B. B. Ochsendorf, L. E. Ellerbroek, R. Chini, O. E. Hartoog, V. Hoffmeister, L. B. F. M. Waters, L. Kaper
The optical to near-infrared (300 - 2500 nm) spectrum of the candidate
massive Young Stellar Object (YSO) B275, embedded in the star forming region
M17, has been observed with X-shooter on the ESO Very Large Telescope. The
spectrum includes both photospheric absorption lines and emission features (H
and Ca triplet emission lines, 1st and 2nd overtone CO bandhead emission), as
well as an infrared excess indicating the presence of a (flaring) circumstellar
disk. The strongest emission lines are double-peaked with a peak separation
ranging between 70 and 105 km s^-1 and provide information on the physical
structure of the disk. The underlying photospheric spectrum is classified as
B6-B7, significantly cooler than a previous estimate based on modeling of the
spectral energy distribution. This discrepancy is solved by allowing for a
larger stellar radius (i.e. a bloated star), and thus positioning the star
above the main sequence. This constitutes the first firm spectral
classification of an early-B pre-main-sequence (PMS) star. We discuss the
position of B275 in the Hertzsprung-Russell diagram in terms of PMS evolution.
Although the position is consistent with PMS tracks of heavily accreting
protostars (Macc > 10^-5 Msun yr^-1), the fact that the photosphere of the
object is detectable suggests that the current mass-accretion rate is not very
high.
View original:
http://arxiv.org/abs/1110.5755
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