32 research outputs found
The early B-type star Rho Oph A is an X-ray lighthouse
We present the results of a 140 ks XMM-Newton observation of the B2 star
Ophiuchi A. The star has exhibited strong X-ray variability: a
cusp-shaped increase of rate, similar to that which we partially observed in
2013, and a bright flare. These events are separated in time by about 104 ks,
which likely corresponds to the rotational period of the star (1.2 days). Time
resolved spectroscopy of the X-ray spectra shows that the first event is caused
by an increase of the plasma emission measure, while the second increase of
rate is a major flare with temperatures in excess of 60 MK ( keV).
From the analysis of its rise, we infer a magnetic field of G and a
size of the flaring region of cm, which corresponds
to of the stellar radius. We speculate that either an intrinsic
magnetism that produces a hot spot on its surface or an unknown low mass
companion are the source of such X-rays and variability. A hot spot of magnetic
origin should be a stable structure over a time span of 2.5 years, and
suggests an overall large scale dipolar magnetic field that produces an
extended feature on the stellar surface. In the second scenario, a low mass
unknown companion is the emitter of X-rays and it should orbit extremely close
to the surface of the primary in a locked spin-orbit configuration, almost on
the verge of collapsing onto the primary. As such, the X-ray activity of the
secondary star would be enhanced by its young age, and the tight orbit as in RS
Cvn systems and Ophiuchi would constitute an extreme system that is
worthy of further investigation.Comment: 10 pages, 7 figures, 2 tables, A&A accepted, this is the version
after the language editor correction
A Deep Chandra Observation of the Giant HII Region N11 I. X-ray Sources in the Field
A very sensitive X-ray investigation of the giant HII region N11 in the LMC
was performed using the Chandra X-ray Observatory. The 300ks observation
reveals X-ray sources with luminosities down to 10^32 erg/s, increasing by more
than a factor of 5 the number of known point sources in the field. Amongst
these detections are 13 massive stars (3 compact groups of massive stars, 9
O-stars and one early B-star) with log(Lx/Lbol)~-6.5 to -7, which may suggest
that they are highly magnetic or colliding wind systems. On the other hand, the
stacked signal for regions corresponding to undetected O-stars yields
log(Lx/Lbol)~-7.3, i.e., an emission level comparable to similar Galactic stars
despite the lower metallicity. Other point sources coincide with 11 foreground
stars, 6 late-B/A stars in N11, and many background objects. This observation
also uncovers the extent and detailed spatial properties of the soft, diffuse
emission regions but the presence of some hotter plasma in their spectra
suggests contamination by the unresolved stellar population.Comment: file including online material, accepted for publication by ApJ
Coordinated UV and X-ray spectroscopic observations of the O-type giant xi Per: the connection between X-rays and large-scale wind structure
We present new, contemporaneous HST STIS and XMM observations of the O7
III(n)((f)) star xi Per. We supplement the new data with archival IUE spectra,
to analyze the variability of the wind lines and X-ray flux of xi Pper. The
variable wind of this star is known to have a 2.086 day periodicity. We use a
simple, heuristic spot model which fits the low velocity (near surface) IUE
wind line variability very well, to demonstrate that the low velocity
absorption in the new STIS spectra of N IV 1718 and Si IV 1402 vary with the
same 2.086 day period. It is remarkable that the period and amplitude of the
STIS data agree with those of the IUE spectra obtained 22 years earlier. We
also show that the time variability of the new XMM fluxes are also consistent
with the 2.086 day period. Thus, our new, multi-wavelength coordinated
observations demonstrate that the mechanism which causes the UV wind line
variability is also responsible for a significant fraction of the X-rays in
single O stars. The sequence of events for the multi-wavelength light curve
minima is: Si IV 1402, N IV 1718, and X-ray flux, each separated by a phase of
about 0.06 relative to the 2.086 day period. Analysis of the X-ray fluxes shows
that they become softer as they weaken. This is contrary to expectations if the
variability is caused by periodic excess absorption. Further, the high
resolution X-ray spectra suggest that the individual emission lines at maximum
are more strongly blue shifted.
If we interpret the low velocity wind line light curves in terms of our
model, it implies that there are two bright regions, i.e., regions with less
absorption, separated by 180 deg, on the surface of the star. We note that the
presence and persistent of two spots separated by 180 deg suggests that a weak
dipole magnetic field is responsible for the variability of the UV wind line
absorption and X-ray flux in xi Per.Comment: 23 pages, 14 figure
Testing massive star evolution, star-formation history, and feedback at low metallicity: Photometric analysis of OB stars in the SMC Wing
The supergiant ionized shell SMC-SGS 1 (DEM 167), located in the outer Wing
of the Small Magellanic Cloud (SMC), resembles structures that originate from
an energetic star-formation event and later stimulate star formation as they
expand into the ambient medium. However, stellar populations within and
surrounding SMC-SGS 1 tell a different story. We present a photometric study of
the stellar population encompassed by SMC-SGS 1 in order to trace the history
of this structure and its potential influence on star formation within the
low-density, low-metallicity SMC Wing. For a stellar population that is
physically associated with SMC-SGS 1, we combined near-ultraviolet (NUV)
photometry from the Galaxy Evolution Explorer (GALEX) with archival optical
(V-band) photometry from the ESO Danish 1.54m Telescope. Given their colors and
luminosities, we estimated stellar ages and masses by matching observed
photometry to theoretical stellar isochrone models. We find that the
investigated region supports an active, extended star-formation event spanning
25 - 40 Myr ago, as well as continued star formation into the present.
Using a standard initial mass function (IMF), we infer a lower bound on the
stellar mass from this period of , corresponding
to a star-formation intensity of 6 10 M
kpc yr. The spatial and temporal distributions of young stars
encompassed by SMC-SGS 1 imply a slow, consistent progression of star formation
over millions of years. Ongoing star formation along the edge of and interior
to SMC-SGS 1 suggests a combined stimulated and stochastic mode of star
formation within the SMC Wing. A slow expansion of the shell within this
low-density environment may preserve molecular clouds within the volume of the
shell, leaving them to form stars even after nearby stellar feedback expels
local gas and dust.Comment: 9 pages, 6 figures, 3 table
The Search for Low-mass Companions of B Stars in the Carina Nebula Cluster Trumpler 16
We have developed lists of likely B3--A0 stars (called "late B" stars) in the
young cluster Trumpler 16. The following criteria were used: location within 3'
of Eta Car, an appropriate V and B-V combination, and proper motion (where
available). Color and magnitude cuts have been made assuming an E(B-V) =0.55
mag +/- 0.1, which is a good approximation close to the center of Trumpler 16.
These lists have been cross-correlated with X-ray sources found in the Chandra
Carina Complex Project (CCCP). Previous studies have shown that only very
rarely (if at all) do late main sequence B stars produce X-rays. We present
evidence that the X-ray detected sources are binaries with low-mass companions,
since stars less massive than 1.4 Msun are strong X-ray sources at the age of
the cluster. Both the median X-ray energies and X-ray luminosities of these
sources are in good agreement with values for typical low-mass coronal X-ray
sources. We find that 39% of the late B stars based on a list with proper
motions have low-mass companions. Similarly, 32% of a sample without proper
motions have low-mass companions. We discuss the X-ray detection completeness.
These results on low-mass companions of intermediate mass stars are
complementary to spectroscopic and interferometric results, and probe new
parameter space of low mass companions at all separations. They do not support
a steeply rising distribution of mass ratios to low masses for
intermediate-mass (5 Msun) primaries, such as would be found by random pairing
from the Initial Mass Function.Comment: Accepted for the ApJS Special Issue on the Chandra Carina Complex
Project (CCCP), scheduled for publication in May 2011. All 16 CCCP Special
Issue papers are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at
leas
Phase-dependent study of near-infrared disk emission lines in LB-1
The mass, origin and evolutionary stage of the binary system LB-1 has been
the subject of intense debate, following the claim that it hosts an
70 black hole, in stark contrast with the expectations for
stellar remnants in the Milky Way. We conducted a high-resolution,
phase-resolved spectroscopic study of the near-infrared Paschen lines in this
system, using the 3.5-m telescope at Calar Alto Observatory. We find that
Pa and Pa (after proper subtraction of the stellar absorption
component) are well fitted with a standard double-peaked model, typical of disk
emission. We measured the velocity shifts of the red and blue peaks at 28
orbital phases: the line center has an orbital motion in perfect antiphase with
the stellar motion, and the radial velocity amplitude ranges from 8 to 13 km/s
for different choices of lines and profile modelling. We interpret this curve
as proof that the disk is tracing the orbital motion of the primary, ruling out
the circumbinary disk and the hierarchical triple scenarios. The phase-averaged
peak-to-peak half-separation (proxy for the projected rotational velocity of
the outer disk) is 70 km s, larger than the stellar orbital
velocity and also inconsistent with a circumbinary disk. From those results, we
infer a primary mass 4--8 times higher than the secondary mass. Moreover, we
show that the ratio of the blue and red peaks (V/R intensity ratio) has a
sinusoidal behaviour in phase with the secondary star, which can be interpreted
as the effect of external irradiation by the secondary star on the outer disk.
Finally, we briefly discuss our findings in the context of alternative
scenarios recently proposed for LB-1. Definitive tests between alternative
solutions will require further astrometric data from .Comment: To be submitted to ApJ. Comments are welcom
Carina OB Stars: X-ray Signatures of Wind Shocks and Magnetic Fields
The Chandra Carina Complex contains 200 known O- and B type stars. The
Chandra survey detected 68 of the 70 O stars and 61 of 127 known B0-B3 stars.
We have assembled a publicly available optical/X-ray database to identify OB
stars that depart from the canonical Lx/Lbol relation, or whose average X-ray
temperatures exceed 1 keV. Among the single O stars with high kT we identify
two candidate magnetically confined wind shock sources: Tr16-22, O8.5 V, and LS
1865, O8.5 V((f)). The O4 III(fc) star HD 93250 exhibits strong, hard, variable
X-rays, suggesting it may be a massive binary with a period of >30 days. The
visual O2 If* binary HD 93129A shows soft 0.6 keV and hard 1.9 keV emission
components, suggesting embedded wind shocks close to the O2 If* Aa primary, and
colliding wind shocks between Aa and Ab. Of the 11 known O-type spectroscopic
binaries, the long orbital-period systems HD 93343, HD 93403 and QZ Car have
higher shock temperatures than short-period systems such as HD 93205 and FO 15.
Although the X-rays from most B stars may be produced in the coronae of unseen,
low-mass pre-main-sequence companions, a dozen B stars with high Lx cannot be
explained by a distribution of unseen companions. One of these, SS73 24 in the
Treasure Chest cluster, is a new candidate Herbig Be star.Comment: To be published in a special issue of the Astrophysical Journal
Supplement on the Chandra Carina Complex Projec