A Survey For Planetary-mass Brown Dwarfs in the Taurus and Perseus Star-forming Regions

A Survey For Planetary-mass Brown Dwarfs in the Taurus and Perseus Star-forming Regions

Authors:

Esplin et al

Abstract:
We present the initial results from a survey for planetary-mass brown dwarfs in the Taurus star-forming region. We have identified brown dwarf candidates in Taurus using proper motions and photometry from several ground- and space-based facilities. Through spectroscopy of some of the more promising candidates, we have found 18 new members of Taurus. They have spectral types ranging from mid M to early L and they include the four faintest known members in extinction-corrected K_s, which should have masses as low as ~4-5 M_Jup according to evolutionary models. Two of the coolest new members (M9.25, M9.5) have mid-IR excesses that indicate the presence of disks. Two fainter objects with types of M9-L2 and M9-L3 also have red mid-IR colors relative to photospheres at <=L0, but since the photospheric colors are poorly defined at >L0, it is unclear whether they have excesses from disks. We also have obtained spectra of candidate members of the IC 348 and NGC 1333 clusters in Perseus that were identified by Luhman et al. (2016). Eight candidates are found to be probable members, three of which are among the faintest and least-massive known members of the clusters (~5 M_Jup).

Chemistry in Disks X: The Molecular Content of Proto-planetary Disks in Taurus

Chemistry in Disks X: The Molecular Content of Proto-planetary Disks in Taurus

Authors:

Guilloteau et al

Abstract:

We used the IRAM 30-m to perform a sensitive wideband survey of 30 protoplanetary disks in the Taurus Auriga region. We simultaneously observed HCO+(3-2), HCN(3-2), C2H(3-2), CS(5-4), and two transitions of SO. We combine the results with a previous survey which observed 13CO (2-1), CN(2-1), two o-H2CO lines and one of SO. We use available interferometric data to derive excitation temperatures of CN and C2H in several sources. We determine characteristic sizes of the gas disks and column densities of all molecules using a parametric power-law disk model. Our study is mostly sensitive to molecules at 200-400 au from the stars. We compare the derived column densities to the predictions of an extensive gas-grain chemical disk model, under conditions representative of T Tauri disks. This survey provides 20 new detections of HCO+ in disks, 18 in HCN, 11 in C2H, 8 in CS and 4 in SO. HCO+ is detected in almost all sources, and its J=3-2 line is essentially optically thick, providing good estimates of the disk radii. The other transitions are (at least partially) optically thin. Variations of the column density ratios do not correlate with any specific property of the star or disk. Disks around Herbig Ae stars appear less rich in molecules than those around T Tauri stars, although the sample remains small. SO is only found in the (presumably younger) embedded objects, perhaps reflecting an evolution of the S chemistry due to increasing depletion with time. Overall, the molecular column densities, and in particular the CN/HCN and CN/C2H ratios, are well reproduced by gas-grain chemistry in cold disks. This study provides a census of simple molecules in disks of radii greater than 200−300 au. Extending that to smaller disks, or searching for less abundant or more complex molecules requires a much more sensitive facility, i.e. NOEMA and ALMA.

Two Proto Brown Dwarfs Confirmed in Taurus

Two confirmed class I very low-mass objects in Taurus

Authors:

Dang-Duc et al

Abstract:

[GKH94] 41 and IRAS 04191+1523B were previously identified to be proto-brown dwarf candidates in Taurus. [GKH94] 41 was classified to be a class I object. The dereddened spectral energy distribution of the source was later found to be suggestive of a class II object. IRAS 04191+1523B is a class I object that is the secondary component of a binary. We determine the evolutionary stage of [GKH94] 41 and estimate the final masses of the two proto-brown dwarf candidates. We used archive millimeter observations to produce continuum maps and collected data from the literature to construct the spectral energy distribution of the targets. Our continuum maps revealed that both [GKH94] 41 and IRAS 04191+1523B are surrounded by envelopes. This provides direct evidence that [GKH94] 41 is a class I object, not class II, as previously classified. For IRAS 04191+1523B, our continuum map spatially resolved the binary. Our estimated final masses are below 49+56−27 MJ and 75+40−26 MJ for [GKH94] 41 and IRAS 04191+1523B, respectively. This indicates that both sources will likely become brown dwarfs or very low-mass stars. Therefore, [GKH94] 41 and IRAS 04191+1523B are two new confirmed class I very low-mass objects. Their existence also supports the scenario that brown dwarfs have the same formation stages as low-mass stars.

Comparing T Tauri Stars in Taurus and Orion

X-ray deficiency on strong accreting T Tauri stars - Comparing Orion with Taurus

Authors:

Bustamonte et al

Abstract:

Depending on whether a T Tauri star accretes material from its circumstellar disk or not, different X-ray emission properties can be found. The accretion shocks produce cool heating of the plasma, contributing to the soft X-ray emission from the star. Using X-ray data from the Chandra Orion Ultra-deep Project and accretion rates that were obtained with the Hubble Space Telescope/WFPC2 photometric measurements in the Orion Nebula Cluster, we studied the relation between the accretion processes and the X-ray emissions of a coherent sample of T Tauri sources in the region. We performed regression and correlation analyses of our sample of T Tauri stars between the X-ray parameters, stellar properties, and the accretion measurements. We find that a clear anti-correlation is present between the residual X-ray luminosity and the accretion rates in our samples in Orion that is consistent with that found on the XMM-Newton Extended Survey of the Taurus molecular cloud (XEST) study. We provide a catalog with X-ray luminosities (corrected from distance) and accretion measurements of an Orion Nebula Cluster (ONC) T Tauri stars sample. Although Orion and Taurus display strong differences in their properties (total gas and dust mass, star density, strong irradiation from massive stars), we find that a similar relation between the residual X-ray emission and accretion rate is present in the Taurus molecular cloud and in the accreting samples from the Orion Nebula Cluster. The spread in the data suggests dependencies of the accretion rates and the X-ray luminosities other than the stellar mass, but the similarity between Orion and Taurus hints at the environment not being one of them. The anti-correlation between the residual X-ray luminosity and mass accretion rate is inherent to the T Tauri stars in general, independent of their birthplace and environment, and intrinsic to early stellar evolution.

Taurus Stars Have Long Lasting Protoplanetary Disks

The age of Taurus - environmental effects on disc lifetimes

Authors:

Rees et al

Abstract:

Using semi-empirical isochrones, we find the age of the Taurus star-forming region to be 3-4 Myr. Comparing the disc fraction in Taurus to young massive clusters suggests discs survive longer in this low density environment. We also present a method of photometrically de-reddening young stars using iZJH data.

Demographics Circumstellar Disks in Ophiuchus and Taurus

Demographics of Transition Discs in Ophiuchus and Taurus

Authors:

Najita et al

Abstract:

Transition disc systems are young stars that appear to be on the verge of dispersing their protoplanetary discs. We explore the nature of these systems by comparing the stellar accretion rates and disc masses of transition discs and normal T Tauri stars in Taurus and Ophiuchus. After controlling for the known dependencies of stellar accretion rate and disc mass and on age, stellar accretion rate on stellar mass, and disc mass on the presence of stellar or sub-stellar companions, we find that the normal T Tauri stars show a trend of stellar accretion rate increasing with disc mass. The transition discs tend to have higher average disc masses than normal T Tauri stars as well as lower accretion rates than normal T Tauri stars of the same disc mass. These results are most consistent with the interpretation that the transition discs have formed objects massive enough to alter the accretion flow, i.e., single or multiple giant planets. Several Ophiuchus T Tauri stars that are not known transition disc systems also have very low accretion rates for their disc masses. We speculate on the possible nature of these sources.