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تسجيل مستخدم جديدThe Serpens Molecular Cloud
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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. (abridged)
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We aimed to map the jets and outflows from the Serpens South star forming region and find an empirical relationship between the magnetic field and outflow orientation. Near-infrared H2 v=1-0 S(1) 2.122{mu}m -line imaging of the sim 30-long filamentar
y shaped Serpens South star forming region was carried out. K s broadband imaging of the same region was used for continuum subraction. Candidate driving sources of the mapped jets/outflows are identified from the list of known protostars and young stars in this region, which was derived from studies using recent Spitzer and Herschel telescope observations. 14 Molecular Hydrogen emission-line objects(MHOs) are identified using our continuum-subtracted images. They are found to constitute ten individual flows. Out of these, nine flows are located in the lower-half(southern) part of the Serpens South filament, and one flow is located at the northern tip of the filament. Four flows are driven by well-identified Class 0 protostars, while the remaining six flows are driven by candidate protostars mostly in the Class I stage, based on the Spitzer and Herschel observations. The orientation of the outflows is systematically perpendicular to the direction of the near-infrared polarization vector, recently published in the literature. No significant correlation was observed between the orientation of the flows and the axis of the filamentary cloud.
We report on the results of an optical spectroscopic survey designed to confirm the youth and determine the spectral types among a sample of young stellar object (YSO) candidates in the Serpens Molecular Cloud. We observed 150 infrared excess objects
, previously discovered by the Spitzer Legacy Program From Molecular Cores to Planet-Forming Disks (c2d), bright enough for subsequent Spitzer/IRS spectroscopy. We obtained 78 optical spectra of sufficient S/N for analysis. Extinctions, effective temperatures and luminosities are estimated for this sample, and used to construct H-R diagrams for the population. We identified 20 background giants contaminating the sample, based on their relatively high extinction, position in the H-R diagram, the lack of Halpha emission and relatively low infrared excess. Such strong background contamination (25%) is consistent with the location of Serpens being close to the Galactic plane (5degrees Galactic latitude). The remaining 58 stars (75%) were all confirmed to be young, mostly K and M-type stars that are presumed to belong to the cloud. Individual ages and masses for the YSOs are inferred based on theoretical evolutionary models. The models indicate a spread in stellar ages from 1 to 15 Myr, peaking at 2 - 6 Myr, and a mass distribution of 0.2 to 1.2 Msun with median value around 0.8 Msun. Strong H emission lines (EW[Halpha] > 3 A) have been detected in more than half of the sample (35 stars). The mass accretion rates as derived from the H line widths span a broad distribution over 4 orders of magnitude with median accretion rate of 10^-8 Msun/yr. Our analysis shows that the majority of the infrared excess objects detected in Serpens are actively accreting, young T-Tauri stars.
The relative role of turbulence, magnetic fields, self-gravity in star formation is a subject of intensive debate. We present IRAM 30m telescope observations of the $^{13}$CO (1-0) emission in the Serpens G3-G6 molecular cloud and apply to the data a
set of statistical methods. Those include the probability density functions (PDFs) of column density and the Velocity Gradients Technique (VGT). We combine our data with the Planck 353 GHz polarized dust emission observations, Hershel H$_2$ column density. We suggest that the Serpens G3-G6 south clump is undergoing a gravitational collapse. Our analysis reveals that the gravitational collapse happens at volume density $nge10^3$ $rm cm^{-3}$. We estimate the plane-of-the-sky magnetic field strength of approximately 120 $mu G$ using the traditional Davis-Chandrasekhar-Fermi method and 100 $mu G$ using a new technique proposed in Lazarian et al.(2020). We find the Serpens G3-G6 south clumps total magnetic field energy significantly surpasses kinetic energy and gravitational energy. We conclude that the gravitational collapse could be successfully triggered in a supersonic and sub-Alfv{e}nic cloud.
We present the results of CO ($J=3-2$) and HCO$^+$ ($J=4-3$) mapping observations toward a nearby embedded cluster, Serpens South, using the ASTE 10 m telescope. Our CO ($J=3-2$) map reveals that many outflows are crowded in the dense cluster-forming
clump that can be recognized as a HCO$^+$ clump with a size of $sim$ 0.2 pc and mass of $sim$ 80 M$_odot$. The clump contains several subfragments with sizes of $sim$ 0.05 pc. By comparing the CO ($J=3-2$) map with the 1.1 mm dust continuum image taken by AzTEC on ASTE, we find that the spatial extents of the outflow lobes are sometimes anti-correlated with the distribution of the dense gas and some of the outflow lobes apparently collide with the dense gas. The total outflow mass, momentum, and energy are estimated at 0.6 $M_odot$, 8 $M_odot$ km s$^{-1}$, and 64 $M_odot$ km$^2$ s$^{-2}$, respectively. The energy injection rate due to the outflows is comparable to the turbulence dissipation rate in the clump, implying that the protostellar outflows can maintain the supersonic turbulence in this region. The total outflow energy seems only about 10 percent the clump gravitational energy. We conclude that the current outflow activity is not enough to destroy the whole cluster-forming clump, and therefore star formation is likely to continue for several or many local dynamical times.
We present results from an ISOCAM survey in the two broad band filters LW2 (5-8.5 mu) and LW3 (12-18 mu) of a 0.13 square degree coverage of the Serpens Main Cloud Core. A total of 392 sources were detected in the 6.7 mu band and 139 in the 14.3 mu b
and to a limiting sensitivity of ~ 2 mJy. Only about 50% of the mid-IR excess sources show excesses in the near-IR J-H/H-K diagram. In the central Cloud Core the Class I/Class II number ratio is 19/18, i.e. about 10 times larger than in other young embedded clusters such as rho Ophiuchi or Chamaeleon. The mid-IR fluxes of the Class I and flat-spectrum sources are found to be on the average larger than those of Class II sources. Stellar luminosities are estimated for the Class II sample, and its luminosity function is compatible with a coeval population of about 2 Myr which follows a three segment power-law IMF. For this age about 20% of the Class IIs are found to be young brown dwarf candidates. The YSOs are in general strongly clustered, the Class I sources more than the Class II sources, and there is an indication of sub-clustering. The sub-clustering of the protostar candidates has a spatial scale of 0.12 pc. These sub-clusters are found along the NW-SE oriented ridge and in very good agreement with the location of dense cores traced by millimeter data. The smallest clustering scale for the Class II sources is about 0.25 pc, similar to what was found for rho Ophiuchi. Our data show evidence that star formation in Serpens has proceeded in several phases, and that a ``microburst of star formation has taken place very recently, probably within the last 10^5 yrs.
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