149 research outputs found
Object-X: The Brightest Mid-IR Point Source in M33
We discuss the nature of the brightest mid-IR point source (which we dub
Object X) in the nearby galaxy M33. Although multi-wavelength data on this
object have existed in the literature for some time, it has not previously been
recognized as the most luminous mid-IR object in M33 because it is entirely
unremarkable in both optical and near-IR light. In the Local Group Galaxies
Survey, Object X is a faint red source visible in VRI and H-alpha but not U or
B. It was easily seen at JHK_s in the 2MASS survey. It is the brightest point
source in all four Spitzer IRAC bands and is also visible in the MIPS 24-micron
band. Its bolometric luminosity is 5x10^5 L_sun. The source is optically
variable on short time scales (tens of days) and is also slightly variable in
the mid-IR, indicating that it is a star. Archival photographic plates (from
1949 and 1991) show no optical source, so the star has been obscured for at
least half a century. Its properties are similar to those of the Galactic OH/IR
star IRC+10420 which has a complex dusty circumstellar structure resulting from
episodic low velocity mass ejections. We propose that Object X is a M>30 M_sun
evolved star obscured in its own dust ejected during episodic mass loss events
over at least half a century. It may emerge from its current ultra-short
evolutionary phase as a hotter post-RSG star analogous to M33 Var A. The
existence and rarity of such objects can be an important probe of a very brief
yet eventful stellar evolutionary phase.Comment: 20 pages, 10 figures. Accepted for publication by Ap
An obscured AGN population hidden in the VIPERS galaxies: identification through spectral energy distribution decomposition
The detection of X-ray emission constitutes a reliable and efficient tool for
the selection of Active Galactic Nuclei (AGNs), although it may be biased
against the most heavily absorbed AGNs. Simple mid-IR broad-band selection
criteria identify a large number of luminous and absorbed AGNs, yet again host
contamination could lead to non-uniform and incomplete samples. Spectral Energy
Distribution (SED) decomposition is able to decouple the emission from the AGN
versus that from star-forming regions, revealing weaker AGN components. We aim
to identify the obscured AGN population in the VIPERS survey in the CFHTLS W1
field through SED modelling. We construct SEDs for 6,860 sources and identify
160 AGNs at a high confidence level using a Bayesian approach. Using optical
spectroscopy, we confirm the nature of ~85% of the AGNs. Our AGN sample is
highly complete (~92%) compared to mid-IR colour selected AGNs, including a
significant number of galaxy-dominated systems with lower luminosities. In
addition to the lack of X-ray emission (80%), the SED fitting results suggest
that the majority of the sources are obscured. We use a number of diagnostic
criteria in the optical, infrared and X-ray regime to verify these results.
Interestingly, only 35% of the most luminous mid-IR selected AGNs have X-ray
counterparts suggesting strong absorption. Our work emphasizes the importance
of using SED decomposition techniques to select a population of type II AGNs,
which may remain undetected by either X-ray or IR colour surveys.Comment: Accepted for publication in MNRAS in May 4, 2020. 18 figures, 3
tables
Fundamental Parameters of four Massive Eclipsing Binaries in Westerlund 1
We present fundamental parameters of 4 massive eclipsing binaries in the
young massive cluster Westerlund 1. The goal is to measure accurate masses and
radii of their component stars, which provide much needed constraints for
evolutionary models of massive stars. Accurate parameters can further be used
to determine a dynamical lower limit for the magnetar progenitor and to obtain
an independent distance to the cluster. Our results confirm and extend the
evidence for a high mass for the progenitor of the magnetar.Comment: 2 pages, to appear in the proceedings of the IAUS 282 on "From
Interacting Binaries to Exoplanets:Essential Modelling Tools" (Tatranska
Lomnica, July 18-22, 2011), Cambridge University Pres
Accurate fundamental parameters and distance to a massive early-type eclipsing binary in the Danks 2 cluster
We present a study of the properties of the O-type, massive eclipsing binary
2MASS J13130841-6239275 located in the outskirts of the Danks 2 cluster in the
G305 star-forming complex, using near-infrared spectroscopy from VLT/ISAAC. We
derive the masses and radii to be 24.50.9 M and 9.20.1
R for the primary and 21.70.8 M and 8.70.1
R for the secondary component. In addition, we evaluate the
sensitivity of our parameters to the choice of the spectral features used to
determine the radial velocities. Both components appear to be main-sequence
O6.5O7 type stars at an age of 5 Myr, which is in agreement with the
age of the cluster. A high visual extinction of A=11.90.1 mag is
reported, which is likely attributed to the cold molecular gas contaminating
the north-east region of the cluster. By fitting the spectral energy
distribution of the system to the available photometry, we
determine a distance to the system of 3.520.08 kpc with a precision of
2, which is the most well-determined distance to the Danks 2 cluster and
the host complex reported in the literature.Comment: 13 pages, 9 figures, 6 tables. Accepted for publication in A&
Monitoring luminous yellow massive stars in M33: new yellow hypergiant candidates
The evolution of massive stars surviving the red supergiant (RSG) stage
remains unexplored due to the rarity of such objects. The yellow hypergiants
(YHGs) appear to be the warm counterparts of post-RSG classes located near the
Humphreys-Davidson upper luminosity limit, which are characterized by
atmospheric instability and high mass-loss rates. We aim to increase the number
of YHGs in M33 and thus to contribute to a better understanding of the
pre-supernova evolution of massive stars. Optical spectroscopy of five
dust-enshrouded YSGs selected from mid-IR criteria was obtained with the goal
of detecting evidence of extensive atmospheres. We also analyzed BVI photometry
for 21 of the most luminous YSGs in M33 to identify changes in the spectral
type. To explore the properties of circumstellar dust, we performed SED-fitting
of multi-band photometry of the 21 YSGs. We find three luminous YSGs in our
sample to be YHG candidates, as they are surrounded by hot dust and are
enshrouded within extended, cold dusty envelopes. Our spectroscopy of star 2
shows emission of more than one H component, as well as emission of
CaII, implying an extended atmospheric structure. In addition, the long-term
monitoring of the star reveals a dimming in the visual light curve of amplitude
larger than 0.5 mag that caused an apparent drop in the temperature that
exceeded 500 K. We suggest the observed variability to be analogous to that of
the Galactic YHG Cas. Five less luminous YSGs are suggested as post-RSG
candidates showing evidence of hot or/and cool dust emission. We demonstrate
that mid-IR photometry, combined with optical spectroscopy and time-series
photometry, provide a robust method for identifying candidate YHGs. Future
discovery of YHGs in Local Group galaxies is critical for the study of the late
evolution of intermediate-mass massive stars.Comment: 24 pages, 12 figures, 7 Tables. A&A in pres
Detection of a Variable Infrared Excess Around SDSS 121209.31+013627.7
We present near-infrared photometry and light curves of the candidate
magnetic white dwarf+brown dwarf binary SDSS J121209.31+013627.7 and report on
the detection of near-infrared excess and variability in the band. The
observed near-infrared excess can be explained by the presence of an L7 brown
dwarf and an extra emission source. The and light curves appear flat,
which rules out eclipses deeper than 0.2 mag and the presence of an accretion
hot spot on the white dwarf. From the variable lightcurve, we get a
refined period for the system of 881 minutes. We show that the observed
variability in band can be explained by cyclotron emission, which can be
modeled by a small spot on the surface of the white dwarf. SDSS 1212 exhibits
similarities to the ultra-short period polar EF Eridani, however the lack of
evidence for Roche-lobe overflow accretion suggests it may be a pre-polar.Comment: 13 pages, 2 figures, accepted to ApJ
The massive star population of the Virgo Cluster galaxy NGC 4535
We analyzed the massive star population of the Virgo Cluster galaxy NGC 4535
using archival Hubble Space Telescope Wide Field Planetary Camera 2 images in
filters F555W and F814W, equivalent to Johnson V and Kron-Cousins I. We
performed high precision point spread function fitting photometry of 24353
sources including 3762 candidate blue supergiants, 841 candidate yellow
supergiants and 370 candidate red supergiants. We estimated the ratio of blue
to red supergiants as a decreasing function of galactocentric radius. Using
Modules for Experiments in Stellar Astrophysics isochrones at solar
metallicity, we defined the luminosity function and estimated the star
formation history of the galaxy over the last 60 Myrs. We conducted a
variability search in the V and I filters using three variability indexes: the
median absolute deviation, the interquartile range and the inverse von-Neumann
ratio. This analysis yielded 120 new variable candidates with absolute
magnitudes ranging from M = 4 to 11 mag. We used the MESA
evolutionary tracks at solar metallicity, to classify the variables based on
their absolute magnitude and their position on the color-magnitude diagram.
Among the new candidate variable sources are eight candidate variable red
supergiants, three candidate variable yellow supergiants and one candidate
luminous blue variable, which we suggest for follow-up observations.Comment: Accepted by A&A, 7 pages, 7 Tables, 53 figure
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