24,250 research outputs found
A molecular line study towards massive extended green object clumps in the southern sky: chemical properties
We present a molecular line study towards 31 extended green object (EGO)
clumps in the southern sky using data from MALT90 (Millimetre Astronomy Legacy
Team 90 GHz). According to previous multiwavelength observations, we divide our
sample into two groups: massive young stellar objects (MYSOs) and HII regions.
Our results seem to support that N2H+ and C2H emissions mainly come from the
gas inside quiescent clumps. In addition, we also find that the [N2H+]/[H13CO+]
and [C2H]/[H13CO+] relative abundance ratios decrease from MYSOs to HII
regions. These results suggest depletion of N2H+ and C2H in the late stages of
massive-star formation, probably caused by the formation of HII regions inside.
N2H+ and C2H might be used as chemical clocks for massive-star formation by
comparing with other molecules such as H13CO+ and HC3N
Molecular line study of massive star forming regions from the RMS survey
In this paper we selected a sample of massive star forming regions from the
Red MSX Source (RMS) survey, to study star formation activities (mainly outflow
and inflow signatures)
A multiwavelength study of the star forming H II region Sh2-82
Based on a multiwavelength study, the interstellar medium and young stellar
objects (YSOs) around the HII region Sh2-82 have been analyzed. Two molecular
clumps were found from the archival data of the Galactic Ring Survey, and using
the Two Micron All-Sky Survey catalog, we found two corresponding young
clusters embedded in the molecular clumps. The very good relations between CO
emission, infrared shells and YSOs suggest that it is probably a triggered star
formation region from the expansion of Sh2-82. We further used the data from
the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire from Spitzer to
study the YSOs within the two clumps, confirming star formation in this region.
By spectral energy distribution fits to each YSO candidate with infrared
excess, we derived the slope of the initial mass function. Finally, comparing
the HII region's dynamical age and the fragmentation time of the molecular
shell, we discard the "collect and collapse" process as being the triggering
mechanism for YSO formation. Sh2-82 can be a mixture of other processes such as
radiative-driven implosion and/or collisions with pre-existing clumps.Comment: in raa, 2012, 12, 65
Neutrinos from gamma-ray bursts: propagation of cosmic rays in their host galaxies
Gamma-ray bursts (GRBs) are proposed as candidate sources of ultra-high
energy cosmic rays (UHECRs). We study the possibility that the PeV neutrinos
recently observed by IceCube are produced by GRB cosmic rays interacting with
the interstellar gas in the host galaxies. By studying the relation between the
X-ray absorption column density N_H and the surface star-formation rate of GRB
host galaxies, we find that N_H is a good indicator of the surface gas density
of the host galaxies. Then we are able to calculate the neutrino production
efficiency of CRs for GRBs with known N_H. We collect a sample of GRBs that
have both measurements of N_H and accurate gamma-ray fluence, and attempt to
calculate the accumulated neutrino flux based on the current knowledge about
GRBs and their host galaxies. When the CR intensity produced by GRBs is
normalized with the observed UHECR flux above , the
accumulated neutrino flux at PeV energies is estimated to be about
(per flavor)
under the assumption that GRB energy production rate follows the cosmic
star-formation rate and the favorable assumption about the CR diffusion
coefficient. This flux is insufficient to account for the IceCube observations,
but the estimate suffers from some assumptions in the calculation and thus we
can not rule out this scenario at present.Comment: 8 pages, 3 figures, 2 tables, to appear in ApJ
A multi-transition molecular line study of infrared dark cloud G331.71+00.59
Using archive data from the Millimeter Astronomy Legacy Team Survey at 90 GHz
(MALT90), carried out using the Mopra 22-m telescope, we made the first
multi-transition molecular line study of infrared dark cloud (IRDC) MSXDC
G331.71+00.59. Two molecular cores were found embedded in this IRDC. Each of
these cores is associated with a known extended green object (EGO), indicating
places of massive star formation. The HCO+ (1-0) and HNC (1-0) transitions show
prominent blue or red asymmetric structures, suggesting outflow and inflow
activities of young stellar objects (YSOs). Other detected molecular lines
include H13CO+ (1-0), C2H (1-0), HC3N (10-9), HNCO(404-303) and SiO (2-1),
which are typical of hot cores and outflows. We regard the two EGOs as evolving
from the IRDC to hot cores. Using public GLIMPS data, we investigate the
spectral energy distribution of EGO G331.71+0.60. Our results support this EGO
being a massive YSO driving the outflow. G331.71+0.58 may be at an earlier
evolutionary stage
HNCO: A Molecule Traces Low-velocity Shock
Using data from MALT90 (Millimetre Astronomy Legacy Team Survey at 90 GHz),
we present molecular line study of a sample of ATLASGAL (APEX Telescope Large
Area Survey of the Galaxy) clumps. Twelve emission lines have been detected in
all. We found that in most sources, emissions of HCN, HNC, CHCN,
HNCO and SiO show more compact distributions than those of HCO, HNC, HCN
and NH. By comparing with other molecular lines, we found that the
abundance of HNCO ((HNCO)) correlates well with other species such as
HC3N, HNC, C2H, H13CO+ and N2H+. Previous studies indicate the HNCO abundance
could be enhanced by shocks. However, in this study, we found the abundance of
HNCO does not correlate well with that of SiO, which is also a good tracer of
shocks. We suggest this may be because HNCO and SiO trace different parts of
shocks. Our analysis indicates that the velocity of shock traced by HNCO tends
to be lower than that traced by SiO. In the low-velocity shocks traced by HNCO,
the HNCO abundance increases faster than that of SiO. While in the relatively
high-velocity shocks traced by SiO, the SiO abundance increases faster than
that of HNCO. We suggest that in the infrared dark cloud (IRDC) of MSXDC
G331.71+00.59, high-velocity shocks are destroying the molecule of HNCO
Jiamusi Pulsar Observations: I. Abnormal emission events of PSR B0919+06
PSR B0919+06 generally radiates radio pulses in a normal phase range. It has
been known for its occasional perplexing abnormal emission events wherein
individual pulses come to an earlier phase range for a few tens of periods and
then returns to its usual phase. Heretofore, only a few such events have been
available for study. We observed PSR B0919+06 for about 30 hours using the
Jiamusi 66-m telescope at Jiamusi Deep Space Station at S-band, and detected 92
abnormal emission events. We identify four types of events based on the
abrupted or gradual phase-shifting of individual pulses. The abnormal emission
events are seen to occur randomly some every 1000 to 3000 periods, and they
affect the leading edge of the mean profile by up to 2\% in amplitude. The
abnormal emission events are probably related to gradual changes of emission
processing in the pulsar magnetosphere.Comment: 9 pages, 13 figures. MNRAS Publishe
Primordial gravastar from inflation
The dS bubbles can nucleate spontaneously during inflation, and will be
stretched by the cosmological expansion to astrophysical scale. We report on a
novel phenomenon that such a bubble might develop into a gravastar (an
ultra-compact object with a dS core) after inflation, which witnessed the
occurrence of inflation and would survive till today. It is pointed out that if
a primordial gravastar was involved in one of the LIGO/Virgo gravitational wave
(GW) events, the post-merger object could be a gravastar that will eventually
collapse into a black hole. As a result, the late-time GW ringdown waveform
will exhibit a series of "echoes" with intervals increasing with time.Comment: 10 pages, 4 figure
On-chip Rotated Polarization Directional Coupler Fabricated by Femtosecond Laser Direct Writing
We present a rotated polarization directional coupler (RPDC) on a photonic
chip. We demonstrate a double-track approach to modify the distribution of
refractive index between adjacent tracks and form a single waveguide with
arbitrary birefringent optical axis. We construct a RPDC with the two
axis-rotated waveguides coupled in a strong regime. The obtained extinction
ratios on average are about 16dB and 20dB for the corresponding orthogonal
polarizations. We perform the reconstruction of Stokes vector to test the
projection performance of our RPDC, and observe the average fidelities up to
98.1% and 96.0% for the perfectly initialized states in 0 and
45 RPDCs respectively.Comment: 6 pages, 4 figure
Optimal Subsampling Algorithms for Big Data Regressions
To fast approximate maximum likelihood estimators with massive data, this
paper studies the Optimal Subsampling Method under the A-optimality Criterion
(OSMAC) for generalized linear models. The consistency and asymptotic normality
of the estimator from a general subsampling algorithm are established, and
optimal subsampling probabilities under the A- and L-optimality criteria are
derived. Furthermore, using Frobenius norm matrix concentration inequalities,
finite sample properties of the subsample estimator based on optimal
subsampling probabilities are also derived. Since the optimal subsampling
probabilities depend on the full data estimate, an adaptive two-step algorithm
is developed. Asymptotic normality and optimality of the estimator from this
adaptive algorithm are established. The proposed methods are illustrated and
evaluated through numerical experiments on simulated and real datasets
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