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Register a new userChemical composition in the IRAS 16562--3959 high-mass star-forming region
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Kotomi Taniguchi Dr.
Publication date
2020
fields
Physics
and research's language is
English
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We have analyzed the Atacama Large Millimeter/submillimeter Array (ALMA) cycle 2 data of band 6 toward the G345.4938+01.4677 massive young protostellar object (G345.5+1.47 MYSO) in the IRAS 16562--3959 high-mass star-forming region with an angular resolution of $sim 0.3$, corresponding to $sim 760$ au. We spatially resolve the central region which consists of three prominent molecular emission cores. A hypercompact (HC) H$_{rm {II}}$ region (Core A) and two molecule-rich cores (Core B and Core C) are identified using the moment zero images of the H30$alpha$ line and a CH$_{3}$OH line, respectively. Various oxygen-bearing complex organic molecules (COMs), such as (CH$_{3}$)$_{2}$CO and CH$_{3}$OCHO, have been detected toward the positions of Core B and Core C, while nitrogen-bearing species, CH$_{3}$CN, HC$_{3}$N and its $^{13}$C isotopologues, have been detected toward all of the cores. We discuss the formation mechanisms of H$_{2}$CO by comparing the spatial distribution of C$^{18}$O with that of H$_{2}$CO. The $^{33}$SO emission, on the other hand, shows a ring-like structure surrounding Core A, and it peaks on the outer edge of the H30$alpha$ emission region. These results imply that SO is enhanced in a shock produced by the expanding motion of the ionized region.
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Nitrogen (N) fractionation is used as a tool to search for a link between the chemical history of the Solar System and star-forming regions. A large variation of $^{14}$N/$^{15}$N is observed towards different astrophysical sources, and current chemical models cannot reproduce it. With the advent of high angular resolution radiotelescopes it is now possible to search for N-fractionation at core scales. We present IRAM NOEMA observations of the J=1-0 transition of N$_{2}$H$^{+}$, $^{15}$NNH$^{+}$ and N$^{15}$NNH$^{+}$ towards the high-mass protocluster IRAS 05358+3543. We find $^{14}$N/$^{15}$N ratios that span from $sim$100 up to $sim$220 and these values are lower or equal than those observed with single-dish observations towards the same source. Since N-fractionation changes across the studied region, this means that it is regulated by local environmental effects. We find also the possibility, for one of the four cores defined in the protocluster, to have a more abundant $^{15}$NNH$^{+}$ with respect to N$^{15}$NNH$^{+}$. This is another indication that current chemical models may be missing chemical reactions or may not take into account other mechanisms, like photodissociation or grain surface chemistry, that could be important.
A multi-wavelength investigation of the star forming complex IRAS 20286+4105, located in the Cygnus-X region, is presented here. Near-infrared K-band data is used to revisit the cluster / stellar group identified in previous studies. The radio continuum observations, at 610 and 1280 MHz show the presence of a HII region possibly powered by a star of spectral type B0 - B0.5. The cometary morphology of the ionized region is explained by invoking the bow-shock model where the likely association with a nearby supernova remnant is also explored. A compact radio knot with non-thermal spectral index is detected towards the centre of the cloud. Mid-infrared data from the Spitzer Legacy Survey of the Cygnus-X region show the presence of six Class I YSOs inside the cloud. Thermal dust emission in this complex is modelled using Herschel far-infrared data to generate dust temperature and column density maps. Herschel images also show the presence of two clumps in this region, the masses of which are estimated to be {sim} 175 M{sun} and 30 M{sun}. The mass-radius relation and the surface density of the clumps do not qualify them as massive star forming sites. An overall picture of a runaway star ionizing the cloud and a triggered population of intermediate-mass, Class I sources located toward the cloud centre emerges from this multiwavelength study. Variation in the dust emissivity spectral index is shown to exist in this region and is seen to have an inverse relation with the dust temperature.
The formation process of high-mass stars (>8M$_odot$) is poorly constrained, particularly, the effects of clump fragmentation creating multiple systems and the mechanism of mass accretion onto the cores. We study the fragmentation of dense gas clumps, and trace the circumstellar rotation and outflows by analyzing observations of the high-mass (~500M$_odot$) star-forming region IRAS 23033+5951. Using the Northern Extended Millimeter Array (NOEMA) in three configurations and the IRAM 30-m single-dish telescope at 220GHz, we probe the gas and dust emission at an angular resolution of ~0.45arcsec, corresponding to 1900au. In the mm continuum emission, we identify a protostellar cluster with at least four mm-sources, where three of them show a significantly higher peak intensity well above a signal-to-noise ratio of 100. Hierarchical fragmentation from large to small spatial scales is discussed. Two fragments are embedded in rotating structures and drive molecular outflows, traced by $^{13}$CO (2-1) emission. The velocity profiles across two of the cores are similar to Keplerian but are missing the highest velocity components close to the center of rotation, which is a common phenomena from observations like these, and other rotation scenarios are not excluded entirely. Position-velocity diagrams suggest protostellar masses of ~6 and 19M$_sun$. Rotational temperatures from fitting CH$_3$CN ($12_K-11_K$) spectra are used for estimating the gas temperature and by that the disk stability against gravitational fragmentation, utilizing Toomres $Q$ parameter. [We] identify only one candidate disk to be unstable against gravitational instability caused by axisymmetric perturbations. The dominant sources cover different evolutionary stages within the same maternal gas clump. The appearance of rotation and outflows of the cores are similar to those found in low-mass star-forming regions.
We have carried out interferometric observations of cyanopolyynes, HC$_{3}$N, HC$_{5}$N, and HC$_{7}$N, in the 36 GHz band toward the G28.28$-$0.36 high-mass star-forming region using the Karl G. Jansky Very Large Array (VLA) Ka-band receiver. The spatial distributions of HC$_{3}$N and HC$_{5}$N are obtained. HC$_{5}$N emission is coincident with a 450 $mu$m dust continuum emission and this clump with a diameter of $sim 0.2$ pc is located at the east position from the 6.7 GHz methanol maser by $sim 0.15$ pc. HC$_{7}$N is tentatively detected toward the clump. The HC$_{3}$N : HC$_{5}$N : HC$_{7}$N column density ratios are estimated at 1.0 : $sim 0.3$ : $sim 0.2$ at an HC$_{7}$N peak position. We discuss possible natures of the 450 $mu$m continuum clump associated with the cyanopolyynes. The 450 $mu$m continuum clump seems to contain deeply embedded low- or intermediate-mass protostellar cores, and the most possible formation mechanism of the cyanopolyynes is the warm carbon chain chemistry (WCCC) mechanism. In addition, HC$_{3}$N and compact HC$_{5}$N emission is detected at the edge of the 4.5 $mu$m emission, which possibly implies that such emission is the shock origin.
We present high angular resolution observations (0.5x0.3) carried out with the Submillimeter Array (SMA) toward the AFGL2591 high-mass star forming region. Our SMA images reveal a clear chemical segregation within the AFGL2591 VLA 3 hot core, where different molecular species (Type I, II and III) appear distributed in three concentric shells. This is the first time that such a chemical segregation is ever reported at linear scales <3000 AU within a hot core. While Type I species (H2S and 13CS) peak at the AFGL2591 VLA 3 protostar, Type II molecules (HC3N, OCS, SO and SO2) show a double-peaked structure circumventing the continuum peak. Type III species, represented by CH3OH, form a ring-like structure surrounding the continuum emission. The excitation temperatures of SO2, HC3N and CH3OH (185+-11 K, 150+-20 K and 124+-12 K, respectively) show a temperature gradient within the AFGL2591 VLA 3 envelope, consistent with previous observations and modeling of the source. By combining the H2S, SO2 and CH3OH images, representative of the three concentric shells, we find that the global kinematics of the molecular gas follow Keplerian-like rotation around a 40 Mo-star. The chemical segregation observed toward AFGL2591 VLA 3 is explained by the combination of molecular UV photo-dissociation and a high-temperature (~1000 K) gas-phase chemistry within the low extinction innermost region in the AFGL2591 VLA 3 hot core.
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