A Catalog of Optically Selected Cores

Abstract

We present a new catalogue of 406 dense cores optically selected by using the STScI Digitized Sky Survey (DSS). In this catalogue 306 cores have neither an Embedded YSO (EYSO) nor a Pre-Main-Sequence (PMS) star, 94 cores have EYSOs (1 core has both an EYSO and a PMS star), and 6 cores have PMS star only. Our sample of dense cores in the catalogue is fairly complete within a category of northern Lynds class 5, 6 clouds, and southern Hartley et al. (1986)'s class A clouds, providing a database useful for the systematic study of dense cores. Most of the cores listed in the catalogue have diameters between 0.05 − 0.36 pc with a mean of ∼ 0.24 pc. The sizes (∼ 0.33 pc in the mean ) of cores with EYSOs are found to be usually larger than the sizes (∼ 0.22 pc in the mean) of starless cores. The typical mean gas density of the cores is ∼ 7 × 10 3 cm −3 . Most of the cores are more likely elongated than spherical (mean aspect ratio: ∼ 2.4). The ratio of the number of cores with EYSOs to the number of starless cores for our sample is about 0.3, suggesting that the typical lifetime of starless cores is 0.3 − 1.6 Myr, about 3 times longer than the duration of the Class 0 and Class I phases. This lifetime is shorter than expected from models of ambipolar diffusion, by factors of 2 − 44.

Chemical modeling of the L1498 and L1517B prestellar cores: CO and HCO+depletion

“…Combining these upper limits with the lower limits we obtain form the chemical ages, we found that the two cores ages are comprised between 0.3 and 1.6 Myr. In addition, we note that the lower limits in the age we obtain here are comparable to shortest lifetime obtained by Lee & Myers (1999) (0.3 × 10 6 yr); it is therefore possible that two cores are at a relatively evolved stage, and may quickly evolve to form protostars. It is interesting to note that, using an independant technique, Pagani et al (2013) obtained an age of <0.7 My for the L183 core, based on its observed N 2 D + /N 2 H + ratio; this value is consistent with the chemical ages derived in this study.…”

“…We argue that these chemical ages are lower limits on time elapsed since the core has formed from their parent molecular cloud. Combining this lower limit with upper limits from Lee & Myers (1999) and Enoch et al (2008) core lifetimes, we estimate that the core ages lie within 0.3 and 1.6 Myr. Our lower limits are comparable to the shortest lifetimes, suggesting that these cores are at a relatively evolved stage, and may shortly collapse to form protostars.…”

Dynamic Cores in Hydrostatic Disguise

“…There also exist both observational evidence (Crutcher 1999;Andre, WardThompson, & Barsony 2000;Bourke et al 2001;Hartmann et al 2001) and theoretical arguments (Nakano 1998;Hartmann et al 2001; as summarized by Mac Low & Klessen 2003) showing that the overwhelming majority of cores must be supercritical. Moreover, the long lifetimes that quasi-static cores would have are difficult to reconcile with observational statistics of cloud cores (Taylor et al 1996;Lee & Myers 1999;Visser, Richer, & Chandler 2002), and with the suggestion of short molecular cloud formation timescales (about a few Myr), based on the observed lack of post-T Tauri stars in Taurus (Herbig 1978;Ballesteros-Paredes, Hartmann, & Vázquez-Semadeni 1999a;Hartmann 2003).…”

Absence of coreshine in the Gum/Vela region

“…(a) A * indicates that the object is associated to Vela (Reipurth 1983;Zealey et al 1983). Other distances are obtained from the analysis of the star reddening (Bourke et al 1995;Lee & Myers 1999;Löhr et al 2007). Three objects have no known distance.…”