69 research outputs found
An X-ray and Infrared Survey of the Lynds 1228 Cloud Core
The nearby Lynds 1228 (L1228) dark cloud at a distance of ~200 pc is known to
harbor several young stars including the driving sources of the giant HH 199
and HH 200 Herbig-Haro outflows. L1228 has been previously studied at optical,
infrared, and radio wavelengths but not in X-rays. We present results of a
sensitive 37 ks Chandra ACIS-I X-ray observation of the L1228 core region.
Chandra detected 60 X-ray sources, most of which are faint (<40 counts) and
non-variable. Infrared counterparts were identified for 53 of the 60 X-ray
sources using archival data from 2MASS, Spitzer, and WISE. Object classes were
assigned using mid-IR colors for those objects with complete photometry, most
of which were found to have colors consistent with extragalactic background
sources. Seven young stellar object (YSO) candidates were identified including
the class I protostar HH 200-IRS which was detected as a faint hard X-ray
source. No X-ray emission was detected from the luminous protostar HH 199-IRS.
We summarize the X-ray and infrared properties of the detected sources and
provide IR spectral energy distribution modeling of high-interest objects
including the protostars driving the HH outflows.Comment: 38 pages, 7 tables, 8 figures; to appear in A
X-ray astronomy of stellar coronae (Review)
X-ray emission from stars in the cool half of the Hertzsprung-Russell diagram
is generally attributed to the presence of a magnetic corona that contains
plasma at temperatures exceeding 1 million K. Coronae are ubiquitous among
these stars, yet many fundamental mechanisms operating in their magnetic fields
still elude an interpretation through a detailed physical description. Stellar
X-ray astronomy is therefore contributing toward a deeper understanding of the
generation of magnetic fields in magnetohydrodynamic dynamos, the release of
energy in tenuous astrophysical plasmas through various plasma-physical
processes, and the interactions of high-energy radiation with the stellar
environment. Stellar X-ray emission also provides important diagnostics to
study the structure and evolution of stellar magnetic fields from the first
days of a protostellar life to the latest stages of stellar evolution among
giants and supergiants. The discipline of stellar coronal X-ray astronomy has
now reached a level of sophistication that makes tests of advanced theories in
stellar physics possible. This development is based on the rapidly advancing
instrumental possibilities that today allow us to obtain images with
sub-arcsecond resolution and spectra with resolving powers exceeding 1000.
High-resolution X-ray spectroscopy has, in fact, opened new windows into
astrophysical sources, and has played a fundamental role in coronal research.
The present article reviews the development and current status of various
topics in the X-ray astronomy of stellar coronae, focusing on observational
results and on theoretical aspects relevant to our understanding of coronal
magnetic structure and evolution.Comment: 168 journal-format pages, 50 figures, partly in color. Accepted for
publication in Astronomy and Astrophysics Review. Many figures are reduced.
Download a full-resolution (gzipped PS) version here:
http://www.astro.phys.ethz.ch/papers/guedel/aar/AARev.ps.g
X-ray emission from an FU Ori star in early outburst: HBC 722
Aims: We conducted the first X-ray observations of the newly erupting FU
Ori-type outburst in HBC 722 (V2493 Cyg) with the aim to characterize its X-ray
behavior and near-stellar environment during early outburst. Methods: We used
data from the XMM-Newton and Chandra X-ray observatories to measure X-ray
source temperatures and luminosities as well as the gas column densities along
the line of sight toward the source. Results: We report a Chandra X-ray
detection of HBC 722 with an X-ray luminosity of LX ~ 4E30 erg s-1. The gas
column density exceeds values expected from optical extinction and standard
gas-to-dust ratios. We conclude that dust-free gas masses are present around
the star, such as strong winds launched from the inner disk, or massive
accretion columns. A tentative detection obtained by XMM-Newton two years
earlier after an initial optical peak revealed a fainter X-ray source with only
weak absorption.Comment: Accepted for Astronomy & Astrophysics Letters on September 17, 201
Formation of freely floating sub-stellar objects via close encounters
We numerically studied close encounters between a young stellar system
hosting a massive, gravitationally fragmenting disk and an intruder diskless
star with the purpose to determine the evolution of fragments that have formed
in the disk prior to the encounter. Numerical hydrodynamics simulations in the
non-inertial frame of reference of the host star were employed to simulate the
prograde and retrograde co-planar encounters. The initial configuration of the
target system (star plus disk) was obtained via a separate numerical simulation
featuring the gravitational collapse of a solar-mass pre-stellar core. We found
that close encounters can lead to the ejection of fragments that have formed in
the disk of the target prior to collision. In particular, prograde encounters
are more efficient in ejecting the fragments than the retrograde encounters.
The masses of ejected fragments are in the brown-dwarf mass regime. They also
carry away an appreciable amount of gas in their gravitational radius of
influence, implying that these objects may possess extended disks or envelopes,
as also suggested by Thies et al. (2015). Close encounters can also lead to the
ejection of entire spiral arms, followed by fragmentation and formation of
freely-floating objects straddling the planetary mass limit. However, numerical
simulations with a higher resolution are needed to confirm this finding.Comment: 12 pages, 7 figures, accepted for publication by Astronomy &
Astrophysic
- …