907 research outputs found
Excess gamma-rays in the direction of the rho Ophiuchi cloud: An exotic object?
The COS-B X-ray data in the direction of the rho Oph dark cloud show an extended structure; at the same time, the region of highest intensity has a spatial distribution compatible with a localized source; 2CG353+16 which is designated Oph gamma. The possibility of an excess gamma ray flux over what is expected on the basis of the interaction of average density cosmic rays with an estimated cloud mass of 2 to 4 000 M is still open, pending an extended CO survey matching the gamma ray data. Estimates for this excess factor are in the range 2 to 4. While the cloud mass may be underestimated, it should be noted that an excess of the same order appears to be present in the nearby Oph-Sag area, well surveyed in CO with the Columbia dish. Possible reasons for a gamma ray excess, in view of two recent observational developments: an Einstein X-ray survey and a VLA radio survey, both covering the approx 2 deg diameter Oph gamma error box. Current interpretations link the gamma ray excess to the cloud gas, in which some active agent is present: stellar winds, or interaction with the North Polar Spur
Parsec-scale X-ray Flows in High-mass Star-forming Regions
The Chandra X-ray Observatory is providing remarkable new views of massive
star-forming regions, revealing all stages in the life cycle of high-mass stars
and their effects on their surroundings. We present a Chandra tour of several
high-mass star-forming regions, highlighting physical processes that
characterize the life of a cluster of high-mass stars, from deeply-embedded
cores too young to have established an HII region to superbubbles so large that
they shape our views of galaxies. Along the way we see that X-ray observations
reveal hundreds of stellar sources powering great HII region complexes,
suffused by both hard and soft diffuse X-ray structures caused by fast O-star
winds thermalized in wind-wind collisions or by termination shocks against the
surrounding media. Finally, we examine the effects of the deaths of high-mass
stars that remained close to their birthplaces, exploding as supernovae within
the superbubbles that these clusters created. We present new X-ray results on
W51 IRS2E and 30 Doradus and we introduce new data on Trumpler 14 in Carina and
the W3 HII region complexes W3 Main and W3(OH).Comment: 6 pages, 3 figures, to appear in the proceedings of IAU Symposium
227,"Massive Star Birth - A Crossroads of Astrophysics," eds. R. Cesaroni, E.
Churchwell, M. Felli, and C.M. Walmsle
Two component model for X-ray emission of radio selected QSO's
Using a large database of radio, optical, and x ray luminosities of AGNs with survival analysis, it was found that the x ray emission of the radio selected quasars has two components. One is related to the optical luminosity and the other is related to the radio luminosity
Gaia Stellar Kinematics in the Head of the Orion A Cloud: Runaway Stellar Groups and Gravitational Infall
This work extends previous kinematic studies of young stars in the Head of
the Orion A cloud (OMC-1/2/3/4/5). It is based on large samples of infrared,
optical, and X-ray selected pre-main sequence stars with reliable radial
velocities and Gaia-derived parallaxes and proper motions. Stellar kinematic
groups are identified assuming they mimic the motion of their parental gas.
Several groups are found to have peculiar kinematics: the NGC 1977 cluster and
two stellar groups in the Extended Orion Nebula (EON) cavity are caught in the
act of departing their birthplaces. The abnormal motion of NGC 1977 may have
been caused by a global hierarchical cloud collapse, feedback by massive Ori
OB1ab stars, supersonic turbulence, cloud-cloud collision, and/or slingshot
effect; the former two models are favored by us. EON groups might have
inherited anomalous motions of their parental cloudlets due to small-scale
`rocket effects' from nearby OB stars. We also identify sparse stellar groups
to the east and west of Orion A that are drifting from the central region,
possibly a slowly expanding halo of the Orion Nebula Cluster. We confirm
previously reported findings of varying line-of-sight distances to different
parts of the cloud's Head with associated differences in gas velocity.
Three-dimensional movies of star kinematics show contraction of the groups of
stars in OMC-1 and global contraction of OMC-123 stars. Overall, the Head of
Orion A region exhibits complex motions consistent with theoretical models
involving hierarchical gravitational collapse in (possibly turbulent) clouds
with OB stellar feedback.Comment: Accepted for publication in MNRAS. 26 pages, 13 figures. The two 3-D
stellar kinematic movies, aimed as Supplementary Materials, can be found on
YouTube at: https://youtu.be/B4GHCVvCYfo (`restricted' sample) and
https://youtu.be/6fUu8sP0QFI (`full' sample
Continuum radiation from active galactic nuclei: A statistical study
The physics of the continuum spectrum of active galactic nuclei (AGNs) was examined using a large data set and rigorous statistical methods. A data base was constructed for 469 objects which include radio selected quasars, optically selected quasars, X-ray selected AGNs, BL Lac objects, and optically unidentified compact radio sources. Each object has measurements of its radio, optical, X-ray core continuum luminosity, though many of them are upper limits. Since many radio sources have extended components, the core component were carefully selected out from the total radio luminosity. With survival analysis statistical methods, which can treat upper limits correctly, these data can yield better statistical results than those previously obtained. A variety of statistical tests are performed, such as the comparison of the luminosity functions in different subsamples, and linear regressions of luminosities in different bands. Interpretation of the results leads to the following tentative conclusions: the main emission mechanism of optically selected quasars and X-ray selected AGNs is thermal, while that of BL Lac objects is synchrotron; radio selected quasars may have two different emission mechanisms in the X-ray band; BL Lac objects appear to be special cases of the radio selected quasars; some compact radio sources show the possibility of synchrotron self-Compton (SSC) in the optical band; and the spectral index between the optical and the X-ray bands depends on the optical luminosity
Methods for Estimating Fluxes and Absorptions of Faint X-ray Sources
X-ray sources with very few counts can be identified with low-noise X-ray
detectors such as ACIS onboard the Chandra X-ray Observatory. These sources are
often too faint for parametric spectral modeling using well-established methods
such as fitting with XSPEC. We discuss the estimation of apparent and intrinsic
broad-band X-ray fluxes and soft X-ray absorption from gas along the line of
sight to these sources, using nonparametric methods. Apparent flux is estimated
from the ratio of the source count rate to the instrumental effective area
averaged over the chosen band. Absorption, intrinsic flux, and errors on these
quantities are estimated from comparison of source photometric quantities with
those of high S/N spectra that were simulated using spectral models
characteristic of the class of astrophysical sources under study. The concept
of this method is similar to the long-standing use of color-magnitude diagrams
in optical and infrared astronomy, with X-ray median energy replacing color
index and X-ray source counts replacing magnitude. Our nonparametric method is
tested against the apparent spectra of 2000 faint sources in the Chandra
observation of the rich young stellar cluster in the M17 HII region. We show
that the intrinsic X-ray properties can be determined with little bias and
reasonable accuracy using these observable photometric quantities without
employing often uncertain and time-consuming methods of non-linear parametric
spectral modeling. Our method is calibrated for thermal spectra characteristic
of stars in young stellar clusters, but recalibration should be possible for
some other classes of faint X-ray sources such as extragalactic AGN.Comment: Accepted for publication in The Astrophysical Journal. 39 pages, 15
figure
The Elephant Trunk Nebula and the Trumpler 37 cluster: Contribution of triggered star formation to the total population of an HII region
Rich young stellar clusters produce HII regions whose expansion into the
nearby molecular cloud is thought to trigger the formation of new stars.
However, the importance of this mode of star formation is uncertain. This
investigation seeks to quantify triggered star formation (TSF) in IC 1396A
(a.k.a., the Elephant Trunk Nebula), a bright rimmed cloud (BRC) on the
periphery of the nearby giant HII region IC 1396 produced by the Trumpler 37
cluster. X-ray selection of young stars from Chandra X-ray Observatory data is
combined with existing optical and infrared surveys to give a more complete
census of the TSF population. Over 250 young stars in and around IC 1396A are
identified; this doubles the previously known population. A spatio-temporal
gradient of stars from the IC 1396A cloud toward the primary ionizing star HD
206267 is found. We argue that the TSF mechanism in IC 1396A is the
radiation-driven implosion process persisting over several million years.
Analysis of the X-ray luminosity and initial mass functions indicates that >140
stars down to 0.1 Msun were formed by TSF. Considering other BRCs in the IC
1396 HII region, we estimate the TSF contribution for the entire HII region
exceeds 14-25% today, and may be higher over the lifetime of the HII region.
Such triggering on the periphery of HII regions may be a significant mode of
star formation in the Galaxy.Comment: Accepted for publication in MNRAS; 28 pages, 18 figure
Coronal X-ray emission from an intermediate-age brown dwarf
We report the X-ray detection of the brown dwarf (BD) companion TWA 5B in a
Myr old pre-main sequence binary system. We clearly resolve the
faint companion (35 photons) separated from the X-ray luminous primary by 2
arcsec in a {\it Chandra} ACIS image. TWA 5B shows a soft X-ray spectrum with a
low plasma temperature of only 0.3 keV and a constant flux during the 3 hour
observation, of which the characteristics are commonly seen in the solar
corona. The X-ray luminosity is 4 erg s (0.1--10 keV
band) or .
Comparing these properties to both younger and older BDs, we discuss the
evolution of the X-ray emission in BDs. During their first few Myr, they
exhibit high levels of X-ray activity as seen in higher mass pre-main sequence
stars. The level in TWA 5B is still high at Myr in while has already substantially cooled
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