1,168 research outputs found
A Simultaneous Optical and X-ray Variability Study of the Orion Nebula Cluster. II. A Common Origin in Magnetic Activity
We present a statistical analysis of simultaneous optical and X-ray light
curves, spanning 600 ks, for 814 pre-main-sequence (PMS) stars in the Orion
Nebula Cluster. The aim of this study is to establish the relationship, if any,
between the sites of optical and X-ray variability, and thereby to elucidate
the origins of X-ray production in PMS stars. In a previous paper we showed
that optical and X-ray variability in PMS stars are very rarely
time-correlated. Here, using time-averaged variability indicators to examine
the joint occurrences of optical and X-ray variability, we confirm that the two
forms of variability are not directly causally related. However, a strong and
highly statistically significant correlation is found between optical
variability and X-ray luminosity. As this correlation is found to be
independent of accretion activity, we argue that X-ray production in PMS stars
must instead be intimately connected with the presence and strength of
optically variable, magnetically active surface regions (i.e. spots) on these
stars. Moreover, because X-ray variability and optical variability are rarely
time-correlated, we conclude that the sites of X-ray production are not
exclusively co-spatial with these regions. We argue that solar-analog coronae,
heated by topologically complex fields, can explain these findings.Comment: To appear in the Astrophysical Journal. 33 pages, 3 figure
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
A deep XMM-Newton X-ray observation of the Chamaeleon I dark cloud
Methods. The northern-eastern fringe of the Chameleon I dark cloud was
observed with XMM-Newton, revisiting a region observed with ROSAT 15 years ago.
Centered on the extended X-ray source CHXR49 we are able to resolve it into
three major contributing components and to analyse their spectral properties.
Furthermore, the deep exposure allows not only the detection of numerous,
previously unknown X-ray sources, but also the investigation of variability and
the study of the X-ray properties for the brighter targets in the field. We use
EPIC spectra, to determine X-ray brightness, coronal temperatures and emission
measures for these sources, compare the properties of classical and weak-line T
Tauri stars and make a comparison with results from the ROSAT observation.
Results. X-ray properties of T Tauri stars in Cha I are presented. The
XMM-Newton images resolve some previously blended X-ray sources, confirm
several possible ones and detect many new X-ray targets, resulting in the most
comprehensive list with 71 X-ray sources in the northern Cha I dark cloud. The
analysis of medium resolution spectra shows an overlapping distribution of
spectral properties for classical and weak-line T Tauri stars, with the X-ray
brighter stars having hotter coronae and a higher L_X/L_bol ratio. X-ray
luminosity correlates with bolometric luminosity, whereas the L_X/L_bol ratio
is slightly lower for the classical T Tauri stars. Large flares as well as a
low iron and a high neon abundance are found in both types of T Tauri stars.
Abundance pattern, plasma temperatures and emission measure distributions
during quiescent phases are attributed toa high level of magnetic activity as
the dominant source of their X-ray emission.Comment: 10 pages, 11 figures, accepted by A&
Simultaneous X-ray, radio, near-infrared, and optical monitoring of Young Stellar Objects in the Coronet cluster
Multi-wavelength (X-ray to radio) monitoring of Young Stellar Objects (YSOs)
can provide important information about physical processes at the stellar
surface, in the stellar corona, and/or in the inner circumstellar disk regions.
While coronal processes should mainly cause variations in the X-ray and radio
bands, accretion processes may be traced by time-correlated variability in the
X-ray and optical/infrared bands. Several multi-wavelength studies have been
successfully performed for field stars and approx. 1-10 Myr old T Tauri stars,
but so far no such study succeeded in detecting simultaneous X-ray to radio
variability in extremely young objects like class I and class 0 protostars.
Here we present the first simultaneous X-ray, radio, near-infrared, and optical
monitoring of YSOs, targeting the Coronet cluster in the Corona Australis
star-forming region, which harbors at least one class 0 protostar, several
class I objects, numerous T Tauri stars, and a few Herbig AeBe stars. [...]
Seven objects are detected simultaneously in the X-ray, radio, and
optical/infrared bands; they constitute our core sample. While most of these
sources exhibit clear variability in the X-ray regime and several also display
optical/infrared variability, none of them shows significant radio variability
on the timescales probed. We also do not find any case of clearly
time-correlated optical/infrared and X-ray variability. [...] The absence of
time-correlated multi-wavelength variability suggests that there is no direct
link between the X-ray and optical/infrared emission and supports the notion
that accretion is not an important source for the X-ray emission of these YSOs.
No significant radio variability was found on timescales of days.Comment: 11 pages, 11 figures, accepted for publication in A&A (06 Dec 2006
Chandra/ACIS-I study of the X-ray properties of the NGC 6611 and M16 stellar population
Mechanisms regulating the origin of X-rays in YSOs and the correlation with
their evolutionary stage are under debate. Studies of the X-ray properties in
young clusters allow to understand these mechanisms. One ideal target for this
analysis is the Eagle Nebula (M16), with its central cluster NGC6611. At 1750
pc from the Sun, it harbors 93 OB stars, together with a population of low-mass
stars from embedded protostars to disk-less Class III objects, with age <=
3Myrs. We study an archival 78 ksec Chandra/ACIS-I observation of NGC6611, and
two new 80ksec observations of the outer region of M16, one centered on the
Column V, and one on a region of the molecular cloud with ongoing
star-formation. We detect 1755 point sources, with 1183 candidate cluster
members (219 disk-bearing and 964 disk-less). We study the global X-ray
properties of M16 and compare them with those of the Orion Nebula Cluster. We
also compare the level of X-ray emission of Class II and Class III stars, and
analyze the X-ray spectral properties of OB stars. Our study supports the lower
level of X-ray activity for the disk-bearing stars with respect to the
disk-less members. The X-ray Luminosity Function (XLF) of M16 is similar to
that of Orion, supporting the universality of the XLF in young clusters. 85% of
the O stars of NGC6611 have been detected in X-rays. With only one possible
exception, they show soft spectra with no hard component, indicating that
mechanisms for the production of hard X-ray emission in O stars are not
operating in NGC 6611.Comment: Accepted in Ap
A 1.3 cm wavelength radio flare from a deeply embedded source in the Orion BN/KL region
Aims: Our aim was to measure and characterize the short-wavelength radio
emission from young stellar objects (YSOs) in the Orion Nebula Cluster and the
BN/KL star-forming region. Methods: We used the NRAO Very Large Array at a
wavelength of 1.3 cm and we studied archival X-ray, infrared, and radio data.
Results: During our observation, a strong outburst (flux increasing >10 fold)
occurred in one of the 16 sources detected at a wavelength of 1.3cm, while the
others remained (nearly) constant. This source does not have an infrared
counterpart, but has subsequently been observed to flare in X-rays. Curiously,
a very weak variable double radio source was found at other epochs near this
position, one of whose components is coincident with it. A very high extinction
derived from modeling the X-ray emission and the absence of an infrared
counterpart both suggest that this source is very deeply embedded.Comment: 7 pages, 5 figures, accepted for publication in A&
Probing grain boundaries in ceramic scintillators using x-ray radioluminescence microscopy
X-ray radioluminescence microscopy (XRLM), a novel fluorescence microscopy technique under focused x-ray excitation, was used to characterize micro-scale luminescence of Eu:Y2O3 and Ce:YAG transparent ceramics and bicrystals. The diffusion length of a known semiconductor measured by XRLM was found to be in agreement with previously measured values, illustrating its use for characterizing charge carrier transport. Emission intensity was found to drop at the boundaries in both Eu:Y2O3 and Ce:YAG ceramics and bicrystals. The depletion in emission at grain boundaries was ultimately found to be related to charge carrier depletion (through either deep trapping or non-radiative recombination). A charge carrier diffusion model was used to understand the effect of grain boundaries on charge carrier transport in these scintillators. The diffusion model was found to accurately predict the spatial distribution of emission in a Ce:YAG single-crystal as a function of x-ray excitation energy. Structural and chemical characterization of grain boundaries in an Eu:Y2O3 ceramic using transmission electron microscopy and secondary ion mass spectrometry mapping showed an ordered boundary region and no detectable segregation of impurities or Eu, justifying the use of an abrupt boundary condition to determine boundary recombination velocities in these materials. The boundary recombination velocities were then used to show that, for ceramics with grain sizes \u3e similar to 20 mu m, there would be a minimal effect from the detected charge carrier depletion at grain boundaries on their bulk x-ray radioluminescence intensity. Ultimately, this study illustrates how this new XRLM technique can be used to measure charge carrier diffusion properties and how it may be coupled with microstructural and micro-scale chemical analyses to fully investigate the effect of grain boundaries on scintillator properties
Radio and X-ray variability of Young Stellar Objects in the Coronet Cluster
The Coronet Cluster in the nearby R CrA dark cloud offers the rare
opportunity to study at least four "class I" protostellar sources as well as
one candidate "class 0" source, a Herbig Ae star, and a candidate brown dwarf
within a few square arcminutes - most of them detected at radio- and X-ray
wavelengths. These sources were observed with the Very Large Array (VLA) at
3.5cm on nine occasions in 1998, spread over nearly four months. The source IRS
5, earlier shown to emit circularly polarized radio emission, was observed to
undergo a flux increase accompanied by changes in its polarization properties.
Comparison with VLA measurements taken in January 1997 allows for some analysis
of longer-term variability. In addition to this radio monitoring, we analyze
archival Chandra and XMM-Newton X-ray data of these sources. Three class I
protostars are bright enough for X-ray spectroscopy, and we perform a
variability analysis for these sources, covering a total of 154 ksec spread
over more than two and a half years. Also in X-rays, IRS 5 shows the most
pronounced variability, whilst the other two class I protostars IRS 1 and IRS 2
have more stable emission. X-ray data is also analyzed for the recently
identified candidate class 0 source IRS 7E, which shows strong variability as
well as for the Herbig Ae star R CrA for which we find extremely hot
X-ray-emitting plasma. For IRS 1,2 and 5, the hydrogen column densities derived
from the X-ray spectra are at about half the values derived with near-infrared
techniques, a situation similar to what has been observed towards some other
young stellar objects.Comment: 17 pages, 11 figures, accepted for publication in A&
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