849 research outputs found
A statistical analysis of X-ray variability in pre-main sequence objects of the Taurus Molecular Cloud
This work is part of a systematic X-ray survey of the Taurus star forming
complex with XMM-Newton. We study the time series of all X-ray sources
associated with Taurus members, to statistically characterize their X-ray
variability, and compare the results to those for pre-main sequence stars in
the Orion Nebula Cluster and to expectations arising from a model where all the
X-ray emission is the result of a large number of stochastically occurring
flares. We find that roughly half of the detected X-ray sources show
variability above our sensitivity limit, and in ~ 26 % of the cases this
variability is recognized as flares. Variability is more frequently detected at
hard than at soft energies. The variability statistics of cTTS and wTTS are
undistinguishable, suggesting a common (coronal) origin for their X-ray
emission. We have for the first time applied a rigorous maximum likelihood
method in the analysis of the number distribution of flare energies on pre-main
sequence stars. In its differential form this distribution follows a power-law
with index alpha = 2.4 +- 0.5, in the range typically observed on late-type
stars and the Sun. The flare energy distribution is probably steep enough to
explain the heating of stellar coronae by nano-flares (alpha > 2), albeit
associated with a rather large uncertainty that leaves some doubt on this
conclusion.Comment: accepted for publication in Astronomy & Astrophysics; to appear in a
Special Section dedicated to the XMM-Newton Extended Survey of the Taurus
Molecular Cloud (XEST
A detailed study of the rise phase of a long duration X-ray flare in the young star TWA 11B
We analyzed a long duration flare observed in a serendipitous XMM-Newton
detection of the M star CD-39 7717B (TWA 11B), member of the young stellar
association TW Hya (~ 8 Myr). Only the rise phase (with a duration of ~ 35 ks)
and possibly the flare peak were observed. We took advantage of the high
count-rate of the X-ray source to carry out a detailed analysis of its spectrum
during the whole exposure. After a careful analysis, we interpreted the rise
phase as resulting from the ignition of a first group of loops (event A) which
triggered a subsequent two-ribbon flare (event B). Event A was analyzed using a
single-loop model, while a two-ribbon model was applied for event B. Loop
semi-lengths of ~ 4 R* were obtained. Such large structures had been previously
observed in very young stellar objects (~ 1 - 4 Myr). This is the first time
that they have been inferred in a slightly more evolved star. The fluorescent
iron emission line at 6.4 keV was detected during event B. Since TWA 11B seems
to have no disk, the most plausible explanation found for its presence in the
X-ray spectrum of this star is collisional - or photo- ionization. As far as we
are concerned, this is only the third clear detection of Fe photospheric
fluorescence in stars other than the Sun.Comment: Accepted for publication in ApJ. 15 pages, 9 figure
Chemical enrichment of the complex hot ISM of the Antennae Galaxies: II. Physical properties of the hot gas and supernova feedback
We investigate the physical properties of the interstellar medium (ISM) in
the merging pair of galaxies known as The Antennae (NGC 4038/39), using the
deep coadded ~411 ks Chandra ACIS-S data set. The method of analysis and some
of the main results from the spectral analysis, such as metal abundances and
their variations from ~0.2 to ~20-30 times solar, are described in Paper I
(Baldi et al. submitted). In the present paper we investigate in detail the
physics of the hot emitting gas, deriving measures for the hot-gas mass (~10^
M_sun), cooling times (10^7-10^8 yr), and pressure (3.5x10^-11-2.8x10^-10 dyne
cm^-2). At least in one of the two nuclei (NGC 4038) the hot-gas pressure is
significantly higher than the CO pressure, implying that shock waves may be
driven into the CO clouds. Comparison of the metal abundances with the average
stellar yields predicted by theoretical models of SN explosions points to SNe
of Type II as the main contributors of metals to the hot ISM. There is no
evidence of any correlation between radio-optical star-formation indicators and
the measured metal abundances. Although due to uncertainties in the average gas
density we cannot exclude that mixing may have played an important role, the
short time required to produce the observed metal masses (<=2 Myr) suggests
that the correlations are unlikely to have been destroyed by efficient mixing.
More likely, a significant fraction of SN II ejecta may be in a cool phase, in
grains, or escaping in hot winds. In each case, any such fraction of the ejecta
would remain undetectable with soft X-ray observations.Comment: 29 pages, 6 figures, accepted by the Astrophysical Journa
How black holes get their kicks: Gravitational radiation recoil revisited
Gravitational waves from the coalescence of binary black holes carry away
linear momentum, causing center of mass recoil. This "radiation rocket" effect
has important implications for systems with escape speeds of order the recoil
velocity. We revisit this problem using black hole perturbation theory,
treating the binary as a test mass spiraling into a spinning hole. For extreme
mass ratios (q = m1/m2 << 1) we compute the recoil for the slow inspiral epoch
of binary coalescence very accurately; these results can be extrapolated to q ~
0.4 with modest accuracy. Although the recoil from the final plunge contributes
significantly to the final recoil, we are only able to make crude estimates of
its magnitude. We find that the recoil can easily reach ~ 100-200 km/s, but
most likely does not exceed ~ 500 km/s. Though much lower than previous
estimates, this recoil is large enough to have important astrophysical
consequences. These include the ejection of black holes from globular clusters,
dwarf galaxies, and high-redshift dark matter halos.Comment: 4 pages, 2 figures, emulateapj style; minor changes made; accepted to
ApJ Letter
Results from DROXO. III. Observation, source list and X-ray properties of sources detected in the "Deep Rho Ophiuchi XMM-Newton Observation"
X-rays from very young stars are powerful probes to investigate the
mechanisms at work in the very first stages of the star formation and the
origin of X-ray emission in very young stars. We present results from a 500 ks
long observation of the Rho Ophiuchi cloud with a XMM-Newton large program
named DROXO, aiming at studying the X-ray emission of deeply embedded Young
Stellar Objects (YSOs). The data acquired during the DROXO program were reduced
with SAS software, and filtered in time and energy to improve the signal to
noise of detected sources; light curves and spectra were obtained. We detected
111 sources, 61 of them associated with rho Ophiuchi YSOs as identified from
infrared observations with ISOCAM. Specifically, we detected 9 out of 11 Class
I, 31 out of 48 Class II and 15 out 16 Class III objects. Six objects out of 21
classified Class III candidates are also detected. At the same time we suggest
that 15 Class III candidates that remain undetected at log Lx < 28.3 are not
related to the cloud population. The global detection rate is ~64%. We have
achieved a flux sensitivity of ~5 x 10^{-15} erg s^{-1} cm^{-2}. The Lx to
L_bol ratio shows saturation at a value of ~ -3.5 for stars with T_eff <= 5000
K or 0.7 M_sun as observed in the Orion Nebula. The plasma temperatures and the
spectrum absorption show a decline with YSO class, with Class I YSOs being
hotter and more absorbed than Class II and III YSOs. In one star (GY 266) with
infrared counterpart in 2MASS and Spitzer catalogs we have detected a soft
excess in the X-ray spectrum which is best fitted by a cold thermal component
less absorbed than the main thermal component of the plasma. Such a soft
component hints to the presence of plasma heated by shocks due to jets outside
the dense circumstellar material.Comment: Accepted for publication on Astronomy & Astrophysics journa
An Infrared Coronagraphic Survey for Substellar Companions
We have used the F160W filter (1.4-1.8 um) and the coronagraph on the
Near-InfraRed Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space
Telescope (HST) to survey 45 single stars with a median age of 0.15 Gyr, an
average distance of 30 pc, and an average H-magnitude of 7 mag. For the median
age we were capable of detecting a 30 M_Jup companion at separations between 15
and 200 AU. A 5 M_Jup object could have been detected at 30 AU around 36% of
our primaries. For several of our targets that were less than 30 Myr old, the
lower mass limit was as low as a Jupiter mass, well into the high mass planet
region. Results of the entire survey include the proper motion verification of
five low-mass stellar companions, two brown dwarfs (HR7329B and TWA5B) and one
possible brown dwarf binary (Gl 577B/C).Comment: 11 figures, accepted by A
On the MBM12 Young Association
I present a comprehensive study of the MBM12 young association (MBM12A). By
combining infrared (IR) photometry from the Two-Micron All-Sky Survey (2MASS)
survey with new optical imaging and spectroscopy, I have performed a census of
the MBM12A membership that is complete to 0.03 Msun (H~15) for a 1.75deg X
1.4deg field encompassing the MBM12 cloud. I find five new members with masses
of 0.1-0.4 Msun and a few additional candidates that have not been observed
spectroscopically. From an analysis of optical and IR photometry for stars in
the direction of MBM12, I identify M dwarfs in the foreground and background of
the cloud. By comparing the magnitudes of these stars to those of local field
dwarfs, I arrive at a distance modulus 7.2+/-0.5 (275 pc) to the MBM12 cloud;
it is not the nearest molecular cloud and is not inside the local bubble of hot
ionized gas as had been implied by previous distance estimates of 50-100 pc. I
have also used Li strengths and H-R diagrams to constrain the absolute and
relative ages of MBM12A and other young populations; these data indicate ages
of 2 +3/-1 Myr for MBM12A and 10 Myr for the TW Hya and Eta Cha associations.
MBM12A may be a slightly evolved version of the aggregates of young stars
within the Taurus dark clouds (~1 Myr) near the age of the IC 348 cluster (~2
Myr).Comment: to be published in The Astrophysical Journal, 41 pages, 14 figures,
also found at http://cfa-www.harvard.edu/sfgroup/preprints.htm
Accelerated Electrons in Cassiopeia A: An Explanation for the Hard X-ray Tail
We propose a model for the hard X-ray (> 10 keV) emission observed from the
supernova remnant Cas A. Lower hybrid waves are generated in strong (mG)
magnetic fields, generally believed to reside in this remnant, by shocks
reflected from density inhomogeneities. These then accelerate electrons to
energies of several tens of keV. Around 4% of the x-ray emitting plasma
electrons need to be in this accelerated distribution, which extends up to
electron velocities of order the electron Alfven speed, and is directled along
magnetic field lines. Bremsstrahlung from these electrons produces the observed
hard x-ray emission. Such waves and accelerated electrons have been observed in
situ at Comet Halley, and we discuss the viability of the extrapolation from
this case to the parameters relevant to Cas A.Comment: 20 pages, 3 figures, aasTeX502, accepted in Ap
Spectral properties of X-ray bright variable sources in the Taurus Molecular Cloud
We analyze 19 bright variable X-ray sources detected in the XMM-Newton
Extended Survey of the Taurus Molecular Cloud (XEST), in order to characterize
the variations with time of their coronal properties and to derive informations
on the X-ray emitting structures. We performed time-resolved spectroscopy of
the EPIC PN and MOS spectra of the XEST sources, using a model with one or two
thermal components, and we used the time evolution of the temperatures and
emission measures during the decay phase of flares to derive the size of the
flaring loops. The light curves of the selected sources show different types of
variability: flares, long-lasting decay or rise through the whole observation,
slow modulation or complex flare-like variability. Spectral analysis shows
typical quiescent plasma temperatures of 5-10 MK and 15-35 MK; the cool
component generally remains constant, while the observed flux changes are due
to variations of the hot component. During flares the plasma reaches
temperatures up to 100 MK and luminosities up to erg s.
Loop sizes inferred from flare analysis are generally smaller than or
comparable to the stellar radius.Comment: 18 pages, 11 figures, 3 tables. Accepted by A&A, to appear in a
special section/issue dedicated to the XMM-Newton Extended Survey of the
Taurus Molecular Cloud (XEST
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