99 research outputs found
CEN34 -- High-Mass YSO in M17 or Background Post-AGB Star?
We investigate the proposed high-mass young stellar object (YSO) candidate
CEN34, thought to be associated with the star forming region M17. Its optical
to near-infrared (550-2500 nm) spectrum reveals several photospheric absorption
features, such as H{\alpha}, Ca triplet and CO bandheads but lacks any emission
lines. The spectral features in the range 8375-8770{\AA} are used to constrain
an effective temperature of 5250\pm250 (early-/mid-G) and a surface gravity of
2.0\pm0.3 (supergiant). The spectral energy distribution of CEN34 resembles the
SED of a high-mass YSO or an evolved star. Moreover, the observed temperature
and surface gravity are identical for high-mass YSOs and evolved stars. The
radial velocity relative to LSR (V_LSR) of CEN34 as obtained from various
photospheric lines is of the order of -60 km/s and thus distinct from the +25
km/s found for several OB stars in the cluster and for the associated molecular
cloud. The SED modeling yields ~ 10^{-4} M_sun of circumstellar material which
contributes only a tiny fraction to the total visual extinction (11 mag). In
the case of a YSO, a dynamical ejection process is proposed to explain the
V_LSR difference between CEN34 and M17. Additionally, to match the temperature
and luminosity, we speculate that CEN34 had accumulated the bulk of its mass
with accretion rate > 4x10^{-3} M_sun/yr in a very short time span (~ 10^3
yrs), and currently undergoes a phase of gravitational contraction without any
further mass gain. However, all the aforementioned characteristics of CEN34 are
compatible with an evolved star of 5-7 M_sun and an age of 50-100 Myrs, most
likely a background post-AGB star with a distance between 2.0 kpc and 4.5 kpc.
We consider the latter classification as the more likely interpretation.
Further discrimination between the two possible scenarios should come from the
more strict confinement of CEN34's distance.Comment: 8 pages, 8 figures, 2 tables; accepted by A&
A Chemical Study of Nine Star-forming Regions with Evidence of Infall Motion
The study of the physical and chemical properties of gas infall motion in the
molecular clumps helps us understand the initial stages of star formation. We
used the FTS wide-sideband mode of the IRAM 30-m telescope to observe nine
infall sources with significant double peaked blue line profile. The
observation frequency range are 83.7 - 91.5 GHz and 99.4 - 107.2 GHz. We have
obtained numbers of molecular line data. Using XCLASS, a total of 7 to 27
different molecules and isotopic transition lines have been identified in these
nine sources, including carbon chain molecules such as CCH, c-C3H2 and HC3N.
According to the radiation transfer model, we estimated the rotation
temperatures and column densities of these sources. Chemical simulations
adopting a physical model of HMSFRs are used to fit the observed molecular
abundances. The comparison shows that most sources are in the early HMPO stage,
with the inner temperature around several ten K
Disks around massive young stellar objects: are they common?
We present K-band polarimetric images of several massive young stellar
objects at resolutions 0.1-0.5 arcsec. The polarization vectors around
these sources are nearly centro-symmetric, indicating they are dominating the
illumination of each field. Three out of the four sources show elongated
low-polarization structures passing through the centers, suggesting the
presence of polarization disks. These structures and their surrounding
reflection nebulae make up bipolar outflow/disk systems, supporting the
collapse/accretion scenario as their low-mass siblings. In particular, S140
IRS1 show well defined outflow cavity walls and a polarization disk which
matches the direction of previously observed equatorial disk wind, thus
confirming the polarization disk is actually the circumstellar disk. To date, a
dozen massive protostellar objects show evidence for the existence of disks;
our work add additional samples around MYSOs equivalent to early B-type stars.Comment: 9 pages, including 2 figures, 1 table, to appear on ApJ
FacetClumps: A Facet-based Molecular Clump Detection Algorithm
A comprehensive understanding of molecular clumps is essential for
investigating star formation. We present an algorithm for molecular clump
detection, called FacetClumps. This algorithm uses a morphological approach to
extract signal regions from the original data. The Gaussian Facet model is
employed to fit the signal regions, which enhances the resistance to noise and
the stability of the algorithm in diverse overlapping areas. The introduction
of the extremum determination theorem of multivariate functions offers
theoretical guidance for automatically locating clump centers. To guarantee
that each clump is continuous, the signal regions are segmented into local
regions based on gradient, and then the local regions are clustered into the
clump centers based on connectivity and minimum distance to identify the
regional information of each clump. Experiments conducted with both simulated
and synthetic data demonstrate that FacetClumps exhibits great recall and
precision rates, small location error and flux loss, a high consistency between
the region of detected clump and that of simulated clump, and is generally
stable in various environments. Notably, the recall rate of FacetClumps in the
synthetic data, which comprises () emission line of the
MWISP within , and 5 km s 35 km s and simulated
clumps, reaches 90.2\%. Additionally, FacetClumps demonstrates satisfactory
performance when applied to observational data.Comment: 27pages,28figure
Rigorous assessment of Cl−‐based anolytes on electrochemical ammonia synthesis
Many challenges in the electrochemical synthesis of ammonia have been recognized with most effort focused on delineating false positives resulting from unidentified sources of nitrogen. However, the influence of oxidizing anolytes on the crossover and oxidization of ammonium during the electrolysis reaction remains unexplored. Here it is reported that the use of analytes containing halide ions (Cl− and Br−) can rapidly convert the ammonium into N2, which further intensifies the crossover of ammonium. Moreover, the extent of migration and oxidation of ammonium is found to be closely associated with external factors, such as applied potentials and the concentration of Cl−. These findings demonstrate the profound impact of oxidizing anolytes on the electrochemical synthesis of ammonia. Based on these results, many prior reported ammonia yield rates are calibrated. This work emphasizes the significance of avoiding selection of anolytes that can oxidize ammonium, which is believed to promote further progress in electrochemical nitrogen fixation
Anomalous excitonic phase diagram in band-gap-tuned Ta2Ni(Se,S)5
During a band-gap-tuned semimetal-to-semiconductor transition, Coulomb
attraction between electrons and holes can cause spontaneously formed excitons
near the zero-band-gap point, or the Lifshitz transition point. This has become
an important route to realize bulk excitonic insulators -- an insulating ground
state distinct from single-particle band insulators. How this route manifests
from weak to strong coupling is not clear. In this work, using angle-resolved
photoemission spectroscopy (ARPES) and high-resolution synchrotron x-ray
diffraction (XRD), we investigate the broken symmetry state across the
semimetal-to-semiconductor transition in a leading bulk excitonic insulator
candidate system Ta2Ni(Se,S)5. A broken symmetry phase is found to be
continuously suppressed from the semimetal side to the semiconductor side,
contradicting the anticipated maximal excitonic instability around the Lifshitz
transition. Bolstered by first-principles and model calculations, we find
strong interband electron-phonon coupling to play a crucial role in the
enhanced symmetry breaking on the semimetal side of the phase diagram. Our
results not only provide insight into the longstanding debate of the nature of
intertwined orders in Ta2NiSe5, but also establish a basis for exploring
band-gap-tuned structural and electronic instabilities in strongly coupled
systems.Comment: 27 pages, 4 + 9 figure
Sciences for The 2.5-meter Wide Field Survey Telescope (WFST)
The Wide Field Survey Telescope (WFST) is a dedicated photometric survey
facility under construction jointly by the University of Science and Technology
of China and Purple Mountain Observatory. It is equipped with a primary mirror
of 2.5m in diameter, an active optical system, and a mosaic CCD camera of 0.73
Gpix on the main focus plane to achieve high-quality imaging over a field of
view of 6.5 square degrees. The installation of WFST in the Lenghu observing
site is planned to happen in the summer of 2023, and the operation is scheduled
to commence within three months afterward. WFST will scan the northern sky in
four optical bands (u, g, r, and i) at cadences from hourly/daily to
semi-weekly in the deep high-cadence survey (DHS) and the wide field survey
(WFS) programs, respectively. WFS reaches a depth of 22.27, 23.32, 22.84, and
22.31 in AB magnitudes in a nominal 30-second exposure in the four bands during
a photometric night, respectively, enabling us to search tremendous amount of
transients in the low-z universe and systematically investigate the variability
of Galactic and extragalactic objects. Intranight 90s exposures as deep as 23
and 24 mag in u and g bands via DHS provide a unique opportunity to facilitate
explorations of energetic transients in demand for high sensitivity, including
the electromagnetic counterparts of gravitational-wave events detected by the
second/third-generation GW detectors, supernovae within a few hours of their
explosions, tidal disruption events and luminous fast optical transients even
beyond a redshift of 1. Meanwhile, the final 6-year co-added images,
anticipated to reach g about 25.5 mag in WFS or even deeper by 1.5 mag in DHS,
will be of significant value to general Galactic and extragalactic sciences.
The highly uniform legacy surveys of WFST will also serve as an indispensable
complement to those of LSST which monitors the southern sky.Comment: 46 pages, submitted to SCMP
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