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Register a new userA necklace of dense cores in the high-mass star forming region G35.20-0.74N: ALMA observations
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Alvaro Sanchez-Monge
Publication date
2014
fields
Physics
and research's language is
English
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The present study aims at characterizing the massive star forming region G35.20N, which is found associated with at least one massive outflow and contains multiple dense cores, one of them recently found associated with a Keplerian rotating disk. We used ALMA to observe the G35.20N region in the continuum and line emission at 350 GHz. The observed frequency range covers tracers of dense gas (e.g. H13CO+, C17O), molecular outflows (e.g. SiO), and hot cores (e.g. CH3CN, CH3OH). The ALMA 870 um continuum emission map reveals an elongated dust structure (0.15 pc long and 0.013 pc wide) perpendicular to the large-scale molecular outflow detected in the region, and fragmented into a number of cores with masses 1-10 Msun and sizes 1600 AU. The cores appear regularly spaced with a separation of 0.023 pc. The emission of dense gas tracers such as H13CO+ or C17O is extended and coincident with the dust elongated structure. The three strongest dust cores show emission of complex organic molecules characteristic of hot cores, with temperatures around 200 K, and relative abundances 0.2-2x10^(-8) for CH3CN and 0.6-5x10^(-6) for CH3OH. The two cores with highest mass (cores A and B) show coherent velocity fields, with gradients almost aligned with the dust elongated structure. Those velocity gradients are consistent with Keplerian disks rotating about central masses of 4-18 Msun. Perpendicular to the velocity gradients we have identified a large-scale precessing jet/outflow associated with core B, and hints of an east-west jet/outflow associated with core A. The elongated dust structure in G35.20N is fragmented into a number of dense cores that may form massive stars. Based on the velocity field of the dense gas, the orientation of the magnetic field, and the regularly spaced fragmentation, we interpret this elongated structure as the densest part of a 1D filament fragmenting and forming massive stars.
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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. Aims. We aim to study the very early stages of high-mass star formation in two infrared-dark, massive clumps, to identify the core population that they harbour. Methods. We obtained ALMA observations of continuum emission at 0.8mm and of the ortho-$rm H_2D^+$ transition at 372GHz towards the two clumps. We use the SCIMES algorithm to identify cores in the position-position-velocity space, finding 16 cores. We model their observed spectra in the LTE approximation, deriving the centroid velocity, linewidth, and column density maps. We also study the correlation between the continuum and molecular data, which in general do not present the same structure. Results. We report for the first time the detection of ortho-$rm H_2D^+$ in high-mass star-forming regions performed with an interferometer. The molecular emission shows narrow and subsonic lines, suggesting that locally the temperature of the gas is less than 10K. From the continuum emission we estimate the cores total masses, and compare them with the respective virial masses. We also compute the volume density values, which are found to be higher than $10^{6}, rm cm^{-3}$. Conclusions. Our data confirm that ortho-$rm H_2D^+$ is an ideal tracer of cold and dense gas. Interestingly, almost all the $rm H_2D^+$-identified cores are less massive than 13M_sun , with the exception of one core in AG354. Furthermore, most of them are subvirial and larger than their Jeans masses. These results are difficult to explain in the context of the turbulent accretion models, which predict massive and virialised prestellar cores.
We present a possible identification strategy for first hydrostatic core (FHSC) candidates and make predictions of ALMA dust continuum emission maps from these objects. We analyze the results given by the different bands and array configurations and identify which combinations of the two represent our best chance of solving the fragmentation issue in these objects. If the magnetic field is playing a role, the emission pattern will show evidence of a pseudo-disk and even of a magnetically driven outflow, which pure hydrodynamical calculations cannot reproduce.
We investigate at a high angular resolution the spatial and kinematic structure of the S255IR high mass star-forming region, which demonstrated recently the first disk-mediated accretion burst in the massive young stellar object. The observations were performed with ALMA in Band 7 at an angular resolution $ sim 0.1^{primeprime}$, which corresponds to $ sim 180 $ AU. The 0.9 mm continuum, C$^{34}$S(7-6) and CCH $N=4-3$ data show a presence of very narrow ($ sim 1000 $ AU), very dense ($nsim 10^7$ cm$^{-3}$) and warm filamentary structures in this area. At least some of them represent apparently dense walls around the high velocity molecular outflow with a wide opening angle from the S255IR-SMA1 core, which is associated with the NIRS3 YSO. This wide-angle outflow surrounds a narrow jet. At the ends of the molecular outflow there are shocks, traced in the SiO(8-7) emission. The SiO abundance there is enhanced by at least 3 orders of magnitude. The CO(3-2) and SiO(8-7) data show a collimated and extended high velocity outflow from another dense core in this area, SMA2. The outflow is bent and consists of a chain of knots, which may indicate periodic ejections possibly arising from a binary system consisting of low or intermediate mass protostars. The C$^{34}$S emission shows evidence of rotation of the parent core. Finally, we detected two new low mass compact cores in this area (designated as SMM1 and SMM2), which may represent prestellar objects.
Magnetic and energetic properties are presented for 17 dense cores within a few hundred pc of the Sun. Their plane-of-sky field strengths are estimated from the dispersion of polarization directions, following Davis, Chandrasekhar and Fermi (DCF). Their ratio of mass to magnetic critical mass is 0.5-3, indicating nearly critical field strengths. The field strength B_pos is correlated with column density N as B_pos~N^p, where p=1.05+-0.08, and with density n as B_pos~n^q, where q=0.66+-0.05. These magnetic properties are consistent with those derived from Zeeman studies (Crutcher et al. 2010), with less scatter. Relations between virial mass M_V, magnetic critical mass M_B, and Alfven amplitude sigma_B/B match the observed range of M/M_B for cores observed to be nearly virial, with M/M_V=0.5-2, with moderate Alfven amplitudes, and with sigma_B/B=0.1-0.4. The B-N and B-n correlations in the DCF and Zeeman samples can be explained when such bound, Alfvenic, and nearly-critical cores have central concentration and spheroidal shape. For these properties, B~N because M/M_B is nearly constant compared to the range of N, and B~n^(2/3) because M^(1/3) is nearly constant compared to the range of n^(2/3). The observed core fields which follow B~n^(2/3) need not be much weaker than gravity, in contrast to core fields which follow B~n^(2/3) due to spherical contraction at constant mass (Mestel 1966). Instead, the nearly critical values of M/M_B suggest that the observed core fields are nearly as strong as possible, among values which allow gravitational contraction.
Most stars in the Galaxy, including the Sun, were born in high-mass star-forming regions. It is hence important to study the chemical processes in these regions to better understand the chemical heritage of both the Solar System and most stellar systems in the Galaxy. The molecular ion HCNH+ is thought to be a crucial species in ion-neutral astrochemical reactions, but so far it has been detected only in a handful of star-forming regions, and hence its chemistry is poorly known. We have observed with the IRAM-30m Telescope 26 high-mass star-forming cores in different evolutionary stages in the J=3-2 rotational transition of HCNH+. We report the detection of HCNH+ in 16 out of 26 targets. This represents the largest sample of sources detected in this molecular ion so far. The fractional abundances of HCNH+, [HCNH+], w.r.t. H2, are in the range 0.9 - 14 X $10^{-11}$, and the highest values are found towards cold starless cores. The abundance ratios [HCNH+]/[HCN] and [HCNH+]/[HCO+] are both < 0.01 for all objects except for four starless cores, for which they are well above this threshold. These sources have the lowest gas temperature in the sample. We run two chemical models, a cold one and a warm one, which attempt to match as much as possible the average physical properties of the cold(er) starless cores and of the warm(er) targets. The reactions occurring in the latter case are investigated in this work for the first time. Our predictions indicate that in the warm model HCNH+ is mainly produced by reactions with HCN and HCO+, while in the cold one the main progenitor species of HCNH+ are HCN+ and HNC+. The results indicate that the chemistry of HCNH+ is different in cold/early and warm/evolved cores, and the abundance ratios [HCNH+]/[HCN] and [HCNH+]/[HCO+] is a useful astrochemical tool to discriminate between different evolutionary phases in the process of star formation.
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