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Register a new userNew Low-Mass Members of the Taurus Star-Forming Region
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K. L. Luhman
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Briceno et al. recently used optical imaging, data from the Two-Micron All-Sky Survey (2MASS), and follow-up spectroscopy to search for young low-mass stars and brown dwarfs in 8 square degrees of the Taurus star-forming region. By the end of that study, there remained candidate members of Taurus that lacked the spectroscopic observations needed to measure spectral types and determine membership. In this work, we have obtained spectroscopy of the 22 candidates that have A_V<=8, from which we find six new Taurus members with spectral types of M2.75 through M9. The new M9 source has the second latest spectral type of the known members of Taurus (~0.02 M_sun). Its spectrum contains extremely strong emission in H_alpha (W~950 A) as well as emission in He I 6678 A and the Ca II IR triplet. This is the least massive object known to exhibit emission in He I and Ca II, which together with the strong H_alpha are suggestive of intense accretion.
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The structure of protoplanetary disks is thought to be linked to the temperature and chemistry of their dust and gas. Whether the disk is flat or flaring depends on the amount of radiation that it absorbs at a given radius, and on the efficiency with which this is converted into thermal energy. The understanding of these heating and cooling processes is crucial to provide a reliable disk structure for the interpretation of dust continuum emission and gas line fluxes. Especially in the upper layers of the disk, where gas and dust are thermally decoupled, the infrared line emission is strictly related to the gas heating/cooling processes. We aim to study the thermal properties of the disk in the oxygen line emission region, and to investigate the relative importance of X-ray (1-120 Angstrom) and far-UV radiation (FUV, 912-2070 Angstrom) for the heating balance there. We use [OI] 63 micron line fluxes observed in a sample of protoplanetary disks of the Taurus/Auriga star forming region and compare it to the model predictions presented in our previous work. The data were obtained with the PACS instrument on board the Herschel Space Observatory as part of the Herschel Open Time Key Program GASPS (GAS in Protoplanetary diskS), published in Howard et al. (2013). Our theoretical grid of disk models can reproduce the [OI] absolute fluxes and predict a correlation between [OI] and the sum Lx+Lfuv. The data show no correlation between the [OI] line flux and the X-ray luminosity, the FUV luminosity or their sum. The data show that the FUV or X-ray radiation has no notable impact on the region where the [OI] line is formed. This is in contrast with what is predicted from our models. Possible explanations are that the disks in Taurus are less flaring than the hydrostatic models predict, and/or that other disk structure aspects that were left unchanged in our models are important. ..abridged..
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The projected stellar rotational velocity ($v sin i$) is critical for our understanding of processes related to the evolution of angular momentum in pre-main sequence stars. We present $v sin i$ measurements of high-resolution infrared and optical spectroscopy for 70 pre-main sequence stars in the Taurus-Auriga star-forming region, in addition to effective temperatures measured from line-depth ratios, and stellar rotation periods determined from optical photometry. From the literature, we identified the stars in our sample that show evidence of residing in circumstellar disks or multiple systems. The comparison of infrared $v sin i$ measurements calculated using two techniques shows a residual scatter of $sim$ 1.8 km s$^{-1}$, defining a typical error floor for the $v sin i$ of pre-main sequence stars from infrared spectra. A comparison of the $v sin i$ distributions of stars with and without companions shows that binaries/multiples typically have a higher measured $v sin i$, which may be caused by contamination by companion lines, shorter disk lifetimes in binary systems, or tidal interactions in hierarchical triples. A comparison of optical and infrared $v sin i$ values shows no significant difference regardless of whether the star has a disk or not, indicating that CO contamination from the disk does not impact $v sin i$ measurements above the typical $sim$ 1.8 km s$^{-1}$ error floor of our measurements. Finally, we observe a lack of a correlation between the $v sin i$, presence of a disk, and H-R diagram position, which indicates a complex interplay between stellar rotation and evolution of pre-main sequence stars.
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