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Paper: The Serpens Molecular Cloud
Monograph: 5, Handbook of Star Forming Regions:
Volume II, The Southern Sky
Page: 693
Abstract: The Serpens cloud has received considerable attention in the last years, in particular the small region known as the Serpens cloud core where a plethora of star formation related phenomena are found. This review summarizes our current observational knowledge of the cloud, with emphasis on the core. Recent results are converging to a distance for the cloud of ∼ 230±20 pc, an issue which has been controversial over the years. We present the gas and dust properties of the cloud core and describe its structure and appearance at different wavelengths. The core contains a dense, very young, low mass stellar cluster with more than 300 objects in all evolutionary phases, from collapsing gaseous condensations to pre-main sequence stars. We describe the behaviour and spatial distribution of the different stellar populations (mm cores, Classes 0, I and II sources). The spatial concentration and the fraction number of Class 0/Class I/Class II sources is considerably larger in the Serpens core than in any other low mass star formation region, e.g. Taurus, Ophiuchus or Chamaeleon, as also stated in different works. Appropriate references for coordinates and fluxes of all Serpens objects are given. However, we provide for the first time a unified list of all near-IR sources which have up to now been identified as members of the Serpens core cluster; this list includes some members identified in this review. A cross-reference table of the near-IR objects with optical, mid-IR, submillimeter, radio continuum and X-ray surces is also provided. A simple analysis has allowed us to identify a sample of ∼ 60 brown dwarf candidates among the 252 near-IR objects; some of them show near-IR excesses and, therefore, they constitute an attractive sample to study very young substellar objects. The review also refers to the outflows associated with the young sources. A section is dedicated to the relatively small amount of works carried out towards Serpens regions outside the core. In particular, we refer to ISO and to recent Spitzer data. These results reveal new centers of active star formation in the Serpens cloud and the presence of new young clusters, which deserve follow-up observations and studies to determine their characteristics and nature in detail. Finally, we give a short, non-exhaustive list of individually interesting Serpens objects.
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