arXiv subjects

Aya E. Higuchi

Publications and source records attributed to Aya E. Higuchi.

28 records · Page 2Linked to original sources

arXiv2017

L483: Warm Carbon-Chain Chemistry Source Harboring Hot Corino Activity

The Class 0 protostar, L483, has been observed in various molecular lines in the 1.2 mm band at a sub-arcsecond resolution with ALMA. An infalling-rotating envelope is traced by the CS line, while a very compact component with a broad velocity width is observed for the CS, SO, HNCO, NH$_2$CHO, and HCOOCH$_3$ lines. Although this source is regarded as the warm carbon-chain chemistry (WCCC) candidate source at a 1000 au scale, complex organic molecules characteristic of hot corinos such as NH$_2$CHO and HCOOCH$_3$ are detected in the vicinity of the protostar. Thus, both hot corino chemistry and WCCC are seen in L483. Although such a mixed chemical character source has been recognized as an intermediate source in previous single-dish observations, we here report the first spatially-resolved detection. A kinematic structure of the infalling-rotating envelope is roughly explained by a simple ballistic model with the protostellar mass of 0.1--0.2 $M_\odot$ and the radius of the centrifugal barrier (a half of the centrifugal radius) of 30--200 au, assuming the inclination angle of 80\degr\ (0\degr\ for a face-on). The broad line emission observed in the above molecules most likely comes from the disk component inside the centrifugal barrier. Thus, a drastic chemical change is seen around the centrifugal barrier.

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arXiv2016

The intrinsic abundance ratio and X-factor of CO isotopologues in L1551 shielded from FUV photodissociation

L1551 is chosen because it is relatively isolated in the Taurus molecular cloud shielded from FUV photons, providing an ideal environment for studying the target properties. Our observations cover ~40'x40' with resolution ~30", which are the maps with highest spatial dynamical range to date. We derive the X(13CO)/X(C18O) value on the sub-parsec scales in the range of ~3-27 with a mean value of 8.0+-2.8. Comparing to the visual extinction map derived from the Herschel observations, we found that the abundance ratio reaches its maximum at low Av (i.e., Av ~ 1-4mag), and decreases to the typical solar system value of 5.5 inside L1551 MC. The high X(13CO)/X(C18O) value at the boundary of the cloud is most likely due to the selective FUV photodissociation of C18O. This is in contrast with Orion-A where its internal OB stars keep the abundance ratio at a high level greater than ~10. In addition, we explore the variation of the X-factor, because it is an uncertain but widely used quantity in extragalactic studies. We found that X-factor is proportional to N(H2)^1.0 which is consistent with previous simulations. Excluding the high density region, the average X-factor is similar to the Milky Way average value.

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arXiv2015

Sgr B2(N): A bipolar outflow and rotating hot core revealed by ALMA

We present the results of SiO (2-1) and SO2 (12-13) line observations of Sgr B2(N) made with the Atacama Large Millimeter/submillimeter Array (ALMA) at an angular resolution of ~2arcsec. Our analysis of the SiO and SO2 line emission reveals a bipolar molecular outflow in an east-west direction whose driving source is located at K2. In addition, SO2 line core shows a north-south velocity gradient most probably indicating a hot core of molecular gas rotating around K2. Fractional abundances of SO2 and SiO (X(SO2) and X(SiO), respectively) in the outflowing molecular gas are derived from comparisons with the C18O emission. Assuming an excitation temperature of 100+-50 K, we calculate X(SO2) = 2.3X10^-8 and X(SiO) = 1.2X10^-9. The outflow from SgrB2(N) K2 is characterized as a young (5X10^3yr) and massive (~2000Msun), but moderately collimated (~60deg) outflow. We also report a possible detection of the SiO (v=2, J=2-1) maser emission from the position of K2. If confirmed, it would make Sgr B2(N) the 4th star forming region associated with SiO masers.

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arXiv2014

IRAS16547-4247: A New Candidate of a Protocluster Unveiled with ALMA

We present the results of continuum and 12CO(3-2) and CH3OH(7-6) line observations of IRAS16547-4247 made with the Atacama Large Millimeter/submillimeter Array (ALMA) at an angular resolution of ~0.5". The 12CO(3-2) emission shows two high-velocity outflows whose driving sources are located within the dust continuum peak. The alignment of these outflows do not coincide with that of the wide-angle, large scale, bipolar outflow detected with APEX in previous studies. The CH3OH(7-6) line emission traces an hourglass structure associated with the cavity walls created by the outflow lobes. Taking into account our results together with the position of the H2O and class I CH3OH maser clusters, we discuss two possible scenarios that can explain the hourglass structure observed in IRAS16547-4247: (1) precession of a biconical jet, (2) multiple, or at least two, driving sources powering intersecting outflows. Combining the available evidence, namely, the presence of two cross-aligned bipolar outflows and two different H2O maser groups, we suggest that IRAS16547-4247 represents an early formation phase of a protocluster.

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arXiv2014

ALMA view of G0.253+0.016: Can cloud-cloud collision form the cloud?

We present the results of sulfur monoxide, SO line emission observations of G0.253+0.016 with the Atacama Large Millimeter/submillimeter Array (ALMA) at an angular resolution of 1.7 arcsec. The dense and massive molecular cloud of G0.253+0.016 is highly sub-structured, yet shows no obvious signs of cluster formation. We found three outstanding features of the cloud from the SO emission, namely, shell structure of radius 1.3 pc, large velocity gradients of 20 km/s/pc with the cloud, and cores with large velocity dispersions (30-40 km/s) around the shell structure. We suggest that these large-velocity dispersion cores will form high-mass stars in the future. In attempt to explore the formation scenario of the dense cloud, we compared our results with numerical simulations, thus, we propose that G0.253+0.016 may have formed due to a cloud-cloud collision process.

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arXiv2013

The Dynamical State of The Serpens South Filamentary Infrared Dark Cloud

We present the results of N$_2$H$^+$ ($J=1-0$) observations toward Serpens South, the nearest cluster-forming, infrared dark cloud. The physical quantities are derived by fitting the hyperfine structure of N$_2$H$^+$. The Herschel and 1.1-mm continuum maps show that a pc-scale filament fragments into three clumps with radii of $0.1-0.2$ pc and masses of $40-230M_\odot$. We find that the clumps contain smaller-scale ($\sim 0.04$ pc) structures, i.e., dense cores. We identify 70 cores by applying CLUMPFIND to the N$_2$H$^+$ data cube. In the central cluster-forming clump, the excitation temperature and line-width tend to be large, presumably due to protostellar outflow feedback and stellar radiation. However, for all the clumps, the virial ratios are evaluated to be $0.1-0.3$, indicating that the internal motions play only a minor role in the clump support. The clumps exhibit no free-fall, but low-velocity infall, and thus the clumps should be supported by additional forces. The most promising force is the globally-ordered magnetic field observed toward this region. We propose that the Serpens South filament was close to magnetically-critical and ambipolar diffusion triggered the cluster formation. We find that the northern clump, which shows no active star formation, has a mass and radius comparable to the central cluster-forming clump, and therefore, it is a likely candidate of a {\it pre-protocluster clump}. The initial condition for cluster formation is likely to be a magnetically-supported clump of cold, quiescent gas. This appears to contradict the accretion-driven turbulence scenario, for which the turbulence in the clumps is maintained by the accretion flow.

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arXiv2013

Maps of Massive Clumps in the Early Stage of Cluster Formation: Two Modes of Cluster Formation, Coeval or Non-Coeval?

We present maps of 7 young massive molecular clumps within 5 target regions in the C18O (J=1-0) line emission, using the Nobeyama 45m telescope. These clumps, which are not associated with clusters, lie at distances between 0.7 to 2.1 kpc. We find C18O clumps with radii of 0.5-1.7 pc, masses of 470-4200 Msun, and velocity widths of 1.4-3.3 km/s. All of the clumps are massive and approximately in virial equilibrium, suggesting they will potentially form clusters. Three of our target regions are associated with H II regions ("CWHRs" from Clump with H II Regions), and the other two are without H II regions (CWOHRs). The C18O clumps can be classified into two morphological types: CWHRs shape a filamentary or shell-like structure, CWOHRs are spherical. The two CWOHRs have systematic velocity gradients. Using the publicly released WISE database, Class I and Class II protostellar candidates were identified within the C18O clumps. The fraction of the Class I candidates among all YSO candidates (Class I+Class II) is > 50 % in CWHRs, and < 50 % in CWOHRs. We conclude that effects from the H II regions can be seen in (1) spatial distributions of the clumps: filamentary or shell-like structure running along the H II regions, (2) velocity structures of the clumps: large velocity dispersion along shells, and (3) small age spreads of YSOs. The small spread in age of the YSOs show that the presence of H II regions tend to trigger coeval cluster formation.

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arXiv2012

Toward Understanding the Origin of Turbulence in Molecular Clouds: Small Scale Structures as Units of Dynamical Multi-Phase Interstellar Medium

In order to investigate the origin of the interstellar turbulence, detailed observations in the CO J=1--0 and 3--2 lines have been carried out in an interacting region of a molecular cloud with an HII region. As a result, several 1,000 to 10,000 AU scale cloudlets with small velocity dispersion are detected, whose systemic velocities have a relatively large scatter of a few km/s. It is suggested that the cloud is composed of small-scale dense and cold structures and their overlapping effect makes it appear to be a turbulent entity as a whole. This picture strongly supports the two-phase model of turbulent medium driven by thermal instability proposed previously. On the surface of the present cloud, the turbulence is likely to be driven by thermal instability following ionization shock compression and UV irradiation. Those small scale structures with line width of ~ 0.6 km/s have a relatively high CO line ratio of J=3--2 to 1--0, 1 < R(3-2/1-0) < 2. The large velocity gradient analysis implies that the 0.6 km/s width component cloudlets have an average density of 10^{3-4} cm^{-3}, which is relatively high at cloud edges, but their masses are only < 0.05 M_{sun}.

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arXiv2011

Molecular Outflows From the Protocluster, Serpens South

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.

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arXiv2009

A Mapping Survey of Dense Clumps Associated with Embedded Clusters : Evolutionary Stages of Cluster-Forming Clumps

We have carried out a survey of the dense clumps associated with 14 embedded clusters in the C^18O (J=1-0) line emission with the Nobeyama 45m telescope in order to understand the formation and evolution of stellar clusters in dense clumps of molecular clouds. We have selected these clusters at distances from 0.3 to 2.1kpc and have mapped about 6' X 6' to 10' X 10'regions (corresponding to 3.8pc X 3.8pc at 2.1kpc) for all the clumps with 22" resolution (corresponding to Jeans length at 2.1kpc). We have obtained dense clumps with radii of 0.40-1.6pc, masses of 150-4600M_sun, and velocity widths in FWHM of 1.4-3.3kms^-1. Most of the clumps are found to be approximately in virial equilibrium, which implies that C^18O gas represents parental dense clumps for cluster formation. From the spatial relation between the distributions of clumps and clusters, we classified C^18O clumps into three types (Type A, B, and C). The C^18O clumps as classified into Type A have emission distributions with a single peak at the stellar clusters and higher brightness contrast than that of other target sources. Type B clumps have double or triple peaks which are associated with the cluster and moderately high brightness contrast structure. Type C clumps have also multiple peaks although they are not associated with the cluster and low brightness contrast structure. We suggest that our classification represents an evolutionary trend of cluster-forming dense clumps because dense gas in molecular clouds is expected to be converted into stellar constituents, or to be dispersed by stellar activities. Moreover, although there is a scatter, we found a tendency that the SFEs of the dense clumps increase from Type A to Type C, which also supports our scenario.

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