radio bursts have been caught in the act by Parkes telescope, as well as Arecibo in Puerto Rico and the Green Bank Telescope in West Virginia. But until now, there have been no detections in any other wavelengths than radio. In today’s paper, we see for the first time a possible counterpart for an FRB detected in very high energy gamma rays using the Swift telescope. Swift has onboard the Burst Alert Telescope (BAT) that is usually triggered when a gamma-ray burst goes off somewhere in the universe. If typical gamma-ray bursts were responsible for FRBs, then the BAT would have been triggered around the time of an FRB detection, providing almost instantaneous gamma-ray measurements in the same general direction of the the FRB. The authors of today’s paper, however, wondered if perhaps there have been gamma-ray counterparts to the FRBs, but they were not luminous enough to trigger the BAT on Swift. The authors therefore went digging in the archival BAT data, looking for times when Swift just fortuitously happened to be looking in the same direction as an FRB when the radio signal was detected.

The Swift BAT detection of FRB 131104. The top panel (a) shows the portion of the field of view of the BAT where the gamma-ray counterpart was detected, denoted by the black circle near the top of the image. The x- and y-axes denote the position on the sky in right ascension (RA) and declination (dec), and the color bar shows how well-detected the emission is, in units of signal above the noise. The bottom panel (b) gives the number of photons detected per second as a function of time for gamma rays in the 5–15 keV energy range.

Sure enough, there were four times when an FRB was detected within the same field of view as the BAT on Swift. Of these four possibilities, a gamma-ray counterpart was detected for the FRB 131104 (so named for two digits for the year, month, and date of observation.) This detection is shown in the figure above. This FRB is located 3.2 Gigaparsecs away, which corresponds to a redshift of z ~ 0.55. The reason that the BAT was not triggered for this gamma-ray burst was because it was on the very edge of the detector, illuminating only 2.9% of the telescope’s detector. The authors were very careful to rule out other causes for the gamma rays.

Artist’s impression of a magnetar in a young star cluster. [ESO/L. Calçada]

Of course, there are still unanswered questions about this detection. One mystery is that while the FRB lasted only 5 milliseconds, the emission detected in gamma rays lasted several minutes and released significantly more energy — a billion times more — than was detected in the radio burst. One theory posits that the sources of FRBs are brightly flaring magnetars, which are neutron stars with extremely high magnetic fields. However, if magnetars are the culprit, then the gamma-ray emission should not be nearly that long nor that energetic. On the other hand, FRBs with gamma-ray counterparts could be caused by binary neutron stars spiraling into each other. However, models indicate that we should see only about 25 of these events a year, whereas the inferred rate of FRBs is thought to be on the order of thousands per day.

Suffice it to say, I think this is a mystery that would stump even the great detective Holmes himself (minus the small detail that he wasn’t an astrophysicist.) In some ways, this new gamma-ray counterpart discovery is extremely enlightening, giving clues as to where to look next for the source of FRBs. On the other hand, however, this detection has resulted in even more questions about FRB origins. As is often the case in science, more data are needed! In the future, we should be able to fine tune the threshold for triggering the BAT when the next radio burst goes off, allowing us to catch the FRB and its gamma rays in the act.

About the author, Joanna Bridge:

I am a sixth-year Ph.D. candidate at Pennsylvania State University. I study galaxy evolution via emission lines of samples of galaxies that span the entirety of cosmic history. Outside of science, I love to read fiction and do karaoke, among other things! Follow me on Twitter at @bojibridge.

13 December 2016

Catching Galactic Recycling In The Act

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: ALMA Reveals Potential Localized Dust Enrichment from Massive Star Clusters in II Zw 40
Authors: S. Michelle Consiglio, Jean L. Turner, Sara Beck, David S. Meier
First Author’s Institution: University of California, Los Angeles
Status: Published in ApJL, open access

Galaxies are recycling centers for gas. Dense gas collapses under gravity to form stars. When massive stars form, they quickly impact their surroundings with intense radiation and mass loss. This enriches the galaxy with heavy elements forged inside the massive stars and becomes the raw material for future star and planet formation. The evolution of the galaxy is determined by this cycle of gas to stars and back.

To measure the lifecycle of gas in galaxies, astronomers use radio telescopes that are sensitive to the three main phases of gas. First, the dense gas where stars form emits light with millimeter-size wavelengths from carbon monoxide (CO). Next, gas ionized by massive stars gives off radio waves when particles collide, in a process called free-free emission. Finally, heavy elements lost by massive stars, which will fuel the next generation of star formation, coalesce to form grains of dust that emit light in the submillimeter regime. With an unmatched angular resolution, the Atacama Large Millimeter/submillimeter Array (ALMA) is the tool of choice for studying the cycle of gas in galaxies.

The authors of today’s paper used ALMA to measure the gas and dust in the nearby galaxy II Zw 40. This small galaxy is forming stars at a prodigious rate. The starlight in the galaxy is dominated by massive stars. The authors investigate the effect of these stars on the gas and dust in the galaxy.

Observations of Gas and Dust

With ALMA, the authors observed three different components of II Zw 40. At a wavelength of 3 mm, the dominant source is free-free emission from ionized gas around the massive star cluster. At 870 µm, after accounting for free-free emission, most of the light is emitted by dust grains. These two components are shown in Figure 1. The peaks of ionized and dust emission are distinct, and the dust emission is localized in several clumps around the cluster.

Figure 1: The galaxy as seen at 3 mm and 870 µm wavelengths. The 3 mm map (upper right and blue contours) shows the extent of free-free emission from ionized gas around massive stars. Removing the free-free contribution from the total 870 µm emission (upper left) gives a map of dust emission (lower left and red contours). [Consiglio et al. 2016]

Figure 2 shows the dense gas compared to dust emission in II Zw 40. Dense gas is traced by emission from CO molecules. The CO emission is offset from dust emission, with areas of dense gas devoid of dust and vice versa. The authors convert the intensity of CO emission to gas mass using the so-called ‘X-factor’. Dividing the mass in gas and dust, the authors find that the gas-to-dust ratio varies from ~70 to 270 across the galaxy. Not only does the ratio vary, but it’s also low overall — there is more dust per unit gas than the authors expect for a galaxy of this type.

Figure 2: Dense gas and dust emission in the galaxy. Dense gas is traced by emission from CO molecules (blue map). Dust emission is traced by 870 µm emission (orange contours, same as lower left panel in Figure 1). Note the discrepancy between the peaks of dense gas and dust. [Consiglio et al. 2016]

Clumps of Stardust

To explain the low, variable gas-to-dust ratio and the clumpy structure of the dust emission, the authors propose that massive stars enrich nearby clouds with heavy elements and dust. Because this enrichment is ongoing, more dust is joining the dense gas, lowering the total gas-to-dust ratio. The dust is clumpy because it has not yet mixed with the rest of the galaxy. This enrichment model suggests that dust without associated dense gas came from an older star cluster that is not visible in free-free emission (Figure 1) because it no longer ionizes gas.

Pushing Dust With Light

Take another look at Figure 1, and notice how the peaks of dust emission are all slightly offset from the peak of the ionized gas. In addition to ionizing gas, the intense radiation from massive stars can actually push on dust grains. Acting like a multitude of tiny billiard balls, the photons from bright stars exert radiation pressure on the dust. The dust then drifts relative to the gas, which may explain the offset between gas and dust peaks.

ALMA has revolutionized the study of star formation in galaxies. This paper shows that the cycle from dense gas (traced by CO) to massive stars (3 mm) to dust-rich gas (870 µm) is complex. The massive stars in the galaxy have enriched parts of the galaxy, while other areas remain relatively dust-free. The dust-rich products of stellar evolution are pushed by radiation pressure but have not yet mixed into the galaxy. Future studies of the galactic recycling plant will explain the origin and dispersal of the ingredients needed for planets and (perhaps) life. Cue Carl Sagan.

About the author, Jesse Feddersen:

I am a 4th-year graduate student at Yale, where I work with Héctor Arce on the effect of stellar feedback on nearby molecular clouds. I’m a proud Indiana native (ask me what a Hoosier is), and received my B.S. in astrophysics from Indiana University. If I’m not working, you’ll probably find me on a trail or at a concert somewhere.

6 December 2016

X-Ray Beasts and Magnetic Fields

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Revisiting the structure and spectrum of the magnetic-reconnection-heated corona in luminous AGNs
Authors: J.Y. Liu, E.L Qiao, and B.F. Liu
First Author’s Institutions: National Astronomical Observatories, Chinese Academy of Sciences
Status: Accepted to ApJ

Deep in the fathomless centers of galaxies, there lurk fantastic beasts of incredible energy and power. Legend has it that our ancestors have long since been aware of their presence (e.g. Carl Seyfert in 1943 and Maarten Schmidt in 1963), but separated by our great divide and limited by our technologies, little has been known about them. Appearing in various shapes and sizes, some beasts have been spotted to spurt out jets, while others appear more docile. How to find these fantastic beasts, you ask? Well, most give away their presence in the optical and UV, while others show up in the radio. These shape shifters are unified by one thing: the source of their power through the accretion of matter onto supermassive black holes. Astronomers call them active galactic nuclei, or AGNs (shhh, here is a secret guide on their different shape-shifting abilities).

Although AGNs are predominantly discovered in the optical/UV and radio, they actually emit all the way up to X-rays, which is true with most highly energetic astrophysical phenomena. Their spectral energy distributions (SEDs; an example is shown in the figure below), are characterized by different components that arise from different parts of their structures. First, we have a blackbody-like bump — the “big blue bump” — in the optical and UV, which is thought to originate from their accretion disks. The big blue bump is well-fit by the standard accretion disk model, which is geometrically thin and opaque to radiation (“optically thick”). Then, we have some emission in soft X-ray (< 5 keV) whose origin is still unclear. Finally, we have hard X-ray emission (> 5 keV) well-described by a power law. The origin of the hard X-ray emission is not completely solved, but astronomers believe that the up-scattering, or inverse Compton scattering, of accretion disk photons by plasma of hot electrons in a corona surrounding the disk produces these hard X-ray emissions.

An example spectral energy distribution (SED) of an AGN. [Elvis et al. 1994]

The formation of the hot corona and its heating mechanism are still unclear. Leading models posit that it is formed by the evaporation of material from the underlying cool disk and then continuously heated by the disk’s magnetic field. More specifically, magnetic loops produced by the disk emerge and reconnect with other magnetic loops in the corona in a phenomenon known as magnetic reconnection, thereby releasing magnetic energy to maintain the corona at high temperatures. The authors of today’s paper investigate the effect of the magnetic field on the structure and spectrum of the magnetic-reconnection-heated disk-corona model, particularly focusing on luminous AGNs.

The authors encapsulate the effect of magnetic field in their model in a parameter they call the magnetic parameter β₀ = (P_gas + P_rad)/P_B, where magnetic pressure is assumed to be proportional to the sum of the gas pressure and radiation pressure in the disk. Larger β₀ corresponds to weaker magnetic field strength and vice versa. By tuning β₀ and the accretion rate, the authors solve for the disk structure and derive the emergent SED. The figure below shows the simulated SEDs for different β₀ and accretion rates. When β₀ is small, strong magnetic fields are produced, which gives rise to similar-looking spectra regardless of accretion rates. As the magnetic field gets weaker (going down the panel), the model with higher accretion rate (dashed line) produces less and less hard X-ray emission. Eventually, the spectrum is dominated by the disk’s “big blue bump”. This agrees with observational trend that hard X-ray spectra of luminous AGNs become softer at higher accretion rates.

Simulated SEDs of AGN at various β₀ (different panels) and accretion rates (different lines). [Liu et al. 2016]

To compare their models with observations, the authors plotted the observable L_bol/L_x as a function of accretion rate (ṁ) for different magnetic field strengths (shown below). The red crosses are observational samples of luminous AGNs with measured black-hole masses from a different work. The model with β₀= 200 more or less agrees with observations for ṁ < 0.2, but at higher accretion rates this model underpredicts L_x — the corona becomes too wimpy to produce enough X-rays. At ṁ > 0.2, the model with β₀= 100 at slightly higher magnetic field agrees with the last data point, although it fails to match observations at lower accretion rates. Although the models are far from a perfect fit, those with higher β₀ (weaker magnetic fields) agree with observational results at large. As with all things research-related, there remains more work to be done before we can piece together a complete picture of those fantastic beasts called AGNs.

L_bol/L_x as a function of accretion rate for various β₀, overlaid with observational data (red crosses). The β₀= 100 and 200 lines agree with the data while the β₀= 10 and 50 lines do not. [Liu et al. 2016]

About the author, Suk Sien Tie:

I am a third year PhD student at the Department of Astronomy at The Ohio State University. I am currently working on quantitative analyses of various quasar selection methods using the Dark Energy Survey (DES) and quasar variability via microlensing.

29 November 2016

It Turns Out the Sun Is More Chill Than We Previously Thought

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: The Mount Wilson Observatory S-index of the Sun
Authors: Ricky Egeland, Willie Soon, Sallie Baliunas, Jeffrey C. Hall, Alexei A. Pevtsov, and Luca Bertello
First Author’s Institutions: High Altitude Observatory; Montana State University
Status: Accepted to ApJ

One of the few certainties that we have as humans is that the Sun always comes and goes, always in intervals of 24 hours, and will continue to do so for the next 5 billion years or so. Owing to this familiar cycle, we sometimes take for granted that our host star is not itself completely stable. It is active, and this activity shapes the evolution the solar system and life on Earth. Because we use the Sun as a reference for other stars, it is thus crucial that we measure its activity as accurately and precisely as possible.

Blame Magnetic Fields

Mount Wilson Observatory’s 100-inch telescope is one of the telescopes that was used to make solar observations for the HK project. [Ken Spencer]

Stars are big balls of hot gas with lots of moving parts. The ones that are similar to the Sun (i.e., solar-type stars) have large convective atmospheres, which act just like boiling water inside a cooking pot. The convective circulation of plasma generates magnetic fields, and the stellar rotation, in turn, makes field lines wrap around the star, creating a stellar dynamo. When the magnetic field lines concentrate, they produce dark spots in the stellar surface and spectacular mass ejections; the activity of a star is measured by the strength of these episodes.

One precise and accurate way of assessing stellar activity uses the features in stellar spectrum known as the Ca II H & K lines. Their strength can be easily measured with spectrometers, and are then translated into a ratio called the S-index (the higher it is, the more active the star). The most famous survey of stellar activity is the HK Project at Mount Wilson Observatory (MWO), which consisted of assessing the S-index of many stars in the sky and ended up becoming the standard calibration for current studies on activity. The problem is that not all observations are carried out with the same instrument, and hence systematic errors start to become a serious problem.

Context Is Key

In order to understand the role of activity in the physics of stellar and planetary evolution, it is important to place the Sun, the one star we know best, in the same context as the others. In today’s paper, the authors aim to precisely and accurately measure its activity using spectroscopic observations of the Moon — which reflects sunlight — obtained with the same instrument employed at the MWO.

Sun-like stars have activity cycles with periods of the order of a few years. The solar cycle has an 11-year period, encompassing a minimum and a maximum. The authors directly measured the minimum, maximum and mean S-index of the Solar Cycle 23 (1996–2007, although the MWO data goes only up to 2003), and found that they were significantly lower than previous estimates of the same cycle. This result shows that, when using different instruments, systematic errors plague the measurements of activity — but the good news is that now we can correct them by applying a better calibration with results from today’s paper.

The solar activity during Cycle 23. The MWO data are the red dots, and the red curve is a fit to the data. Most of the activity cycle measured by previous studies (dashed curves) are visibly shifted to higher values. [Egeland et al. 2016]

Well, now that we have dealt with accuracy, what about precision? As it turns out, there is no lack of solar activity data in the literature, which are now correctly calibrated. The authors used them to constrain the solar activity minimum, maximum and mean values within less than 1% for all indices.

Composite time series of the S-index of the Sun, using various data sets (colored symbols), which are now correctly calibrated. The colored curves are fits to the data. [Egeland et al. 2016]

Effects on Future Studies

So, the Sun is slightly less active than we previously measured, and this impacts our understanding of solar-type stars the most. By correctly placing our star in the context of others, we can better assess how common it is, which helps us answer questions about the conditions necessary for life to emerge and how it evolves along with the star. These results also rectify some inconsistencies previously observed in the activity of solar-type stars, and again reminds us of a critical aspect of science: systematics matter. The article itself will serve as a guide on measuring stellar activity, paving the way towards better practices in the field.

Bonus if you read to the end:

The Sun from September to November 2016. Larger GIF available here.[ESA/NASA/SOHO]

About the author, Leonardo dos Santos:

Leonardo is a graduate student of Astronomy at University of São Paulo, Brazil, and a visiting student at University of Chicago. His current research consists of studying physical and chemical properties of stars similar to the Sun, hunting for exoplanets and software development for astronomical instrumentation. When not sitting in front of a computer, he enjoys doing night sky photography, cooking, riding his mountain bike and failing miserably at pretty much all these tasks.

22 November 2016

The True Colours of a Blue Straggler

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Formation and Evolution of Blue Stragglers in 47 Tucanae
Authors: J. Parada, H. Richer, J. Heyl, J. Kalirai, R. Goldsbury
First Author’s Institution: Department of Physics & Astronomy, University of British Columbia, Canada
Status: Published in ApJ

Just by looking at a star’s colour, astronomers can learn a few things about its properties. The most direct one is its temperature: red stars like Betelgeuse are cool, blue stars like Rigel are hot. Another property we can estimate by eye is the apparent magnitude, a concept idealised by the Greek Hipparchus almost 2000 years ago. He ranked stars from brightest to faintest in a scale of one to six, one denoting the twenty brightest stars he could see, and six denoting those he could barely spot with the naked eye. This scheme was adapted into a logarithmic scale by N. R. Pogson in 1856, and is still in use nowadays. The apparent magnitude depends on the star’s luminosity and distance, so if we know the latter, we can estimate the former using the measured magnitude. Knowing the luminosity and the temperature we’ve estimated from the star’s colour, we can place it in a Hertzprung-Russell (HR) diagram and infer other properties, like evolutionary stage, mass, and radius. Unfortunately, distance is a very hard thing to measure accurately in astronomy. However, there’s a way to overcome that: studying stars in clusters. In a star cluster, all stars are basically at the same distance from us, so we can ignore the effect of the distance and compare the apparent magnitude of these stars directly. We can then build a colour-magnitude diagram (CMD), the observational version of the HR diagram. Lots of our knowledge about stellar evolution came from comparing theoretical evolutionary models to this kind of diagram.

Blue stragglers in the open star cluster NGC 3766. [ESO]

With the development of high resolution imaging, CMDs revealed the presence of some unexpected stellar populations. A prominent population is blue straggler stars (BSS). These stars appear as a blue extension of the main sequence (MS) in globular clusters, right above the so-called turnoff point, where stars are about to leave the main sequence. They are unexpected because, given the cluster’s age and metallicity, stars with their properties should already have evolved off the main sequence. Blue stragglers are still there though, like they were too lazy to leave the MS. How they are formed and where they go when they finally evolve are two questions still up for debate (bite1, bite2). The authors of today’s paper studied the blue stragglers in the cluster 47 Tucanae with the aim of shedding some light on the subject.

Blue stragglers: what we know so far

The only way to explain the blue stragglers being brighter and bluer than the turnoff is if they went through some rejuvenating process. There are different ways to do that, but two main conditions must be followed: i) one MS star must be involved, and ii) one of the stars involved gained mass to become rejuvenated. This mass gain gives the star more fuel, extending its main sequence lifetime. There are two possible ways for this to occur: mass transfer or merger. The authors considered three different scenarios: i) direct collision of stars, ii) evolution of primordial binaries, and iii) hierarchical evolution of triple systems. The resulting blue straggler can have different characteristics depending on the mechanism that caused it. The populations we observe are a combination of all these mechanisms, but one of them can prevail over the others depending on the environment.

Combining theory and observations: what 47 Tucanae can teach us

In order to try to identify the origins and possible evolution of the blue stragglers in 47 Tucanae, the authors first separated the stars into different stellar populations. First, based on the stars’ location on the CMD, the cluster was split into two blue straggler populations, bright (bBSS) and faint (fBSS), each containing half of the blue straggler sample, main sequence (MS), binary main sequence (MSBn), and red giant branch (RGB). This is illustrated in Fig. 1. The authors then estimated the cumulative distribution of each of these populations along the radius of the cluster, also shown in Fig. 1. The first thing to notice is that the bBSS and the fBSS samples are actually different: the bBSS is more concentrated in the inner regions than the fBSS is. Moreover, the bBSS cumulative radial distribution does not resemble any of the other populations, making it difficult to link its formation to any specific group. The fBSS, on the other hand, show a distribution very similar to the MSBn, pointing to a binary origin for the fBSS. The estimated masses of these two populations are also similar, with the fBSS being slightly smaller, as would be expected for a final product of binary evolution. The mass of the bBSS is in contrast higher, suggesting an origin involving triple or multiple stellar systems.

Figure 1: The left panel shows a colour magnitude diagram using Hubble's filters F255W and F336W. The stellar populations identified are indicated. On the right panel, the cumulative distribution for each population is shown. The numbers of stars in each panel are slightly different because a completeness correction was done on the right panel, taking into account stars that aren't detected because of limitations in the method.

Figure 1: The left panel shows a CMD using Hubble’s filters F255W and F336W. The stellar populations identified are indicated. On the right panel, the cumulative distribution for each population is shown. The numbers of stars in each panel are slightly different because a completeness correction was done on the right panel, taking into account stars that aren’t detected because of limitations in the method.

Next the authors relied on evolutionary models calculated with the code MESA (Modules for Experiments in Stellar Astrophysics) to identify the region occupied by evolved blue straggler stars (see Fig. 2). Calculating the cumulative radial distribution, they noted that what they call evolved blue stragglers (eBSS) follow a similar distribution to the blue stragglers, suggesting they are indeed linked. Moreover, they found an excess of stars in the RGB and the horizontal branch (HB) when compared to the expected number considering only single evolution. According to their estimates, this excess can be explained by stars evolving from blue stragglers into these regions. So it appears that the blue stragglers have a post-MS evolution comparable to that of a normal star of the same mass. There’s still some disagreement between the lifetimes the authors estimated and others found in the literature, indicating a more detailed study of individual blue straggler properties is in order to better constrain these values. Future studies using high quality spectra may help with that.

Figure 2: The left panel shows again the cluster’s CMD, now with some MESA models overplotted. The sequences have initial masses of f 0.9, 1.1, 1.4, and 1.8 solar masses, from bottom to top. The right panel shows the cumulative distribution for the samples selected on the right panel. There’s again a completeness correction that makes the number of stars slightly different in the plots.

In short, the authors verified that different mechanisms leading to blue stragglers can in fact be identified within a cluster. Interactions in multiple systems seem to dominate in the central regions, while binary evolution seems to be the dominant mechanism in the cluster outskirts. The former leads to more massive, brighter objects, while fainter blue stragglers are explained by the latter. The evolution of the blue stragglers seems to be similar to simple MS stars with same mass, which makes it easier to model their evolution. However, this still has some discrepancies with other results, so more detailed studies, focused on individual objects, are needed. The blue stragglers have not shown all their true colours yet.

About the author, Ingrid Pelisoli:

I am a second year PhD student at Universidade Federal do Rio Grande do Sul, in Brazil. I study white dwarf stars and (try to) use what we learn about them to understand more about the structure and evolution of our Galaxy. When I am not sciencing, I like to binge-watch sci-fi and fantasy series, eat pizza, and drink beer.

15 November 2016

Investigating One of the Strongest Starbursts in the Universe

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: SPT0346-52: Negligible AGN Activity in a Compact, Hyper-starburst Galaxy at z = 5.7
Authors: J. Ma, et al.
First Author’s Institution: University of Florida
Status: Accepted to ApJ

In recent years, the South Pole Telescope has discovered an intriguing population of gravitationally lensed dusty star-forming galaxies. These galaxies, some of the most distant and faint in Cosmos, are able to be studied because their light is magnified by a foreground gravitational lens. Using this phenomenon, astronomers can learn more about the stars, gas and dust — not the kind of dust under your bed that you should probably vacuum up some day, but instead the type that blocks, absorbs and scatters light — in the very distant universe.

One galaxy in particular drew the researchers’ attention as it stood out in the 2500 square degree (that’s about 6% of the sky; for comparison: the full Moon subtends about 0.2 square degrees) survey undertaken by the South Pole Telescope: its name is SPT-S J034640-5204.9, though we shall call it SPT0346-52.

A 20″x20″ cutout of SPT-3462-52 showing HST (grey), Spitzer (blue contours), and ALMA observations (red contours). [Ma et al. 2016]

Galaxy SPT0346-52

This galaxy, the main protagonist of today’s astrobite paper, is located at a redshift of z=5.656. That means that, since its light can only travel at a finite speed, we are observing it at a time when the universe itself was a mere 1 billion years old. This makes SPT0346-52 part of the first population of galaxies to have formed in the universe, and one of the most distant of the type of galaxy known as “dusty star-forming galaxies” to have been observed thus far. Because of this it has been studied intensely using a wide range of wavelengths to probe its physical properties: so far it has been observed with HST, Spitzer, Herschel, ALMA, APEX and the VLT!

From ALMA observations it could be determined that the foreground lensing galaxy (a “mere” 8 billion light-years from Earth) was magnifying the light from SPT0346-52 by a factor of about 5.6, and that the intrinsic infrared luminosity of SPT0346-52 was a factor of 1000 higher than that of our nearest neighbour, the Andromeda galaxy. The star formation rate of SPT0346-52 was calculated to be about 4500 solar masses per year — compare that to the Milky Way’s “meagre” rate of 1 solar mass per year! In addition, given that with a radius of about 0.6 kpc it’s about 25 times smaller than our own galaxy, this indicates that SPT0346-52 has one of the highest star formation rate densities of any known galaxy!

While much has been learned about this fascinating galaxy from previous multi-wavelength observations, one intriguing question remains:

What causes its high luminosity?

Does SPT0346-52’s high infrared luminosity arise solely from intense star formation, or does it harbour an obscured active galactic nucleus (AGN)? This is the question explored in today’s paper.

Several indicators appear to favour the presence of an AGN: the dust temperature in SPT0346-52 is found to be about 52 K, making it warmer that that of typical dusty star-forming galaxies. The luminosity ratio of some of its emission lines is consistent with a quasar (the most energetic and distant type of active galactic nucleus), and it shows strong H2O emission lines which are similar to those found in distant quasars. However, SPT0346-52 does not show any indications of an AGN in deep optical spectroscopy.

In order to shed more light on this mystery, the authors of today’s paper undertook two further observations, probing the extremes of the electromagnetic spectrum. They observed SPT0346-52 in X-rays using the Chandra satellite. A clear detection of X-ray photons would be indicative of the signature of an accreting black hole powering an AGN, while a purely star-formation-driven galaxy would emit a lot less X-ray flux. Additionally, the authors undertook radio observations with the Australian Telescope Compact Array (ATCA). Similarly to X-rays, radio waves can be used to distinguish star-forming galaxies from radio-loud active galactic nuclei.

Spectral Energy Distribution of SPT0346-52

The best-fitting spectral energy distribution for the galaxy SPT0346-52. It uses input flux measurements from a range of wavelengths in order to find the best-fitting spectrum given a range of model spectra. Since a galaxy’s spectrum changes with age, mass, dust content and other factors, the best-fitting model can tell us a lot about the physical properties within the galaxy. In this best-fitting model the authors found that AGN likely contribute less than 5% to the total infrared luminosity of this galaxy. [Ma et al. 2016]

Results and What They Can Tell Us

In almost 14 hours of X-ray observations, just over 3 photons were detected, and over one hour of radio observations yielded no detection at the source position…

However, together with the previous multi-wavelength observations, it was possible to use the upper limits from the X-ray and radio part of the spectrum to constrain the shape of the spectrum of SPT0346-52. This indicated that a pure starburst spectrum of the galaxy is the best fit to the observations. Similarly, the upper limits of the ATCA radio observations are consistent with a star-forming galaxy, and would have to be a factor of at least 2 higher for a radio-loud AGN.

One of the Most Extreme Starbursting Galaxies in the Universe

Also, while the presence of an AGN cannot be completely ruled out, the authors conclude that star-formation — and not AGN activity — is the dominant driver behind the far-infrared emission observed. The authors plotted SPT0346-52 on a diagram showing the distribution of X-ray luminosity vs far-infrared luminosity for both star-forming and AGN-dominated galaxies (see Fig. 3). This shows that SPT0346-52 lies left of the dividing line between star-formation and AGN, making it likely that it is indeed powered by very high star formation rates.

One question, however, remains:

Fig. 3: This plot shows the positions of a range of galaxies on a far-infrared vs. X-ray luminosity diagram. SPT0346-52 is the red arrow, while star-forming galaxies are marked as blue S, and AGN as yellow A. Using the dividing lines between AGN and star-formation driven luminosities indicated on the plot suggests that SPT0346-52 does not contain an AGN. Hence it appears as though SPT0346-52’s extreme brightness is powered by star-formation.

What Could Power Such High Star Formation Rates?

The authors offer several possible explanations for the finding that SPT0346-52 has one of the highest star formation rate densities (a measure of how many stars form each year in a given volume) in the universe.

They suggest that SPT0346-52 may have an especially high star formation efficiency; probably higher than about 40%. This means that SPT0346-52 converts 40% of its available hydrogen gas into stars; a more typical values for galaxies like the Milky Way is around 5%. Additionally, the gas fraction — the amount of gas available for this star formation within the galaxy — is also exceptionally high at above 40%. The authors suggest that it might be possible to explain the very high star formation rate in SPT0346-52 with such a high gas fraction, even without a very high star formation efficiency.

Hence, despite the many observations of SPT0346-52, several mysteries still remain unsolved. The authors suggest that this would make SPT0346-52 an ideal candidate for follow-up observations with the JWST, which is due to launch in 2018…

About the author, Steph Greis:

I’m a third (out of four) year PhD student at the University of Warwick, UK, where I study local analogues to redshift z~5 Lyman break galaxies (LBGs) which are some of the earliest galaxies in the Universe. When I’m not thinking about galaxies far, far away, I enjoy reading, cooking, and geocaching.

8 November 2016

Modest Chaos in the Early Solar System

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: The Asteroid Belt as a Relic From a Chaotic Early Solar System
Authors: Andre Izidoro, Sean N. Raymond, Arnaud Pierens, Alessandro Morbidelli, Othon C. Winter, David Nesvorny
First Author’s Institution: Laboratoire d’astrophysique de Bordeaux, Pessac, France
Status: Accepted to ApJ

The puzzling architecture of the Solar system has long been a headache for planetary dynamicists. We can sort of divide its structure (cover image, not to scale) into several zones. First, the terrestrial planets Mercury, Venus, Earth and Mars, which are divided from the gas and ice giant planets by the asteroid belt. Beyond the ice giants there is the Kuiper belt, which spans out to very large distances from the Sun. One longstanding conundrum in this ordering is the relatively small mass of Mars and existence of the asteroid belt in between Mars and Jupiter. It shouldn’t be there, and Mars should be way bigger. In fact, Mars is only 10% of the mass of Earth and therefore seems to have never accreted enough material to become a fully fledged planet. In planet formation, Mars-sized objects are usually termed “planetary embryos”, as we think this is the intermediate stage of a planet’s growth.

Sailing adolescent Jupiter to blame?

To explain the apparent dip in the mass-distance relation of our solar system (increasing mass with planetary distance from the Sun until Jupiter, and then decreasing again), planetary scientists developed a model deeply rooted in dynamical principles: the “Grand Tack”. The idea of the model is that Jupiter, directly after its formation out of the early Solar nebula at ~3.5 AU (AU = distance Sun–Earth), migrated toward the Sun, became “tacked” on at ~1.5 AU and then migrated outward again to its current positon at 5.2 AU. By doing so it depleted the mass concentrated at the locations of nowadays Mars and the asteroid belt, and thus Mars was never able to grow bigger. It also successfully explains some other issues, such as the inclinations and excitations of asteroids and the transition from water-poor to water-rich asteroids in the middle of the belt.

Figure 2: Chaos in the giant planets's orbits and their long-term stability. During the gas disk phase the giant planets migrated inward toward the Sun and are eventually locked in so-called mean-motion resonances. After the gas dissipation Saturn and Jupiter orbit in chaotic, but stable orbits. (Fig. 8 from the paper)

Chaos in the giant planets’ orbits and their long-term stability. During the gas-disk phase the giant planets migrated inward toward the Sun and are eventually locked in so-called mean-motion resonances. After the gas dissipation, Saturn and Jupiter orbit in chaotic but stable orbits. [Izidoro et al. 2016]

A more “primordial” origin

Alternatively, the mass-depletion at Mars’ location could have very early roots, even before (or during) the formation of Jupiter, caused by microphysics in the disk, as was explained in this Astrobite about the pile-up and evolution of dust. Here, the initial small dust grains in the disks did not accumulate everywhere in the disk and start to form planets. Instead, they end up only in specific regions and thus planet formation is concentrated in some narrow zones. In our case, this could be in two locations: one in the inner region, where the terrestrial planets reside nowadays, and one in the outer region, possibly around the region of Jupiter (more details given in this recent review paper). Such a scenario however poses another problem. The Grand Tack scenario from above easily explains the current relatively high inclinations and excitations of the asteroids in the belt: Jupiter drops by and gives everyone a huge gravitational swing due to its enormous mass. In the other scenario this isn’t the case — so we would nominally expect the asteroids to remain in relatively calm and low-excitation orbits.

But Andé Izidoro and collaborators think differently. They propose that instead of migrating (like in the Grand Tack model), Jupiter and Saturn were on chaotic but stable orbits. This scenario is illustrated in the figure above, which shows how the gas and ice giants evolved over time in such a configuration. Essentially what happens is that all the planets roughly formed at approximately where they are today. However, as our early solar system was subject to dynamic evolution (we possibly lost some planetary embryos early on), the orbits of Jupiter and Saturn moved continously and chaotically through the disk (though not nearly as dramatically as in the Grand Tack scenario). Thus, Jupiter came close to the asteroid belt again and its gravity excited the asteroids. You’ll probably ask yourself now where the great difference lies. This is highlighted in the figure below, where the deviations between the Grand Tack and the “chaotic motion” pictures become readily apparent.

In the Grand Tack model, Jupiter is formed within a sea of primordial planetesimals/asteroids (which are divided into S- and C-types: water-poor and water-rich). It migrates inwards and then outwards, and scatters some of the outer asteroids to their current locations. In the “low-mass asteroid belt”/”chaotic motion” model the S- and C-type deviation and the mass-depletion at around Mars’ current location is primordial. Jupiter and Saturn form later and evolve in chaotic but stable orbits, which scatters some of the outer asteroids inside. [Morbidelli & Raymond, 2016, arXiv:1610.07202]

What now?

Instead of one we have now two models, both of which can give us an answer to why our solar system’s architecture looks like it does today. Fortunately, the two models make different predictions about how the dynamics evolved in the disks. In the coming years we may thus be able to distinguish between them by testing their differences, and we may be able to get deeper insights into how our home in the universe evolved into how it looks today.

About the author, Tim Lichtenberg:

I am a graduate student at ETH Zurich in planetary astrophysics. By combining astro- and geophysical numerical approaches I try to understand the influence of star-forming environments on the formation of terrestrial planets. Occasionally, when I am not at the bus swinging back and forth between the institutes, I enjoy doing sports or become involved in science outreach projects.

1 November 2016

A Long Time Ago Around a Star Far, Far Away…

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: OGLE-2012-BLG-0950Lb: The First Planet Mass Measurement from Only Microlens Parallax and Lens Flux
Authors: N. Koshimoto, A. Udalski, J.P. Beaulieu et al.
First Author’s Institution: Department of Earth and Space Sciences, Osaka University, Japan
Status: Accepted in AJ

In today’s paper, we’re looking at a planet discovery. The planet in question is called OGLE-2012-BLG-0950Lb. That’s a bit of a mouthful, so for the purposes of this article I’m going to nickname it Oggy.

Oggy is an unusual planet. It’s intrinsically fairly normal: a planet roughly the size of Neptune, orbiting an M-dwarf star. But Oggy is 3 kiloparsecs, or 9,800 light years, away, making it one of the most distant planets ever discovered. That’s because it was discovered via the microlensing technique.

A Quick Microlensing Primer:

Microlensing is a bit of a forgotten relative of observational exoplanet science — we hear about transit and radial velocity planets all the time, and direct imaging is intuitively simple and makes pretty pictures, so it gets lots of press time too. The microlensing technique, meanwhile, has been quietly churning out planet detections since 2004. And yet, if you asked your average exoplanet scientist how microlensing works, they’d probably panic and mutter something about general relativity before running away as soon as possible.

Microlensing works like this: your system consists of a lens star, which magnifies light from a source star. The source star is moving relative to the lens, and the closer it gets to the lens, the more the light rays are bent, and the more the lens magnifies the source. One could plot contours of equal magnification — they would be circles, traced around the lens star, as I’ve tried to demonstrate in Figure 1. The closer to the lens, the more a source would be magnified. The background source will move across the diagram following the red arrow, and as the flux increases and then decreases a flux peak is observed.

Figure 1: On the left, the blue contours show regions of equal magnification — increasing towards the centre where the star is positioned. On the right is the sort of flux curve you’d observe as the source passes behind the lens, following the red arrow. [Astrobites]

Now, if the lens star also has a planet, this can warp the light rays even more. In fact, the interference between the two gravitational sources can even create places where the magnification of the source is infinite. Alternatively, depending on the system geometry, it can reduce the magnification of the lens for a very short time. As the source moves across the sky, there’s an added peak or trough, on top of the peak from the lens magnifying the source. Figure 2 shows a schematic of this setup.

Figure 2: Three lens diagrams for the three possible scenarios. Left: a source is magnified by a lens star. Centre and right: the lens star now hosts a planet, which also warps the light: either enhancing or reducing the lens effect for a short period of time, and creating an additional bump or dip on top of the lensing curve. [Astrobites]

Confused yet?

Applying this to Oggy:

During the summer of 2012, two different microlensing surveys — one called OGLE and one called MOA — both independently identified a flux ramp-up around the same star. A source was just beginning its journey behind a lens. They monitored the event, and I’ve shown their data in Figure 3. This matches the situation in Figure 2: there’s a wide peak as the source passes behind the star, and a teensy dip at day 6149, where the planet briefly dims the source — a zoom-in of this dip is shown in the bottom panel.

Figure 3: The microlensing event where planet Oggy was identified. Top: the whole event as the source star passes behind the lens. Bottom left: a close up of the effect of the planet on the observed flux. Bottom right: a close up of the final dimming of the event as the source passes out from behind the star. [Koshimoto et al. 2016]

All of these data can be fitted to gain an understanding of the lens and planet system, and work out what’s actually been seen by the team. However, there are a lot of free parameters in the system, and only a few free parameters for your curve, so the fit is degenerate. In other words, the curve isn’t enough to constrain all the parameters, and there are multiple different good fits. There are various ways to solve this problem: for example, the length of time that the source is magnified might tell you about the source diameter (since it will be magnified for slightly longer if it is slightly bigger). However, in the case of Oggy, the source size is consistent with zero — so the errors are too big for the magnification duration to give us any information.

However, the total passage of the star is a fairly long event, with the star being magnified for a total of ~120 days. This means that the orbital motion of the Earth during the microlensing event is actually detectable! The models that include the so-called ‘microlens parallax’ are shown in Figure 3, and they fit better than those that ignore the Earth’s motion. For completeness, the authors also use a third model, one they call the xallarap model, where the orbital motion of the lens is considered (kind of like a backwards parallax). However, this is found to be a less good fit than the parallax model.

By determining the microlens parallax, and with some additional Keck telescope observations that confirm the flux of the lens star, the authors calculate a mass for the planet and for the star. It’s 35 times the mass of the Earth, orbiting an M-dwarf host star, with a projected distance of three times the Earth–Sun distance. Oggy is the first planet for which a mass has been determined with just the parallax and the lens flux.

So, why should you care?

Microlensing is exciting as it’s the only method that can find these extremely far away planets. This allows us to figure out whether our part of the galaxy is representative: does the galactic bulge have as many planets as we do out here in a spiral arm? Are the planets in the galactic bulge generally bigger or smaller than ours? For now, microlensing is the only technique that can answer this question.

Since several microlensing surveys are now being carried out with simultaneous ground and space observations (using the Spitzer and Kepler telescopes), there will be more and more planets identified where the microlens parallax can be very precisely determined — so the method used here will be valuable in calculating masses for these future discoveries. Oggy is a great benchmark, proving that these missions will give valuable scientific output.

About the author, Elisabeth Matthews:

I’m currently a second year PhD student at the University of Exeter, in the south of England, where I’m aiming to detect and characterise extra-solar planets and debris disks via direct imaging. So far this has meant lots of detecting background stars that happen to be near bright, nearby stars and no detecting of actual planets — but hopefully my luck will change soon!

Previous 1 … 32 33 34 35 36 Next