arXiv subjects

Valentin D. Ivanov

Publications and source records attributed to Valentin D. Ivanov.

At least 91 records · Page 5Linked to original sources

arXiv2014

Near-infrared spectroscopy of candidate red supergiant stars in clusters

Clear identifications of Galactic young stellar clusters farther than a few kpc from the Sun are rare, despite the large number of candidate clusters. We aim to improve the selection of candidate clusters rich in massive stars with a multiwavelength analysis of photometric Galactic data that range from optical to mid-infrared wavelengths. We present a photometric and spectroscopic analysis of five candidate stellar clusters, which were selected as overdensities with bright stars (Ks < 7 mag) in GLIMPSE and 2MASS images. A total of 48 infrared spectra were obtained. The combination of photometry and spectroscopy yielded six new red supergiant stars with masses from 10 Msun to 15 Msun. Two red supergiants are located at Galactic coordinates (l,b)=(16.7deg,-0.63deg) and at a distance of about ~3.9 kpc; four other red supergiants are members of a cluster at Galactic coordinates (l,b)=(49.3deg,+0.72deg) and at a distance of ~7.0 kpc. Spectroscopic analysis of the brightest stars of detected overdensities and studies of interstellar extinction along their line of sights are fundamental to distinguish regions of low extinction from actual stellar clusters. The census of young star clusters containing red supergiants is incomplete; in the existing all-sky near-infrared surveys, they can be identified as overdensities of bright stars with infrared color-magnitude diagrams characterized by gaps.

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arXiv2014

Massive stars in the giant molecular cloud G23.3-0.3 and W41

Young massive stars and stellar clusters continuously form in the Galactic disk, generating new HII regions within their natal giant molecular clouds and subsequently enriching the interstellar medium via their winds and supernovae. Massive stars are among the brightest infrared stars in such regions; their identification permits the characterization of the star formation history of the associated cloud as well as constraining the location of stellar aggregates and hence their occurrence as a function of global environment. We present a stellar spectroscopic survey in the direction of the giant molecular cloud G23.3-0.3. This complex is located at a distance of ~ 4-5 kpc, and consists of several HII regions and supernova remnants. We discovered 11 OfK+ stars, one candidate Luminous Blue Variable, several OB stars, and candidate red supergiants. Stars with K-band extinction from ~1.3 - 1.9 mag appear to be associated with the GMC G23.3-0.3; O and B-types satisfying this criterion have spectro-photometric distances consistent with that of the giant molecular cloud. Combining near-IR spectroscopic and photometric data allowed us to characterize the multiple sites of star formation within it. The O-type stars have masses from 25 - 45 Msun, and ages of 5-8 Myr. Two new red supergiants were detected with interstellar extinction typical of the cloud; along with the two RSGs within the cluster GLIMPSE9, they trace an older burst with an age of 20--30 Myr. Massive stars were also detected in the core of three supernova remnants - W41, G22.7-0.2, and G22.7583-0.4917. A large population of massive stars appears associated with the GMC G23.3-0.3, with the properties inferred for them indicative of an extended history of stars formation.

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arXiv2014

The VMC Survey - XII. Star cluster candidates in the Large Magellanic Cloud

In this work we analyse Colour-Magnitude Diagrams (CMDs) of catalogued star clusters located in the Large Magellanic Cloud (LMC), from a YJKs photometric data set obtained by the Visible and Infrared Survey Telescope for Astronomy (VISTA) survey of the Magellanic Clouds system (VMC). We studied a total of 98 objects of small angular size, typically ~ 11.6 pc in diameter projected towards both uncrowded tile LMC 8_8 and crowded tile LMC 5_5. They populate relatively crowded LMC fields with significant fluctuations in the stellar density, the luminosity function, and the colour distribution as well as uncrowded fields. This cluster sample is aimed at actually probing our performance in reaching the CMD features of clusters with different ages in crowded/uncrowded fields. We applied a subtraction procedure to statistically clean the cluster CMDs from field star contamination. We then matched theoretical isochrones to the background-subtracted CMDs to determine the ages and metallicities of the clusters. We estimated the ages of 65 clusters, which resulted to be in the age range 7.3 < log(t/yr) < 9.55. We also classified as chance grouping of stars 19 previoulsy catalogued clusters, two possible cluster-like asterisms, and one unresolved cluster. For other 8 objects, we could not find a clear star concentration in the Ks images either, so we quoted them as cluster-like asterisms. Finally, we found two previously catalogued single star clusters to be unresolved background galaxies (KMHK747, OGLE366), and one to be a triple cluster system (BSDL 2144).

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arXiv2014

Confirmation of a cluster of galaxies hidden behind the Galactic bulge using the VVV Survey

Suzaku and Chandra X-ray observations detected a new cluster of galaxies, Suzaku J1759-3450, at a redshift z=0.13. It is located behind the Milky Way, and the high Galactic dust extinction renders it nearly invisible at optical wavelengths. We attempt here to confirm the galaxy cluster with near-infrared imaging observations, and to characterize its central member galaxies. Images from the VVV survey were used to detect candidate member galaxies of Suzaku J1759-3450 within the central region of the cluster, up to 350 kpc from the X-ray peak emission. Color-magnitude and color-color diagrams and morphology criteria allowed us to select the galaxies among the numerous foreground sources. Fifteen candidate cluster members were found very close to a modeled red-sequence at the redshift of the cluster. Five members are extremely bright, and one is possibly a cD galaxy. The asymmetry in the spatial distribution of the galaxies respect to the X-ray peak emission is an indicator of that this cluster is still suffering a virialization process. Our investigation of Suzaku J1759-3450 demonstrates the potential of the VVV Survey to study the hidden population of galaxies in the Zone of Avoidance.

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arXiv2014

VISTA Variables in the Vía Láctea (VVV): Halfway Status and Results

The VISTA Variables in the Vía Láctea (VVV) survey is one of six public ESO surveys, and is now in its 4th year of observing. Although far from being complete, the VVV survey has already delivered many results, some directly connected to the intended science goals (detection of variables stars, microlensing events, new star clusters), others concerning more exotic objects, e.g. novae. Now, at the end of the fourth observing period, and comprising roughly 50% of the proposed observations, the actual status of the survey, as well some of the results based on the VVV data, are presented.

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arXiv2014

Ground-based transit observations of the super-Earth GJ 1214b

GJ 1214b is one of the few known transiting super-Earth-sized exoplanets with a measured mass and radius. It orbits an M-dwarf, only 14.55 pc away, making it a favorable candidate for follow-up studies. However, the composition of GJ 1214b's mysterious atmosphere has yet to be fully unveiled. Our goal is to distinguish between the various proposed atmospheric models to explain the properties of GJ 1214b: hydrogen-rich or hydrogen-He mix, or a heavy molecular weight atmosphere with reflecting high clouds, as latest studies have suggested. Wavelength-dependent planetary radii measurements from the transit depths in the optical/NIR are the best tool to investigate the atmosphere of GJ 1214b. We present here (i) photometric transit observations with a narrow-band filter centered on 2.14 microns and a broad-band I-Bessel filter centered on 0.8665 microns, and (ii) transmission spectroscopy in the H and K atmospheric windows that cover three transits. The obtained photometric and spectrophotometric time series were analyzed with MCMC simulations to measure the planetary radii as a function of wavelength. We determined radii ratios of 0.1173 for I-Bessel and 0.11735 at 2.14 microns. Our measurements indicate a flat transmission spectrum, in agreement with last atmospheric models that favor featureless spectra with clouds and high molecular weight compositions.

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arXiv2013

Discovery of New Companions to High Proper Motion Stars from the VVV Survey

[abridged] The severe crowding in the direction of the inner Milky Way suggests that the census of stars within a few tens of parsecs in that direction may not be complete. We search for new nearby objects companions of known high proper motion (HPM) stars located towards the densest regions of the Southern Milky Way where the background contamination presented a major problem to previous works. The common proper motion (PM) method was used--we inspected the area around 167 known HPM (>=200 mas/yr) stars: 67 in the disk and 100 in the bulge. Multi-epoch images were provided by 2MASS and the VISTA Variables in Via Lactea (VVV). The VVV is a new on-going ZYJHKs plus multi-epoch Ks survey of ~562 deg^2 of Milky Way's bulge and inner Southern disk. Seven new co-moving companions were discovered around known HPM stars; six known co-moving pairs were recovered; a pair of stars that was thought to be co-moving was found to have different proper motions; published HPMs of eight stars were not confirmed; last but not least, spectral types ranging from G8V to M5V were derived from new infrared spectroscopy for seventeen stars, members of the co-moving pairs. The seven newly discovered stars constitute ~4% of the nearby HPM star list but this is not a firm limit on the HPM star incompleteness because our starting point--the HPM list assembled from the literature--is incomplete itself, missing many nearby HPM M and L type objects, and it is contaminated with non-HPM stars. We have demonstrated, that the superior sub-arcsec spatial resolution, with respect to previous surveys, allows the VVV to examine further the binary nature nature of known HPM stars. The >=5 yr span of VVV will provide sufficient baseline for finding new HPM stars from VVV data alone.

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arXiv2012

Discovery of Two Very Wide Binaries with Ultracool Companions and a New Brown Dwarf at the L/T Transition

We present the discovery and spectroscopic follow-up of a nearby late-type L dwarf (2M0614+3950), and two extremely wide very-low-mass binary systems (2M0525-7425AB and 2M1348-1344AB), resulting from our search for common proper motion pairs containing ultracool components in the Two Micron All Sky Survey (2MASS) and the Wide-field Infrared Survey Explorer (WISE) catalogs. The near-infrared spectrum of 2M0614+3950 indicates a spectral type L$9 \pm 1$ object residing at a distance of $26.0 \pm 1.8$ pc. The optical spectrum of 2M0525-7425A reveals an M$3.0 \pm 0.5$ dwarf primary, accompanied by a secondary previously classified as L2. The system has an angular separation of $\sim 44"$, equivalent to $\sim 2000 $AU at distance of $46.0 \pm 3.0$ pc. Using optical and infrared spectra, respectively, we classify the components of 2M1348-1344AB as M$4.5 \pm 0.5$ and T$5.5 \pm 1$. The angular separation of $\sim 68"$ is equivalent to $\sim 1400 $AU at a distance of $20.7 \pm 1.4$ pc. 2M1348-1344AB is one of only six very wide (separation $>$ 1000 AU) systems containing late T dwarfs known to date.

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arXiv2012

Variability of the Infrared Excess of Extreme Debris Disks

Debris disks with extremely large infrared excesses (fractional luminosities $> 10^{-2}$) are rare. Those with ages between 30 and 130 Myr are of interest because their evolution has progressed well beyond that of protoplanetary disks (which dissipate with a timescale of order 3 Myr), yet they represent a period when dynamical models suggest that terrestrial planet building may still be progressing through large, violent collisions that could yield large amounts of debris and large infrared excesses. For example, our Moon was formed through a violent collision of two large proto-planets during this age range. We report two disks around the solar-like stars ID8 and HD 23514 in this age range where the 24 μm infrared excesses vary on timescales of a few years, even though the stars are not variable in the optical. Variations this rapid are difficult to understand if the debris is produced by collisional cascades, as it is for most debris disks. It is possible that the debris in these two systems arises in part from condensates from silicate-rich vapor produced in a series of violent collisions among relatively large bodies. If their evolution is rapid, the rate of detection of extreme excesses would indicate that major collisions may be relatively common in this age range.

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arXiv2011

New Galactic Star Clusters Discovered in the VVV Survey

VISTA Variables in the V\'ıa Láctea (VVV) is one of the six ESO Public Surveys operating on the new 4-meter Visible and Infrared Survey Telescope for Astronomy (VISTA). VVV is scanning the Milky Way bulge and an adjacent section of the disk, where star formation activity is high. One of the principal goals of the VVV Survey is to find new star clusters of different ages. In order to trace the early epochs of star cluster formation we concentrated our search in the directions to those of known star formation regions, masers, radio, and infrared sources. The disk area covered by VVV was visually inspected using the pipeline processed and calibrated $K_{\rm S}$-band tile images for stellar overdensities. Subsequently, we examined the composite $JHK_{\rm S}$ and $ZJK_{\rm S}$ color images of each candidate. PSF photometry of $15\times15$ arcmin fields centered on the candidates was then performed on the Cambridge Astronomy Survey Unit reduced images. After statistical field-star decontamination, color-magnitude and color-color diagrams were constructed and analyzed. We report the discovery of 96 new infrared open clusters and stellar groups. Most of the new cluster candidates are faint and compact (with small angular sizes), highly reddened, and younger than 5\,Myr. For relatively well populated cluster candidates we derived their fundamental parameters such as reddening, distance, and age by fitting the solar-metallicity Padova isochrones to the color-magnitude diagrams.

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arXiv2009

Model-Independent Diagnostics of Highly Reddened Milky Way Star Clusters: Age Calibration

The next generation near- and mid-infrared Galactic surveys will yield a large number of new highly obscured star clusters. Detailed characterization of these new objects with spectroscopy is time-consuming. Diagnostic tools that will be able to characterize clusters based only on the available photometry will be needed to study large samples of the newly found objects. The brightness difference between the red clump and the main-sequence turn-off point have been used as a model-independent age calibrator for clusters with ages from a few 10$^8$ to 10$^{10}$ yr in the optical. Here we apply for the first time the method in the near-infrared. We calibrated this difference in $K$-band, which is likely to be available for obscured clusters, and we apply it to a number of test clusters with photometry comparable to the one that will be yielded by the current or near-future surveys. The new calibration yields reliable ages over the range of ages for which the red clump is present in clusters. The slope of the relation is smoother than that of the corresponding $V$-band relation, reducing the uncertainty in the age determinations with respect to the optical ones.

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arXiv2009

Near-infrared spectra of Galactic stellar clusters detected on Spitzer/GLIMPSE images

We present near-infrared spectroscopic observations of massive stars in three stellar clusters located in the direction of the inner Galaxy. One of them, the Quartet, is a new discovery while the other two were previously reported as candidate clusters identified on mid-infrared Spitzer images (GLIMPSE20 and GLIMPSE13). Using medium-resolution (R=900-1320) H and K spectroscopy, we firmly establish the nature of the brightest stars in these clusters, yielding new identifications of an early WC and two Ofpe/WN9 stars in the Quartet and an early WC star in GLIMPSE20. We combine this information with the available photometric measurements from 2MASS, to estimate cluster masses, ages, and distances. The presence of several massive stars places the Quartet and GLIMPSE20 among the small sample of known Galactic stellar clusters with masses of a few 10^3 Msun, and ages from 3 to 8 Myr. We estimate a distance of about 3.5 kpc for Glimpse 20, and 6.0 kpc for Quartet. The large number of giant stars identified in GLIMPSE13 indicates that it is another massive (~ 6500 Msun) cluster, but older, with an age between 30 and 100 Myr, at a distance of about 3 kpc.

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arXiv2008

Luminosity-Metallicity Relation for dIrr Galaxies in the Near-Infrared

(abridged) The present work is a first step to collect homogeneous abundances and near-infrared (NIR) luminosities for a sample of dwarf irregular (dIrr) galaxies, located in nearby groups. The use of NIR luminosities is intended to provide a better proxy to mass than the blue luminosities commonly used in the literature; in addition, selecting group members reduces the impact of uncertain distances. Accurate abundances are derived to assess the galaxy metallicity. Optical spectra are collected for Hii regions in the dIrrs, allowing the determination of oxygen abundances by means of the temperature-sensitive method. For each dIrr galaxy H-band imaging is performed and the total magnitudes are measured via surface photometry. This high-quality database allows us to build a well-defined luminosity-metallicity relation in the range -13 >= M(H) >= -20. The scatter around its linear fit is reduced to 0.11 dex, the lowest of all relations currently available. There might exist a difference between the relation for dIrrs and the relation for giant galaxies, although a firm conclusion should await direct abundance determinations for a significant sample of massive galaxies. This new dataset provides an improved luminosity-metallicity relation, based on a standard NIR band, for dwarf star-forming galaxies. The relation can now be compared with some confidence to the predictions of models of galaxy evolution. Exciting follow-ups of this work are (a) exploring groups with higher densities, (b) exploring nearby galaxy clusters to probe environmental effects on the luminosity-metallicity relation, and (c) deriving direct oxygen abundances in the central regions of star-forming giant galaxies, to settle the question of a possible dichotomy between the chemical evolution of dwarfs and that of massive galaxies.

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arXiv2007

Millimagnitude Photometry for Transiting Extrasolar Planetary Candidates IV: The Puzzle of the Extremely Red OGLE-TR-82 Primary Solved

We present precise new V, I, and K-band photometry for the planetary transit candidate star OGLE-TR-82. Good seeing V-band images acquired with VIMOS instrument at ESO VLT allowed us to measure V=20.6+-0.03 mag star in spite of the presence of a brighter neighbour about 1" away. This faint magnitude answers the question why it has not been possible to measure radial velocities for this object. One transit of this star has been observed with GMOS-S instrument of GEMINI-South telescope in i and g-bands. The measurement of the transit allows us to verify that this is not a false positive, to confirm the transit amplitude measured by OGLE, and to improve the ephemeris. The transit is well defined in i-band light curve, with a depth of A_i=0.034 mag. It is however, less well defined, but deeper (A_g=0.1 mag) in the g-band, in which the star is significantly fainter. The near-infrared photometry obtained with SofI array at the ESO-NTT yields K=12.2+-0.1 and V-K=8.4+-0.1, so red that it is unlike any other transit candidate studied before. Due to the extreme nature of this object, we have not yet been able to measure velocities for this star, but based on the new data we consider two different possible configurations:(1) a nearby M7V star, or (2) a blend with a very reddened distant red giant. The nearby M7V dwarf hypothesis would give a radius for the companion of R_p=0.3+-0.1 R_J, i.e. the size of Neptune. Quantitative analysis of near-IR spectroscopy finally shows that OGLE-TR-82 is a distant, reddened metal poor early K giant. This result is confirmed by direct comparison with stellar templates that gives the best match for a K3III star. Therefore, we discard the planetary nature of the companion. Based on all the new data, we conclude that this system is a main-sequence binary blended with a background red giant.

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arXiv2007

Millimagnitude Photometry for Transiting Extrasolar Planetary Candidates: II. Transits of OGLE-TR-113-b in the Optical and Near-IR

We present precise V and Ks-band transit photometry for the planetary host star OGLE-TR-113. Using the Ks-band photometry, we confirm the dwarf nature of OGLE-TR-113, and obtain new estimates for its effective temperature, distance and reddening. We employ the V-band photometry to obtain planetary and orbit parameters from the transit fit, a= (0.0232 \pm 0.0038) AU, orbital period P= (1.4324752 \pm 0.0000015) days, i= 86.7 - 90, R_p= (1.09 \pm 0.09) R_J. These values are in excellent agreement with previous works. Assuming a mass M_p= (1.32 \pm 0.19) M_J for the planet we obtain its mean density ρ= (1.26 \pm 0.50) g cm^{-3}, also in agreement with previous works. The transit observed in the Ks-band has a larger scatter and we find its amplitude to be consistent with that in the V-band. In this way, we find an independent confirmation of the planetary nature of OGLE-TR-113b.

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arXiv2006

Infrared Spectroscopy of the Ultra Low Mass Binary Oph 162225-240515

Binary properties are an important diagnostic of the star and brown dwarf formation processes. While wide binaries appear to be rare in the sub-stellar regime, recent observations have revealed Ophiuchus 162225-240515 (2MASS J16222521-2405139) as a likely young ultra-low-mass binary with an apparent separation of ~240 AU. Here, we present low-resolution near-infrared spectra of the pair from NTT/SOFI (R~600) and VLT/ISAAC (R~1400), covering the 1.0-2.5um spectral region. By comparing to model atmospheres from Chabrier & Baraffe and Burrows et al., we confirm the surface temperatures to be T_A = (2350+/-150) K and T_B = (2100+/-100) K for the two components of the binary, consistent with earlier estimates from optical spectra. Using gravity sensitive K I features, we find the surface gravity to be significantly lower than field dwarfs of the same spectral type, providing the best evidence so far that these objects are indeed young. However, we find that models are not sufficiently reliable to infer accurate ages/masses from surface gravity. Instead, we derive masses of M_A = 13 (+8/-4) M_J and M_B = 10 (+5/-4) M_J for the two objects using the well-constrained temperatures and assuming an age of 1-10 Myr, consistent with the full range of ages reported for the Oph region.

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arXiv2006

Spectroscopy of Young Planetary Mass Candidates with Disks

It is now well established that many young brown dwarfs exhibit characteristics similar to classical T Tauri stars, including infrared excess from disks and emission lines related to accretion. Whether the same holds true for even lower mass objects, namely those near and below the Deuterium-burning limit, is an important question. Here we present optical spectra of six isolated planetary mass candidates in Chamaeleon II, Lupus I and Ophiuchus star-forming regions, recently identified by Allers and collaborators to harbor substantial mid-infrared excesses. Our spectra, from ESO's Very Large Telescope and New Technology Telescope, show that four of the targets have spectral types in the ~M9-L1 range, and three of those also exhibit H_alpha. Their luminosities are consistent with masses of ~5-15 M_{Jupiter} according to models of Chabrier, Baraffe and co-workers, thus placing these four objects among the lowest mass brown dwarfs known to be surrounded by circum-sub-stellar disks. Our findings bolster the idea that free-floating planetary mass objects could have infancies remarkably similar to those of Sun-like stars and suggest the intriguing possibility of planet formation around primaries whose masses are comparable to those of extra-solar giant planets. Another target appears to be a brown dwarf (~M8) with prominent H_alpha emission, possibly arising from accretion. The sixth candidate is likely a background source, underlining the need for spectroscopic confirmation.

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arXiv2006

Common Proper Motion Search for Faint Companions Around Early-Type Field Stars - Progress Report - II

The multiplicity of early-type stars is still not well established. The studies of individual star forming regions suggest a connection with the age and the environment. To fill in this gap, we started the first detailed adaptive-optic-assisted imaging survey of 308 BA-type field stars within 300 pc from the Sun, to derive their multiplicity in a homogeneous way. Our first epoch observations yield 195 companion candidates around 117 sample targets. The second epoch observations are underway.

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