Pre-main sequence accretion in the low metallicity Galactic star-forming region Sh 2-284

Abstract

We present optical spectra of pre-main sequence (PMS) candidates around the H$\alpha$ region taken with the Southern African Large Telescope, SALT, in the low metallicity ($Z$) Galactic region Sh 2-284, which includes the open cluster Dolidze 25 with an atypical low metallicity of $Z$ $\sim$ 1/5 $Z_{\odot }$. It has been suggested on the basis of both theory and observations that PMS mass-accretion rates, ${\dot{M}}_{\mathrm{acc}}$, are a function of $Z$. We present the first sample of spectroscopic estimates of mass-accretion rates for PMS stars in any low-$Z$ star-forming region. Our data-set was enlarged with literature data of H$\alpha$ emission in intermediate-resolution R-band spectroscopy. Our total sample includes 24 objects spanning a mass range between 1 - 2 $M_{\odot }$ and with a median age of approximately 3.5 Myr. The vast majority (21 out of 24) show evidence for a circumstellar disk on the basis of 2MASS and Spitzer infrared photometry. We find ${\dot{M}}_{\mathrm{acc}}$ in the 1 - 2 $M_{\odot }$ interval to depend quasi-quadratically on stellar mass, with ${\dot{M}}_{\mathrm{acc}}$ $\propto$ $M_{\ast }^{2.4\pm 0.35}$, and inversely with stellar age ${\dot{M}}_{\mathrm{acc}}$ $\propto$ $t_{\ast }^{-0.7\pm 0.4}$. Furthermore, we compare our spectroscopic ${\dot{M}}_{\mathrm{acc}}$ measurements with solar $Z$ Galactic PMS stars in the same mass range, but, surprisingly find no evidence for a systematic change in ${\dot{M}}_{\mathrm{acc}}$ with $Z$. We show that literature accretion-rate studies are influenced by detection limits, and we suggest that ${\dot{M}}_{\mathrm{acc}}$ may be controlled by factors other than $Z_{\ast }$, $M_{\ast }$, and age.

arXiv commentAccepted for publication in the Astrophysical Journal. Contains 13 pages, 11 figures, 4 tables

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