Investigation of a Nearby, Cold, and Young Brown Dwarf

Somewhere in between stars, which sustain hydrogen fusion in their cores, and giant gaseous planets, which do not, lies an intermediate class of cool, cloudy objects called brown dwarfs. A new look at a nearby brown dwarf shows how challenging it can be to classify these objects.

Neither Star nor Planet

Brown dwarfs are assigned spectral types M, L, T, and Y, ranging from objects that just barely miss the mass cutoff to be able to fuse hydrogen into helium to those that overlap in mass with the largest planets. The coolest brown dwarfs, those in class Y, might have cloudy atmospheres similar to those of giant planets but easier to study; tiny, faint, and cool, planets are hard to pick out against the bright light of their host stars.

Finding and classifying Y-type brown dwarfs has its own challenges, though. Part of the challenge comes from the fact that even objects assigned the same spectral type show significant differences. To understand whether these variations are due to metallicity, formation mechanism, or other factors, researchers must amass a sample of these hard-to-find objects.

Under Spectral Scrutiny

Enter CWISE J105512.11+544328.3, or W1055+5443 for short. Researchers initially assigned this object a spectral class of T8 — slightly more massive and warmer than a Y-type brown dwarf — but an updated distance measurement placed it closer to Earth, implying a lower luminosity and nudging its spectral type down to Y0. To confirm this spectral-type assignment and learn more about this nearby brown dwarf, Grady Robbins (University of Florida) and collaborators analyzed archival photometry from the Spitzer Space Telescope and collected new spectra using the Keck II telescope.

Oddities were immediately apparent, in both its photometry and its spectra: W1055+5443 appears unusually blue in Spitzer’s filters compared to other Y0 brown dwarfs, and its spectral features were difficult to fit with a single template. Previous research has shown that when brown dwarfs are difficult to classify from their spectra, it can mean that what seems to be a single brown dwarf is actually two — but modeling shows that W1055+5443 is most likely a single Y0 brown dwarf, though a peculiar one.

Unusual Properties

As brown dwarfs age, they cool and contract. This means that brown dwarfs that are young tend to have earlier spectral types (M and L) and lower surface gravity, and brown dwarfs that are old tend to have later spectral types (T and Y) and higher surface gravity. Based on its spectral type, W1055+5443 should be an old brown dwarf with high surface gravity, but modeling suggests that its surface gravity is low — is this object unexpectedly young?

Robbins and collaborators found that W1055+5443 is likely a member of the Crius 197 moving group, a collection of ten stars moving together through space. Many of the stars in this group have ages around 180 million years, implying that W1055+5443 is similarly youthful, though some group members are much older. If the 180-million-year estimate proves correct, this suggests that W1055+5443 is a young brown dwarf with a mass just 4–6 times that of Jupiter, placing it well within the realm of planet masses!

The authors note that these estimates are preliminary and await further investigation. Spectroscopy from JWST would allow researchers to pin down whether the object belongs to the Crius 197 moving group, determine its metallicity, and investigate its unusual spectral features, and longer-wavelength photometry would provide an accurate estimate of its temperature.

Citation

“CWISE J105512.11+544328.3: A Nearby Y Dwarf Spectroscopically Confirmed with Keck/NIRES,” Grady Robbins et al 2023 ApJ 958 94. doi:10.3847/1538-4357/ad0043