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protoplanetary disk

Data from the Atacama Large Millimeter/submillimeter Array (ALMA) has recently revealed the first detection of gas-phase methanol, a derivative of methane, in a protoplanetary disk. This milestone discovery is an important step in understanding the conditions for planet formation that can lead to life-supporting planets like Earth.

Planetary Chemistry

One major goal in the study of exoplanets is to find planets that orbit in their host stars’ habitable zones, a measure that determines whether the planet receives the right amount of sunlight to support liquid water. But there’s another crucial element in the formation of a life-supporting planet: chemistry.

To understand the chemistry of newly born planets, we need to study protoplanetary disks — because it’s from these that young planets form. The elements and molecules contained in these dusty disks are what initially make up the atmospheres of planets forming within the disks.

ALMA

The Atacama Large Millimeter/submillimeter Array under the southern sky. [ESO/B. Tafreshi]

The Hunt for Complexity

The detection of complex molecules in protoplanetary disks is an important milestone, because complex molecules are necessary to build the correct chemistry to support life. Unfortunately, detecting these molecules is very difficult, requiring observations with both high spatial resolution and high sensitivity. Thus far, though we’ve observed elements and simple molecules in protoplanetary disks, detections of complex molecules have been elusive — with only one success before now.

Luckily, we now have an observatory up to the challenge! ALMA’s unprecedented spatial resolution and sensitivity has recently allowed a team of scientists led by Catherine Walsh (Leiden University) to observe gas-phase methanol in a protoplanetary disk for the first time. This detection was made in the disk around the young star TW Hya, and it represents one of the largest molecules that has ever been observed in a disk to date.

Locating Ices

methanol detection

The model (purple line) and data (dashed line) showing the methanol line detection. [Adapted from Walsh et al. 2016]

Since TW Hya’s disk has temperatures of less than ~100K (-173°C), we would expect most of the disk’s methanol to be frozen. The gas-phase methanol observed by Walsh and collaborators was likely released from a larger reservoir of frozen methanol residing on dust grains in the disk. The peak of the methanol emission was detected from a ring located about 30 AU out from the central star, which suggests that the larger dust grains in the disk — located in the inner 50 AU — may host the bulk of the disk ice reservoir.

Walsh and collaborator’s important detection opens a window into studying complex organic chemistry during planetary system formation. This stepping stone can help us to better understand the conditions when Earth formed and what we should look for in the search for life-supporting planets.

Citation

Catherine Walsh et al 2016 ApJ 823 L10. doi:10.3847/2041-8205/823/1/L10

Kepler planets

What was the big deal behind the Kepler news conference yesterday? It’s not just that the number of confirmed planets found by Kepler has more than doubled (though that’s certainly exciting news!). What’s especially interesting is the way in which these new planets were confirmed.

planet discoveries

Number of planet discoveries by year since 1995, including previous non-Kepler discoveries (blue), previous Kepler discoveries (light blue) and the newly validated Kepler planets (orange). [NASA Ames/W. Stenzel; Princeton University/T. Morton]

No Need for Follow-Up

Before Kepler, the way we confirmed planet candidates was with follow-up observations. The candidate could be validated either by directly imaging (which is rare) or obtaining a large number radial-velocity measurements of the wobble of the planet’s host star due to the planet’s orbit. But once Kepler started producing planet candidates, these approaches to validation became less feasible. A lot of Kepler candidates are small and orbit faint stars, making follow-up observations difficult or impossible.

This problem is what inspired the development of what’s known as probabilistic validation, an analysis technique that involves assessing the likelihood that the candidate’s signal is caused by various false-positive scenarios. Using this technique allows astronomers to estimate the likelihood of a candidate signal being a true planet detection; if that likelihood is high enough, the planet candidate can be confirmed without the need for follow-up observations.

A breakdown of the catalog of Kepler Objects of Interest (click for a closer look!). Just over half had previously been identified as false positives or confirmed as candidates. 1284 are newly validated, and another 455 have FPP between 10 and 90%. [Morton et al. 2016]

A breakdown of the catalog of Kepler Objects of Interest. Just over half had previously been identified as false positives or confirmed as candidates. 1284 are newly validated, and another 455 have FPP of 10–90%. [Morton et al. 2016]

Probabilistic validation has been used in the past to confirm individual planet candidates in Kepler data, but now Timothy Morton (Princeton University) and collaborators have taken this to a new level: they developed the first code that’s designed to do fully automated batch processing of a large number of candidates.

In a recently published study — the results of which were announced yesterday — the team applied their code to the entire catalog of 7,470 Kepler objects of interest.

New Planets and False Positives

The team’s code was able to successfully evaluate the total false-positive probability (FPP) for 7,056 of the objects of interest. Of these, 428 objects previously identified as candidates were found to have FPP of more than 90%, suggesting that they are most likely false positives.

Confirmed and candidates

Periods and radii of candidate and confirmed planets in the Kepler Objects of Interest catalog. Blue circles have previously been identified as confirmed planets. Candidates (orange) are shaded by false positive probability; more transparent means more likely to be a false positive. [Morton et al. 2016]

In contrast, 1,935 candidates were found to have FPP of less than 1%, and were therefore declared validated planets. Of these confirmations, 1,284 were previously unconfirmed, more than doubling Kepler’s previous catalog of 1,041 confirmed planets. Morton and collaborators believe that 9 of these newly confirmed planets may fall within the habitable zone of their host stars.

While the announcement of 1,284 newly confirmed planets is huge, the analysis presented in this study is the real news. The code used is publicly available and can be applied to any transiting exoplanet candidate. This means that this analysis technique can be used to find batches of exoplanets in data from the extended Kepler mission (K2) or from the future TESS and PLATO transit missions.

Citation

Timothy D. Morton et al 2016 ApJ 822 86. doi:10.3847/0004-637X/822/2/86

Giant impact

Earth has experienced a large number of impacts, from the cratering events that may have caused mass extinctions to the enormous impact believed to have formed the Moon. A new study examines whether our planet’s impact history is typical for Earth-like worlds.

N-Body Challenges

Timeline

Timeline placing the authors’ simulations in context of the history of our solar system (click for a closer look). [Quintana et al. 2016]

The final stages of terrestrial planet formation are thought to be dominated by giant impacts of bodies in the protoplanetary disk. During this stage, protoplanets smash into one another and accrete, greatly influencing the growth, composition, and habitability of the final planets.

There are two major challenges when simulating this N-body planet formation. The first is fragmentation: since computational time scales as N^2, simulating lots of bodies that split into many more bodies is very computationally intensive. For this reason, fragmentation is usually ignored; simulations instead assume perfect accretion during collisions.

Total bodies

Total number of bodies remaining within the authors’ simulations over time, with fragmentation included (grey) and ignored (red). Both simulations result in the same final number of bodies, but the ones that include fragmentation take more time to reach that final number. [Quintana et al. 2016]

The second challenge is that many-body systems are chaotic, which means it’s necessary to do a large number of simulations to make statistical statements about outcomes.

Adding Fragmentation

A team of scientists led by Elisa Quintana (NASA NPP Senior Fellow at the Ames Research Center) has recently pushed at these challenges by modeling inner-planet formation using a code that does include fragmentation. The team ran 140 simulations with and 140 without the effects of fragmentation — using similar initial conditions — to understand how including fragmentation affects the outcome.

Quintana and collaborators then used the fragmentation-inclusive simulations to examine the collisional histories of Earth-like planets that form. Their goal is to understand if our solar system’s formation and evolution is typical or unique.

How Common Are Giant Impacts?

Number of giant impacts

Histogram of the total number of giant impacts received by the 164 Earth-like worlds produced in the authors’ fragmentation-inclusive simulations. [Quintana et al. 2016]

The authors find that including fragmentation does not affect the final number of planets that are formed in the simulation (an average of 3–4 in each system, consistent with our solar system’s terrestrial planet count). But when fragmentation is included, fewer collisions end in merger — which results in typical accretion timescales roughly doubling. So the effects of fragmentation influence the collisional history of the system and the length of time needed for the final system to form.

Examining the 164 Earth-analogs produced in the fragmentation-inclusive simulations, Quintana and collaborators find that impacts large enough to completely strip a planet’s atmosphere are rare; fewer than 1% of the Earth-like worlds experienced this.

But giant impacts that are able to strip ~50% of an Earth-analog’s atmosphere — roughly the energy of the giant impact thought to have formed our Moon — are more common. Almost all of the authors’ Earth-analogs experienced at least 1 giant impact of this size in the 2-Gyr simulation, and the average Earth-like world experienced ~3 such impacts.

These results suggest that our planet’s impact history — with the Moon-forming impact likely being the last giant impact Earth experienced — is fairly typical for Earth-like worlds. The outcomes also indicate that smaller impacts that are still potentially life-threatening are much more common than bulk atmospheric removal. Higher-resolution simulations could be used to examine such smaller impacts.

Citation

Elisa V. Quintana et al 2016 ApJ 821 126. doi:10.3847/0004-637X/821/2/126

CME

Coronal mass ejections (CMEs) and solar flares are two examples of major explosions from the surface of the Sun — but they’re not the same thing, and they don’t have to happen at the same time. A recent study examines whether we can predict which solar flares will be closely followed by larger-scale CMEs.

solar flare

Image of a solar flare from May 2013, as captured by NASA’s Solar Dynamics Observatory. [NASA/SDO]

Flares as a Precursor?

A solar flare is a localized burst of energy and X-rays, whereas a CME is an enormous cloud of magnetic flux and plasma released from the Sun. We know that some magnetic activity on the surface of the Sun triggers both a flare and a CME, whereas other activity only triggers a confined flare with no CME.

But what makes the difference? Understanding this can help us learn about the underlying physical drivers of flares and CMEs. It also might help us to better predict when a CME — which can pose a risk to astronauts, disrupt radio transmissions, and cause damage to satellites — might occur.

In a recent study, Monica Bobra and Stathis Ilonidis (Stanford University) attempt to improve our ability to make these predictions by using a machine-learning algorithm.

Classification by Computer

number of features

Using a combination of 6 or more features results in a much better predictive success (measured by the True Skill Statistic; higher positive value = better prediction) for whether a flare will be accompanied by a CME. [Bobra & Ilonidis 2016]

Bobra and Ilonidis used magnetic-field data from an instrument on the Solar Dynamics Observatory to build a catalog of solar flares, 56 of which were accompanied by a CME and 364 of which were not. The catalog includes information about 18 different features associated with the photospheric magnetic field of each flaring active region (for example, the mean gradient of the horizontal magnetic field).

The authors apply a machine-learning algorithm known as a binary classifier to this catalog. This algorithm tries to predict, given a set of features, whether an active region that produces a flare will also produce a CME. Bobra and Ilonidis then use a feature-selection algorithm to try to understand which features distinguish between flaring regions that don’t produce a CME and those that do.

Predictors of CMEs

The authors reach several interesting conclusions:

  1. Under the right conditions, their algorithm is able to predict whether an active region with a given set of features will produce a CME as well as a flare with a fairly high rate of success.
  2. None of the 18 features they tested are good predictors in isolation: it’s necessary to look at a combination of at least 6 features to have success predicting whether a flare will be accompanied by a CME.
  3. The features that are the best predictors are all intensive features — ones that stay the same independent of the active region’s size. Extensive features — ones that change as the active region grows or shrinks — are less successful predictors.

Only the magnetic field properties of the photosphere were considered, so a logical next step is to extend this study to consider properties of the solar corona above active regions as well. In the meantime, these are interesting first results that may well help us better predict these major solar eruptions.

Bonus

Check out this video for a great description from NASA of the difference between solar flares and CMEs (as well as some awesome observations of both).

Citation

M. G. Bobra and S. Ilonidis 2016 ApJ 821 127. doi:10.3847/0004-637X/821/2/127

A new type of galaxy has just been added to the galaxy zoo: a small, compact, and old elliptical galaxy that shows signs of a monster black hole actively accreting material in its center. What can this unusual discovery tell us about how compact elliptical galaxies form?

A New Galactic Beast

Compact elliptical galaxies are an extremely rare early-type dwarf galaxy. Consistent with their name, compact ellipticals are small, very compact collections of ancient stars; these galaxies exhibit a high surface brightness and aren’t actively forming stars.

SDSS J085431.18+173730.5

Optical view of the ancient compact elliptical galaxy SDSS J085431.18+173730.5 (center of image) in an SDSS color composite image. [Adapted from Paudel et al. 2016]

Most compact ellipticals are found in dense environments, particularly around massive galaxies. This has led astronomers to believe that compact ellipticals might form via the tidal stripping of a once-large galaxy in interactions with another, massive galaxy. In this model, once the original galaxy’s outer layers are stripped away, the compact inner bulge component would be left behind as a compact elliptical galaxy. Recent discoveries of a few isolated compact ellipticals, however, have strained this model.

Now a new galaxy has been found to confuse our classification schemes: the first-ever compact elliptical to also display signs of an active galactic nucleus. Led by Sanjaya Paudel (Korea Astronomy and Space Science Institute), a team of scientists discovered SDSS J085431.18+173730.5 serendipitously in Sloan Digital Sky Survey data. The team used SDSS images and spectroscopy in combination with data from the Canada-France-Hawaii Telescope to learn more about this unique galaxy.

Puzzling Characteristics

SDSS J085431.18+173730.5 presents an interesting conundrum. Ancient compact ellipticals are supposed to be devoid of gas, with no fuel left to trigger nuclear activity. Yet SDSS J085431.18+173730.5 clearly shows the emission lines that indicate active accretion onto a supermassive black hole of ~2 million solar masses, according to the authors’ estimates. Paudel and collaborators show that this mass is consistent with the low-mass extension of the known scaling relation between central black-hole mass and brightness of the host galaxy.

BH mass v. bulge brightness scaling relation

Central black hole mass vs. bulge K-band magnitude. SDSS J085431.18+173730.5 (red dot) falls right on the low-mass extension of the observed scaling relation. It has similar properties to M32, another compact elliptical galaxy. [Adapted from Paudel et al. 2016]

To add to the mystery, SDSS J085431.18+173730.5 has no nearby neighbors: like the few other isolated compact ellipticals recently discovered, there are no massive galaxies in the immediate vicinity that could have led to its tidal stripping. So how was this puzzling ancient galaxy formed?

The authors of this study support a previously proposed “flyby” scenario: isolated compact ellipticals may simply be tidally stripped systems that ran away from their hosts. Paudel and collaborators suggest that SDSS J085431.18+173730.5 might have long ago interacted with NGC 2672 — a galaxy group located a whopping 6.5 million light-years away — before being flung out to its current location.

Further studies of this unique galaxy’s emission profile, as well as efforts to learn about its underlying stellar population and central kinematics, will hopefully help us to better understand not only the origins of this galaxy, but how all compact ellipticals form and evolve.

Citation

Sanjaya Paudel et al 2016 ApJ 820 L19. doi:10.3847/2041-8205/820/1/L19

TYC-2505-672-1

A new record holder exists for the longest-period eclipsing binary star system: TYC-2505-672-1. This intriguing system contains a primary star that is eclipsed by its companion once every 69 years — with each eclipse lasting several years!

120 Years of Observations

In a recent study, a team of scientists led by Joseph Rodriguez (Vanderbilt University) characterizes the components of TYC-2505-672-1. This binary star system consists of an M-type red giant star that undergoes a ~3.45-year-long, near-total eclipse with a period of ~69.1 years. This period is more than double that of the previous longest-period eclipsing binary!

Rodriguez and collaborators combined photometric observations of TYC-2505-672-1 by the Kilodegree Extremely Little Telescope (KELT) with a variety of archival data, including observations by the American Association of Variable Star Observers (AAVSO) network and historical data from the Digital Access to a Sky Century @ Harvard (DASCH) program.

In the 120 years spanned by these observations, two eclipses are detected: one in 1942-1945 and one in 2011-2015. The authors use the observations to analyze the components of the system and attempt to better understand what causes its unusual light curve.

Characterizing an Unusual System

TYC-2505-672-1 light curve

Observations of TYC-2505-672-1 plotted from 1890 to 2015 reveal two eclipses. (The blue KELT observations during the eclipse show upper limits only.) [Rodriguez et al. 2016]

By modeling the system’s emission, Rodriguez and collaborators establish that TYC-2505-672-1 consists of a 3600-K primary star — that’s the M giant — orbited by a small, hot, dim companion that’s a toasty 8000 K. But if the companion is small, why does the eclipse last several years?

The authors argue that the best model of TYC-2505-672-1 is one in which the small companion star is surrounded by a large, opaque circumstellar disk. Rodriguez and collaborators suggest that the companion could be a former red giant whose atmosphere was stripped from it, leaving behind the small, hot core shrouded by a large, cool disk of stripped gas. The large size of the disk causes the eclipse of the primary to last for years, as viewed from Earth.

The authors estimate the properties such a disk would need to produce the observed light curve. They find that if the companion were surrounded by a disk several AU in diameter, it could orbit at a distance of ~20-30 AU from the primary and reproduce the emission we see.

The next eclipse of TYC-2505-672-1 will begin in April 2080. We needn’t wait until then to gather more information about this system, however! Radial velocity measurements will help establish the masses of the two components, and high-cadence UV observations could reveal more about the evolutionary state of the system. Studying this extreme binary provides an excellent opportunity to learn more about the environments in late-life star systems.

Citation

Joseph E. Rodriguez et al 2016 AJ 151 123. doi:10.3847/0004-6256/151/5/123

Stephan's Quintet

In this age of large astronomical surveys, one major scientific bottleneck is the analysis of enormous data sets. Traditionally, this task requires human input — but could computers eventually take over? A pair of scientists explore this question by testing whether computers can classify galaxies as well as humans.

classification disagreement

Examples of disagreement: galaxies that Galaxy-Zoo humans classified as spirals with >95% agreement, but the computer algorithm classified as ellipticals with >70% certainty. Most are cases where the computer got it wrong — but not all of them. [Adapted from Kuminski et al. 2016]

Limits of Citizen Science

Galaxy Zoo is an internet-based citizen science project that uses non-astronomer volunteers to classify galaxy images. This is an innovative way to provide more manpower, but it’s still only practical for limited catalog sizes. How do we handle the data from upcoming surveys like the Large Synoptic Survey Telescope (LSST), which will produce billions of galaxy images when it comes online?

In a recent study by Evan Kuminski and Lior Shamir, two computer scientists at Lawrence Technological University in Michigan, a machine learning algorithm known as Wndchrm was used to classify a dataset of Sloan Digital Sky Survey (SDSS) galaxies into ellipticals and spirals. The authors’ goal is to determine whether their algorithm can classify galaxies as accurately as the human volunteers for Galaxy Zoo.

Automatic Classification

After training their classifier on a small set of spiral and elliptical galaxies, Kuminski and Shamir set it loose on a catalog of ~3 million SDSS galaxies. The classifier first computes a set of 2,885 numerical descriptors (like textures, edges, and shapes) for each galaxy image, and then uses these descriptors to categorize the galaxy as spiral or elliptical.

Rate of agreement

Rate of agreement of the computer classification with human classification (for the Galaxy Zoo “superclean” subset) for different ranges of computed classification certainties. For certainties above 54% for spirals and 80% for ellipticals, the agreement is over 98%. [Kuminski et al. 2016]

In addition, the classifier calculates a certainty level for each classification, with the certainties adding to 100%: a galaxy categorized as spiral at 85% certainty is categorized as elliptical at 15% certainty. This provides a quantity/quality tradeoff, allowing for the creation of subcatalogs by cutting at specific certainty levels. Selecting for a high level of certainty decreases the sample size, but increases the sample’s classification accuracy.

Comparing the Outcome

To evaluate the accuracy of the algorithm’s findings, the authors examined SDSS galaxies that had also been classified by Galaxy Zoo. In particular, they used a 45,000-galaxy subset that consists only of “superclean” Galaxy Zoo galaxies — meaning the human volunteers who categorized them were in agreement at a level of 95% or higher.

Numbers of galaxies

Number of spiral and elliptical galaxies classified above different certainty levels. Cutting at the 54% certainty level for spirals and 80% for ellipticals leaves ~900,000 and ~600,000 spiral and elliptical galaxies, respectively. [Kuminski et al. 2016]

In this set, Kuminski and Shamir found that if they draw a cut-off at the 54% certainty level for spiral galaxies and the 80% certainty level for ellipticals, they find 98% agreement between the computer classification of the galaxies and the human classification via Galaxy Zoo. Applying these cuts to the entire sample resulted in the identification of ~900,000 spiral galaxies and ~600,000 ellipticals, representing the largest catalog of its kind.

The authors acknowledge that completeness is a problem; half the data had to be cut to achieve this level of accuracy. Sacrificing some data can still result in very large catalogs, however — and as surveys become more powerful and large databases become more prevalent, algorithms such as this one will likely become critical to the scientific process.

Citation

Evan Kuminski and Lior Shamir 2016 ApJS 223 20. doi:10.3847/0067-0049/223/2/20

Planet Nine

Recent studies have identified signs of an unseen, distant ninth planet in our solar system. How might we find the elusive Planet Nine? A team of scientists suggests the key might be cosmology experiments.

A Hypothetical Planet

Kuiper orbits

Orbits of six distant Kuiper-belt objects. Their clustered perihelia and orbital orientations suggest they may have been shepherded by a massive object, hypothesized to be Planet Nine. [Caltech/Robert Hurt]

Early this year, a study was published that demonstrated that the clustered orbits of distant Kuiper belt objects (and several other features of our solar system) can be explained by the gravitational tug of a yet-undiscovered planet. This hypothetical Planet Nine is predicted to be a giant planet similar to Neptune or Uranus, with a mass of more than ~10 Earth masses, currently orbiting ~700 AU away.

In a recent study, a team of scientists led by Nicolas Cowan (McGill University in Canada) has estimated the blackbody emission expected from Planet Nine. The team proposes how we might be able to search for this distant body using its heat signature.

Heat from an Icy World

Cowan and collaborators first estimate Planet Nine’s effective temperature, based on the solar flux received at ~700 AU and assuming its internal heating is similar to Uranus or Neptune. They find that Planet Nine’s effective temperature would likely be an icy ~30–50 K, corresponding to a blackbody peak at 50–100 micrometers.

Planet Nine search space

Search space for Planet Nine. Based on its millimeter flux and annual parallax motion, several current and future cosmology experiments may be able to detect it. Experiments’ resolution ranges are shown with blue boxes. [Cowan et al. 2016]

How can we detect an object with emission that peaks in this range? Intriguingly, cosmology experiments monitoring the cosmic microwave background (CMB) radiation are optimized for millimeter flux. At a wavelength of 1mm, Cowan and collaborators estimate that Planet Nine would have a very detectable flux level of ~30 mJy. The authors propose that CMB experiments with high enough resolution (~5m telescopes and larger) could have the ability to detect Planet Nine!

There’s one major catch: how can we differentiate between Planet Nine and the ~4000 foreground asteroids that are brighter than 30 mJy at millimeter wavelengths?

Cowan and collaborators argue that this can be done using a combination of asteroid databases and parallax measurements. The authors calculate that Planet Nine should move roughly a few arcseconds per day, mostly due to parallax. In comparison, asteroids will move ~10 arcminutes per day in a combination of proper motion and parallax — an order of magnitude faster than Planet Nine.

Resolution Constraints

To hunt down Planet Nine, we therefore need telescopes that can not only resolve a 30 mJy point source, but can also resolve an annual parallax motion of ~5 arcminutes per year.

The authors demonstrate that several current and planned CMB experiments have the resolution and ability to detect Planet Nine, provided that they map large swatches of the sky and return to the same regions every few months. These experiments include CCAT, the South Pole Telescope, the Atacama Cosmology Telescope, CMB-S4, and even possibly Planck.

With the astronomical community coming together to brainstorm ways to track down this elusive possible planet, the use of CMB experiments is an intriguing option. And even if Planet Nine is discovered by other means, measuring its heat signature will teach us more about the internal workings of giant planets.

Citation

Nicolas B. Cowan et al 2016 ApJ 822 L2. doi:10.3847/2041-8205/822/1/L2

Fornax cluster

Traditionally, dense cluster centers are cannibalistic environments, with larger galaxies stripping stars from smaller interlopers in minor mergers and dynamical harassment. A recent survey of the Fornax cluster, one example of such an environment, reveals how this cluster may have been built.

Clues in Halos

Fornax cluster context

Context for the southern constellation Fornax (“the furnace”). The Fornax cluster is marked with a red circle. [ESO, IAU and Sky & Telescope]

Deep surveys of dense cluster environments are necessary because the imprint of mass assembly is hidden in galactic halos, the faint outer regions of galaxies. Deep observations can reveal answers to questions about how the galaxies in these extreme environments formed and evolved — for instance, did the majority of the galaxies’ stars form in situ, or were they accreted from interactions with other galaxies?

The Fornax Deep Survey (FDS) is just such a campaign. FDS uses the European Southern Observatory’s VLT Survey Telescope to obtain deep photometry of the entire 26 square degrees of the Fornax cluster, a spectacular galaxy cluster located 65 million light-years away.

Central Observations

The FDS team plans to release the full results from the survey soon. For now, in an initial study led by Enrichetta Iodice (INAF’s Astronomical Observatory of Capodimonte, Italy), the team presents their first findings from the two square degrees around NGC 1399, a supergiant elliptical galaxy in the cluster center.

The two main results from this study are:

  1. The discovery of a faint stellar bridge between NGC 1399 and a nearby galaxy, NGC 1387.
  2. The characterization of NGC 1399’s light profile, which shows that the galaxy consists of two main components separated by a strong break. The bright central galaxy is likely composed of stars that formed in situ, whereas the exponential outer component is a stellar halo composed of stars likely captured from accretion events.

What do these points tell us about the history of the center of the Fornax cluster? These observations are indications that the Fornax cluster was built up by mergers and accretion events.

A Violent Past

The light profile the authors found is consistent with those of simulated galaxies whose halos were formed through the multiple accretion of progenitors. This suggests that the stellar halo of NGC 1399 has been through a major merging event.

This enlarged view of NGC 1399 and 1387 in the g band (top) and g–i band (bottom) gives a better view of the faint stellar stream connecting the two galaxies. [Iodice et al. 2016]

This enlarged view of NGC 1399 and 1387 in the g band (top) and g–i band (bottom) gives a better view of the faint stellar stream connecting the two galaxies. North is up and east is left. [Iodice et al. 2016]

The faint stellar bridge is likely a sign of an ongoing interaction between NGC 1399 and NGC 1387, in which NGC 1387’s outer envelope on its east side is being stripped away. But besides this indication, there is little evidence for recent merger activity, which would usually produce a significant number of luminous stellar streams and tidal tails.

The authors argue that this means that any major mergers in the Fornax cluster center probably happened in an early formation epoch. The cluster is now in a more dynamically evolved stage, in which most of the gravitational interactions between galaxies have already taken place.

Follow-up kinematics studies will be crucial to further interpreting these photometric observations from the center of the Fornax cluster. In the meantime, keep an eye out for future results from FDS!

Citation

E. Iodice et al 2016 ApJ 820 42. doi:10.3847/0004-637X/820/1/42

M-dwarf planets

M-dwarf stars are excellent targets for planet searches because the signal of an orbiting planet is relatively larger (and therefore easier to detect!) around small, dim M dwarfs, compared to Sun-like stars. But are there better or worse stars to target within this category when searching for habitable, Earth-like planets?

Confusing the Signal

Radial velocity campaigns search for planets by looking for signatures in a star’s spectra that indicate the star is “wobbling” due to the gravitational pull of an orbiting planet. Unfortunately, stellar activity can mimic the signal of an orbiting planet in a star’s spectrum — something that is particularly problematic for M dwarfs, which can remain magnetically active for billions of years. To successfully detect planets that orbit in their stars’ habitable zones, we have to account for this problem.

In a recent study led by Elisabeth Newton (Harvard-Smithsonian Center for Astrophysics), the authors use literature measurements to examine the rotation periods for main-sequence, M-type stars. They focus on three factors that are important for detecting and characterizing habitable planets around M dwarfs:

  1. Whether the habitable-zone orbital periods coincide with the stellar rotation
    False planet detections caused by stellar activity often appear as a “planet” with an orbital period that’s a multiple of the stellar rotation period. If a star’s rotation period coincides with the range of orbital periods corresponding to its habitable zone, it’s therefore possible to obtain false detections of habitable planets.
  2. How long stellar activity and rapid rotation last in the star
    All stars become less magnetically active and rotate more slowly as they age, but the rate of this decay depends on their mass: lower-mass stars stay magnetically active for longer and take longer to spin down.
  3. Whether detailed atmospheric characterization will be possible
    It’s ideal to be able to follow up on potentially habitable exoplanets, and search for biosignatures such as oxygen in the planetary atmosphere. This type of detection will only be feasible for low-mass dwarfs, however, due to the relative size of the star and the planet.

An Ideal Range

rotation period vs. mass

Stellar rotation period as a function of stellar mass. The blue shaded region shows the habitable zone as a function of stellar mass. For M dwarfs between ~0.25 and ~0.5 solar mass, the habitable-zone period overlaps with the stellar rotation period. [Newton et al. 2016]

Newton and collaborators find that stars in the mass range of 0.25 to 0.5 solar mass (stellar class M1V-M4V) are non-ideal targets, because their stellar rotation periods (or a multiple thereof) coincide with the orbital periods of their habitable zones. In addition, atmospheric characterization will only be feasible in the near future for stars with mass less than ~0.25 solar mass.

On the other hand, dwarfs with mass less than ~0.1 solar masses (stellar classes later than M6V) will retain their stellar activity and faster rotation rates throughout most of their lifetimes, making them non-ideal targets as well.

When searching for habitable exoplanets, the best targets are therefore the mid M dwarfs in the mass range of 0.1 to 0.25 solar mass (stellar class M4V-M6V). Building a sample focused on these stars will reduce the likelihood that planets found in the stars’ habitable zones are false detections. This will hopefully produce a catalog of potentially habitable exoplanets that we can eventually follow up with atmospheric observations.

Citation

Elisabeth R. Newton et al 2016 ApJ 821 L19. doi:10.3847/2041-8205/821/1/L19

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