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Register a new userBrown dwarf disks with ALMA: evidence for truncated dust disks in Ophiuchus
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The study of the properties of disks around young brown dwarfs can provide important clues on the formation of these very low-mass objects and on the possibility of forming planetary systems around them. The presence of warm dusty disks around brown dwarfs is well known, based on near- and mid-infrared studies. High angular resolution observations of the cold outer disk are limited; we used ALMA to attempt a first survey of young brown dwarfs in the $rho$-Oph star-forming region. All 17 young brown dwarfs in our sample were observed at 890 $mu $m in the continuum at $sim0.!^{primeprime}5$ angular resolution. The sensitivity of our observations was chosen to detect $sim0.5$ M$_oplus$ of dust. We detect continuum emission in 11 disks ($sim65$% of the total), and the estimated mass of dust in the detected disks ranges from $sim0.5$ to $sim6$ M$_oplus$. These disk masses imply that planet formation around brown dwarfs may be relatively rare and that the supra-Jupiter mass companions found around some brown dwarfs are probably the result of a binary system formation. We find evidence that the two brightest disks in $rho$-Oph have sharp outer edges at R<~25 AU, in contrast to disks around Taurus brown dwarfs. This difference may suggest that the different environment in $rho$-Oph may lead to significant differences in disk properties. A comparison of the M$_{disk}$/M$_ast$ ratio for brown dwarf and solar-mass systems also shows a possible deficit of mass in brown dwarfs, which could support the evidence for dynamical truncation of disks in the substellar regime. These findings are still tentative and need to be put on firmer grounds by studying the gaseous disks around brown dwarfs and by performing a more systematic and unbiased survey of the disk population around the more massive stars.
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We present a high angular resolution ($sim 0.2^{primeprime}$), high sensitivity ($sigma sim 0.2$ mJy) survey of the 870 $mu$m continuum emission from the circumstellar material around 49 pre-main sequence stars in the $rho$ Ophiuchus molecular cloud. Because most millimeter instruments have resided in the northern hemisphere, this represents the largest high-resolution, millimeter-wave survey of the circumstellar disk content of this cloud. Our survey of 49 systems comprises 63 stars; we detect disks associated with 29 single sources, 11 binaries, 3 triple systems and 4 transition disks. We present flux and radius distributions for these systems; in particular, this is the first presentation of a reasonably complete probability distribution of disk radii at millimeter-wavelengths. We also compare the flux distribution of these protoplanetary disks with that of the disk population of the Taurus-Auriga molecular cloud. We find that disks in binaries are both significantly smaller and have much less flux than their counterparts around isolated stars. We compute truncation calculations on our binary sources and find that these disks are too small to have been affected by tidal truncation and posit some explanations for this. Lastly, our survey found 3 candidate gapped disks, one of which is a newly identified transition disk with no signature of a dip in infrared excess in extant observations.
We present a study of the radial distribution of dust species in young brown dwarf disks. Our work is based on a compositional analysis of the 10 and 20 micron silicate emission features for brown dwarfs in the Taurus-Auriga star-forming region. A fundamental finding of our work is that brown dwarfs exhibit stronger signs of dust processing in the cold component of the disk, compared to the higher mass T Tauri stars in Taurus. For nearly all of our targets, we find a flat disk structure, which is consistent with the stronger signs of dust processing observed in these disks. For the case of one brown dwarf, 2M04230607, we find the forsterite mass fraction to be a factor of ~3 higher in the outer disk compared to the inner disk region. Simple large-scale radial mixing cannot account for this gradient in the dust chemical composition, and some local crystalline formation mechanism may be effective in this disk. The relatively high abundance of crystalline silicates in the outer cold regions of brown dwarf disks provides an interesting analogy to comets. In this context, we have discussed the applicability of the various mechanisms that have been proposed for comets on the formation and the outward transport of high-temperature material. We also present Chandra X-ray observations for two Taurus brown dwarfs, 2M04414825 and CFHT-BD-Tau 9. We find 2M04414825, which has a ~12% crystalline mass fraction, to be more than an order of magnitude brighter in X-ray than CFHT-BD-Tau 9, which has a ~35% crystalline mass fraction. Combining with previous X-ray data, we find the inner disk crystalline mass fractions to be anti-correlated with the X-ray strength.
We present a census of the disk population for UKIDSS selected brown dwarfs in the 5-10 Myr old Upper Scorpius OB association. For 116 objects originally identified in UKIDSS, the majority of them not studied in previous publications, we obtain photometry from the WISE database. The resulting colour-magnitude and colour-colour plots clearly show two separate populations of objects, interpreted as brown dwarfs with disks (class II) and without disks (class III). We identify 27 class II brown dwarfs, 14 of them not previously known. This disk fraction (27 out of 116 or 23%) among brown dwarfs was found to be similar to results for K/M stars in Upper Scorpius, suggesting that the lifetimes of disks are independent of the mass of the central object for low-mass stars and brown dwarfs. 5 out of 27 disks (19%) lack excess at 3.4 and 4.6 microns and are potential transition disks (i.e. are in transition from class II to class III). The transition disk fraction is comparable to low-mass stars. We estimate that the timescale for a typical transition from class II to class III is less than 0.4 Myr for brown dwarfs. These results suggest that the evolution of brown dwarf disks mirrors the behaviour of disks around low-mass stars, with disk lifetimes on the order of 5-10 Myr and a disk clearing timescale significantly shorter than 1 Myr.
We present the results of ALMA band 7 observations of dust and CO gas in the disks around 7 objects with spectral types ranging between M5.5 and M7.5 in Upper Scorpius OB1, and one M3 star in Ophiuchus. We detect unresolved continuum emission in all but one source, and the $^{12}$CO J=3-2 line in two sources. We constrain the dust and gas content of these systems using a grid of models calculated with the radiative transfer code MCFOST, and find disk dust masses between 0.1 and 1 M$_oplus$, suggesting that the stellar mass / disk mass correlation can be extrapolated for brown dwarfs with masses as low as 0.05 M$_odot$. The one disk in Upper Sco in which we detect CO emission, 2MASS J15555600, is also the disk with warmest inner disk as traced by its H - [4.5] photometric color. Using our radiative transfer grid, we extend the correlation between stellar luminosity and mass-averaged disk dust temperature originally derived for stellar mass objects to the brown dwarf regime to $langle T_{dust} rangle approx 22 (L_{*} /L_{odot})^{0.16} K$, applicable to spectral types of M5 and later. This is slightly shallower than the relation for earlier spectral type objects and yields warmer low-mass disks. The two prescriptions cross at 0.27 L$_odot$, corresponding to masses between 0.1 and 0.2 M$_odot$ depending on age.
We present 0.25 arcsec resolution (35 au) ALMA 1.3 mm dust polarization observations for 37 young stellar objects (YSOs) in the Ophiuchus cloud. These data encompass all the embedded protostars in the cloud and several Flat and Class II objects to produce the largest, homogeneous study of dust polarization on disk scales to date. The goal of this study is to study dust polarization down to disk scales. We find that 14/37 (38%) of the YSOs are detected in polarization. Nine of these sources have uniform polarization angles and four sources have azimuthal polarization. The sources with uniform polarization tend to have steeper inclinations (> 60 degree) than those with azimuthal polarization (< 60 degree). The majority (9/14) of the detected sources have polarization morphologies and disk properties consistent with dust self-scattering in optically thick disks. The remaining sources may be instead tracing magnetic fields. Their inferred field directions from rotating the polarization vectors by 90 degree are mainly poloidal or hourglass shaped. We find no evidence of a strong toroidal field component toward any of our disks. For the 23 YSOs that are undetected in polarization, roughly half of them have 3-sigma upper limits of < 2%. These sources also tend to have inclinations < 60 degree and they are generally compact. Since lower inclination sources tend to have azimuthal polarization, these YSOs may be undetected in polarization due to unresolved polarization structure within our beam. We propose that disks with inclinations > 60 degree are the best candidates for future polarization studies of dust self-scattering as these systems will generally show uniform polarization vectors that do not require very high resolution to resolve. We release the continuum and polarization images for all the sources with this publication. Data from the entire survey can be obtained from Dataverse.
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