14 research outputs found
Evolution of High-Energy Particle Distribution in Mature Shell-Type Supernova Remnants
Multi-wavelength observations of mature supernova remnants (SNRs), especially
with recent advances in gamma-ray astronomy, make it possible to constrain
energy distribution of energetic particles within these remnants. In
consideration of the SNR origin of Galactic cosmic rays and physics related to
particle acceleration and radiative processes, we use a simple one-zone model
to fit the nonthermal emission spectra of three shell-type SNRs located within
2 degrees on the sky: RX J1713.7-3946, CTB 37B, and CTB 37A. Although radio
images of these three sources all show a shell (or half-shell) structure, their
radio, X-ray, and gamma-ray spectra are quite different, offering an ideal case
to explore evolution of energetic particle distribution in SNRs. Our spectral
fitting shows that 1) the particle distribution becomes harder with aging of
these SNRs, implying a continuous acceleration process, and the particle
distributions of CTB 37A and CTB 37B in the GeV range are harder than the
hardest distribution that can be produced at a shock via the linear diffusive
shock particle acceleration process, so spatial transport may play a role; 2)
the energy loss timescale of electrons at the high-energy cutoff due to
synchrotron radiation appears to be always a bit (within a factor of a few)
shorter than the age of the corresponding remnant, which also requires
continuous particle acceleration; 3) double power-law distributions are needed
to fit the spectra of CTB 37B and CTB 37A, which may be attributed to shock
interaction with molecular clouds.Comment: Accepted for publication in The Astrophysical Journal, 11 pages, 3
figures, 1 tabl
Hierarchical Structure of YSO Clusters in the W40 and Serpens South Region: Group Extraction and Comparison with Fractal Clusters
Young stellar clusters are believed to inherit the spatial distribution like
hierarchical structures of their natal molecular cloud during their formation.
However, the change of the structures between the cloud and the young clusters
is not well constrained observationally. We select the W40 - Serpens South
region (~ 7 9 pc) of the Aquila Rift as a testbed and
investigate hierarchical properties of spatial distribution of young stellar
objects (YSOs) in this region. We develop a minimum spanning tree (MST)-based
method to group stars into several levels by successively cutting down edges
longer than an algorithmically determined critical value. A total of 832 YSOs
are divided into 5 levels with 23 groups. For describing the hierarchical
properties in a controlled way, we construct a set of synthetic source
distributions at various fractal dimensions, and apply the same method to
explore their group characters. By comparing the parameter and the surface
density profiles of the observed and the synthetic data, we find that the YSO
observation matches spatial patterns from multi-fractal dimensions. In the
periphery region where the molecular clouds are more diffuse, the YSO structure
is close to a fractal dimension of 2.0, while in the core regions the fractal
dimensions are close to 1.6 and 1.4 for the W40 and the Serpens South regions,
respectively. Therefore, the YSOs may inherit the fractal pattern of the dense
part of the molecular clouds, but such pattern dissipates slowly in several
Myr.Comment: 12 pages, 9 figures, Submitted to MNRA
Hierarchical Structure of YSO Clusters in the W40 and Serpens South Region: Group Extraction and Comparison with Fractal Clusters
Young stellar clusters are believed to inherit the spatial distribution like hierarchical structures of their natal molecular cloud during their formation. However, the change of the structures between the cloud and the young clusters is not well constrained observationally. We select the W40–Serpens South region (∼7 × 9 pc2) of the Aquila Rift as a testbed and investigate hierarchical properties of spatial distribution of young stellar objects (YSOs) in this region. We develop a minimum spanning tree (MST) based method to group stars into several levels by successively cutting down edges longer than an algorithmically determined critical value. A total of 832 YSOs are divided into 5 levels with 23 groups. For describing the hierarchical properties in a controlled way, we construct a set of synthetic source distributions at various fractal dimensions, and apply the same method to explore their group characters. By comparing the Q parameter and the surface density profiles of the observed and the synthetic data, we find that the YSO observation matches spatial patterns from multifractal dimensions. In the periphery region where the molecular clouds are more diffuse, the YSO structure is close to a fractal dimension of 2.0. While in the core regions, the fractal dimensions are close to 1.6 and 1.4 for the W40 and the Serpens South regions, respectively. Therefore, the YSOs may inherit the fractal pattern of the dense part of the molecular clouds, but such pattern dissipates slowly in several Myr
Deep Near-Infrared Survey Toward the W40 and Serpens South Region in Aquila Rift: A Comprehensive Catalog of Young Stellar Objects
Active star forming regions are excellent laboratories for studying the
origins and evolution of young stellar object (YSO) clustering. The W40 -
Serpens South region is such a region, and we compile a super
near-and-mid-infrared catalog of point sources in it, based on deep NIR
observations of CFHT in combination with 2MASS, UKIDSS, and Spitzer catalogs.
From this catalog, we identify 832 YSOs, and classify 15, 135, 647, and 35 of
them to be the deeply embedded sources, Class I, Class II YSOs, and transition
disk sources, respectively. In general, these YSOs are well correlated with the
filamentary structures of molecular clouds, especially the deeply embedded
sources and the Class I YSOs. The W40 central region is dominated by Class II
YSOs, but in the Serpens South region, a half of the YSOs are Class I. We
further generate a minimum spanning tree (MST) for all the YSOs. Around the W40
cluster, there are eight prominent MST branches that may trace vestigial
molecular gas filaments that once fed gas to the central natal gas clump. Of
the eight, only two now include detectable filamentary gas in Herschel data and
corresponding Class I YSOs, while the other six are exclusively populated with
Class II. Four MST branches overlap with the Serpens South main filament, and
where they intersect, molecular gas "hubs" and more Class I YSOs are found. Our
results imply a mixture of YSO distributions composed of both primordial and
somewhat evolved YSOs in this star forming region.Comment: 13 pages, 10 figures, Accepted for publication in MNRA
Two methods for separating the magnetospheric solar wind charge exchange soft X-ray emission from the diffuse X-ray background
Solar wind charge exchange (SWCX) is the process of solar wind high-valence ions exchanging charges with neutral components and generating soft X-rays. Recently, detecting the SWCX emission from the magnetosphere is proposed as a new technique to study the magnetosphere using panoramic soft X-ray imaging. To better prepare for the data analysis of upcoming magnetospheric soft X-ray imaging missions, this paper compares the magnetospheric SWCX emission obtained by two methods in an XMM-Newton observation, during which the solar wind changed dramatically. The two methods differ in the data used to fit the diffuse X-ray background (DXB) parameters in spectral analysis. The method adding data from the ROSAT All-Sky Survey (RASS) is called the RASS method. The method using the quiet observation data is called the Quiet method, where quiet observations usually refer to observations made by the same satellite with the same target but under weaker solar wind conditions. Results show that the spectral compositions of magnetospheric SWCX emission obtained by the two methods are very similar, and the changes in intensity over time are highly consistent, although the intensity obtained by the RASS method is about \begin{document}\end{document} keV \begin{document}\end{document} higher than that obtained by the Quiet method. Since the DXB intensity obtained by the RASS method is about \begin{document}\end{document} keV \begin{document}\end{document} lower than that obtained by the Quiet method, and the linear correlation coefficient between the difference of SWCX and DXB obtained by the two methods in different energy band is close to −1, the differences in magnetospheric SWCX can be fully attributed to the differences in the fitted DXB. The difference between the two methods is most significant when the energy is less than 0.7 keV, which is also the main energy band of SWCX emission. In addition, the difference between the two methods is not related to the SWCX intensity and, to some extent, to solar wind conditions, because SWCX intensity typically varies with the solar wind. In summary, both methods are robust and reliable, and should be considered based on the best available options
Charge exchange emission and cold clumps in multi-phase galactic outflows
Large-scale outflows from starburst galaxies are multi-phase, multi-component
fluids. Charge-exchange lines which originate from the interfacing surface
between the neutral and ionised components are a useful diagnostic of the cold
dense structures in the galactic outflow. From the charge-exchange lines
observed in the nearby starburst galaxy M82, we conduct surface-to-volume
analyses and deduce that the cold dense clumps in its galactic outflow have
flattened shapes, resembling a hamburger or a pancake morphology rather than
elongated shapes. The observed filamentary H features are therefore not
prime charge-exchange line emitters. They are stripped material torn from the
slow moving dense clumps by the faster moving ionised fluid which are
subsequently warmed and stretched into elongated shapes. Our findings are
consistent with numerical simulations which have shown that cold dense clumps
in galactic outflows can be compressed by ram pressure, and also progressively
ablated and stripped before complete disintegration. We have shown that some
clumps could survive their passage along a galactic outflow. These are advected
into the circumgalactic environment, where their remnants would seed
condensation of the circumgalactic medium to form new clumps. The infall of
these new clumps back into the galaxy and their subsequent re-entrainment into
the galactic outflow form a loop process of galactic material recycling.Comment: 17 pages, 6 figures; accepted for publication in MNRA