8 research outputs found
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
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
A Coordinated X-Ray and Optical Campaign of the Nearest Massive Eclipsing Binary, δ Orionis Aa. III. Analysis of Optical Photometric (MOST) and Spectroscopic (Ground-based) Variations
We report on both high-precision photometry from the Microvariability and Oscillations of Stars (MOST) space telescope and ground-based spectroscopy of the triple system δ Ori A, consisting of a binary O9.5II+early-B (Aa1 and Aa2) with P = 5.7 days, and a more distant tertiary (O9 IV P\gt 400 years). This data was collected in concert with X-ray spectroscopy from the Chandra X-ray Observatory. Thanks to continuous coverage for three weeks, the MOST light curve reveals clear eclipses between Aa1 and Aa2 for the first time in non-phased data. From the spectroscopy, we have a well-constrained radial velocity (RV) curve of Aa1. While we are unable to recover RV variations of the secondary star, we are able to constrain several fundamental parameters of this system and determine an approximate mass of the primary using apsidal motion. We also detected second order modulations at 12 separate frequencies with spacings indicative of tidally influenced oscillations. These spacings have never been seen in a massive binary, making this system one of only a handful of such binaries that show evidence for tidally induced pulsations
The Hot and Energetic Universe: A White Paper presenting the science theme motivating the Athena+ mission
This White Paper, submitted to the recent ESA call for science themes to define its future large missions, advocates the need for a transformational leap in our understanding of two key questions in astrophysics: 1) How does ordinary matter assemble into the large scale structures that we see today? 2) How do black holes grow and shape the Universe? Hot gas in clusters, groups and the intergalactic medium dominates the baryonic content of the local Universe. To understand the astrophysical processes responsible for the formation and assembly of these large structures, it is necessary to measure their physical properties and evolution. This requires spatially resolved X-ray spectroscopy with a factor 10 increase in both telescope throughput and spatial resolving power compared to currently planned facilities. Feedback from supermassive black holes is an essential ingredient in this process and in most galaxy evolution models, but it is not well understood. X-ray observations can uniquely reveal the mechanisms launching winds close to black holes and determine the coupling of the energy and matter flows on larger scales. Due to the effects of feedback, a complete understanding of galaxy evolution requires knowledge of the obscured growth of supermassive black holes through cosmic time, out to the redshifts where the first galaxies form. X-ray emission is the most reliable way to reveal accreting black holes, but deep survey speed must improve by a factor ~100 over current facilities to perform a full census into the early Universe. The Advanced Telescope for High Energy Astrophysics (Athena+) mission provides the necessary performance (e.g. angular resolution, spectral resolution, survey grasp) to address these questions and revolutionize our understanding of the Hot and Energetic Universe. These capabilities will also provide a powerful observatory to be used in all areas of astrophysics