We recently had another article accepted for publication in the Astronomical Journal! A pre-print is now available here. This paper presents the discovery a nearby young brown dwarf with a disk, W1200-7845.
Brown dwarfs are substellar objects that range in mass from about 13-80 Jupiter masses. They don’t have enough mass to sustain hydrogen burning in their cores, so they don’t qualify as stars, but they are just massive enough to burn molecular hydrogen (H2) in their cores, making them also distinct from planets. Understanding brown dwarfs is therefore key to understanding the connection between stars and planets. However, we still don’t know if brown dwarfs form the way stars do (gravitational collapse of a molecular cloud) or the way planets do (forming in the disks of larger stars and then getting ejected from orbit). Research on this is ongoing.
Identifying the Brown Dwarf
We first picked out W1200-7845 after classification on the Disk Detective website by estimating the effective temperature and the fractional infrared luminosity of the system, using fits to the system’s SED. The fractional infrared luminosity measures the amount of light in the object’s infrared excess as a fraction of the host’s light–a larger fractional infrared luminosity suggests an object with a significant amount of warm gas and dust around it. W1200-7845 has a fairly high fractional infrared luminosity but a low effective temperature, making an interesting object for further study.
We then cross-matched W1200-7845 with BANYAN Σ, a tool to estimate the likelihood that an object in in a young moving group based on its position, proper motion, and (if available) parallax and radial velocity. BANYAN Σ found that W1200-7845 had a 99.8% probability of being a member of the 3.7 million-year-old ε Chamaeleontis (ε Cha) association. Its Gaia DR2 parallax puts W1200-7845 at a distance of 102 parsecs or 333 lightyears, which is within the Solar neighborhood.
W1200-7845 Has a Circumstellar Disk
After confirming that W1200-7845 is a member of ε Cha, we moved on to better characterizing the host’s mass and effective temperature and classifying the circumstellar disk. To estimate the mass of W1200-7845, we plotted its SED, as shown below. Using W1200-7845’s SED, we fit the observed data (grey points) with a model for both the source (the brown dwarf itself) and its circumstellar disk. We found the best fit model for the source to be a substellar object with a mass of 42 Jupiter masses and an effective temperature of 2784 K.
We used three different disk models to find the best fit to the disk. We first tried a single blackbody fit (a disk with one approximate temperature and one fractional infrared luminosity). We then tried a power-law disk fit (the disk has a temperature gradient, with the inner material hotter). We finally tried a two-blackbody fit, meaning two disk temperatures for the inner and outer portions of the disk. We found that the best model was the power-law disk fit, which had a negative slope of -0.94. This value of the slope makes the disk a Class II YSO, common for young objects.
W1200-7845’s Near Infrared Spectrum
We also obtained a near infrared spectrum for W1200-7845 using the Magellan 6.5-meter telescopes at Las Campanas Observatory. We compared W1200-7845 to young brown dwarf spectral templates in order to find the best match. We found that the best match to W1200-7845 was a M6.0γ ± 0.5 spectral type, as shown in the spectrum below. This further confirmed that W1200-7845 is consistent with other young brown dwarfs. We weren’t able to resolve any signatures of accretion of disk material onto the host. However, accretion levels for brown dwarfs are very low due to their low mass, so it’s possible that the signatures of accretion are too low to be detected in our spectrum.
Why W1200-7845 Is Interesting
Objects like W1200-7845 are important because of their youth and proximity. Youth is important because we need to look at young objects at various ages to determine whether brown dwarfs form more like planets or stars. Proximity is important because brown dwarfs are inherently faint due to their low mass, so the closer these objects are the more detail we will be able to see. As W1200-7845 is both very young at ~3.7 million years old and nearby at 102 parsecs, it can serve as a benchmark object for future studies of brown dwarf formation and evolution.
Our next steps for W1200-7845 are to obtain longer millimeter observations in order to measure the mass of the disk, the disk’s radius, and to search for a cold dust component to the disk. Because W1200-7845 is so nearby, it is possible to resolve the disk to within a few AU (astronomical units) using larger radio interferometers like ALMA. This would allow for an unprecedented look at a key point in the evolution of brown dwarf disks. Obtaining an optical spectrum of W1200-7845 would allow for a more robust determination of the accretion level as the most prominent accretion line for brown dwarfs is Hα at .6563 microns.