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تسجيل مستخدم جديدUbiquitous $rm NH_3$ supersonic component in L1688 coherent cores
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Context : Star formation takes place in cold dense cores in molecular clouds. Earlier observations have found that dense cores exhibit subsonic non-thermal velocity dispersions. In contrast, CO observations show that the ambient large-scale cloud is warmer and has supersonic velocity dispersions. Aims : We aim to study the ammonia ($rm NH_3$) molecular line profiles with exquisite sensitivity towards the coherent cores in L1688 in order to study their kinematical properties in unprecedented detail. Methods : We used $rm NH_3$ (1,1) and (2,2) data from the first data release (DR1) in the Green Bank Ammonia Survey (GAS). We first smoothed the data to a larger beam of 1 to obtain substantially more extended maps of velocity dispersion and kinetic temperature, compared to the DR1 maps. We then identified the coherent cores in the cloud and analysed the averaged line profiles towards the cores. Results : For the first time, we detected a faint (mean $rm NH_3$(1,1) peak brightness $<$0.25 K in $T_{MB}$), supersonic component towards all the coherent cores in L1688. We fitted two components, one broad and one narrow, and derived the kinetic temperature and velocity dispersion of each component. The broad components towards all cores have supersonic linewidths ($mathcal{M}_S ge 1$). This component biases the estimate of the narrow dense core components velocity dispersion by $approx$28% and the kinetic temperature by $approx$10%, on average, as compared to the results from single-component fits. Conclusions : Neglecting this ubiquitous presence of a broad component towards all coherent cores causes the typical single-component fit to overestimate the temperature and velocity dispersion. This affects the derived detailed physical structure and stability of the cores estimated from $rm NH_3$ observations.
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Stars form in cold dense cores showing subsonic velocity dispersions. The parental molecular clouds display higher temperatures and supersonic velocity dispersions. The transition from core to cloud has been observed in velocity dispersion, but tempe
rature and abundance variations are unknown. We aim to study the transition from cores to ambient cloud in temperature and velocity dispersion using a single tracer. We use NH3 (1,1) and (2,2) maps in L1688 from the Green Bank Ammonia Survey, smoothed to 1, and determine the physical properties from fits. We identify the coherent cores and study the changes in temperature and velocity dispersion from cores to the surrounding cloud. We obtain a kinetic temperature map tracing the extended cloud, improving from previous maps tracing mostly the cores. The cloud is 4-6 K warmer than the cores, and shows a larger velocity dispersion (diff. = 0.15-0.25 km/s). Comparing to Herschel-based measurements, we find that cores show kinetic temperature $approx$1.8 K lower than the dust temperature; while the gas temperature is higher than the dust temperature in the cloud. We find an average p-NH3 fractional abundance (with respect to H2) of $(4.2pm0.2) times 10^{-9}$ towards the coherent cores, and $(1.4pm0.1) times 10^{-9}$ outside the core boundaries. Using stacked spectra, we detect two components, one narrow and one broad, towards cores and their neighbourhoods. We find the turbulence in the narrow component to be correlated to the size of the structure (Pearson-r=0.54). With these unresolved regional measurements, we obtain a turbulence-size relation of ${sigma}_{v,NT}propto r^{0.5}$, similar to previous findings using multiple tracers. We discover that the subsonic component extends up to 0.15 pc beyond the typical coherent boundaries, unveiling larger extents of the coherent cores and showing gradual transition to coherence over ~0.2 pc.
We present the observation and analysis of newly discovered coherent structures in the L1688 region of Ophiuchus and the B18 region of Taurus. Using data from the Green Bank Ammonia Survey (GAS), we identify regions of high density and near-constant,
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The Galactic Center 50 km s$^{-1}$ Molecular Cloud (50MC) is the most remarkable molecular cloud in the Sagittarius A region. This cloud is a candidate for the massive star formation induced by cloud-cloud collision (CCC) with a collision velocity of
$sim30rm~km~s^{-1}$ that is estimated from the velocity dispersion. We observed the whole of the 50MC with a high angular resolution ($sim2.0times1.4$) in ALMA cycle 1 in the H$^{13}$CO$^+~J=1-0$ and ${rm C^{34}S}~J=2-1$ emission lines. We identified 241 and 129 bound cores with a virial parameter of less than 2, which are thought to be gravitationally bound, in the H$^{13}$CO$^+$ and ${rm C^{34}S}$ maps using the clumpfind algorithm, respectively. In the CCC region, the bound ${rm H^{13}CO^+}$ and ${rm C^{34}S}$ cores are 119 and 82, whose masses are $68~%$ and $76~%$ of those in the whole 50MC, respectively. The distribution of the core number and column densities in the CCC are biased to larger densities than those in the non-CCC region. The distributions indicate that the CCC compresses the molecular gas and increases the number of the dense bound cores. Additionally, the massive bound cores with masses of $>3000~M_{odot}$ exist only in the CCC region, although the slope of the core mass function (CMF) in the CCC region is not different from that in the non-CCC region. We conclude that the compression by the CCC efficiently formed massive bound cores even if the slope of the CMF is not changed so much by the CCC.
Context. The different theoretical models concerning the formation of high-mass stars make distinct predictions regarding their progenitors, i.e. the high-mass prestellar cores. However, so far no conclusive observation of such objects has been made.
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