arXiv subjects

A. Z. Bonanos

Publications and source records attributed to A. Z. Bonanos.

68 records · Page 4Linked to original sources

arXiv2010

Spitzer SAGE-SMC Infrared Photometry of Massive Stars in the Small Magellanic Cloud

We present a catalog of 5324 massive stars in the Small Magellanic Cloud (SMC), with accurate spectral types compiled from the literature, and a photometric catalog for a subset of 3654 of these stars, with the goal of exploring their infrared properties. The photometric catalog consists of stars with infrared counterparts in the Spitzer, SAGE-SMC survey database, for which we present uniform photometry from 0.3-24 um in the UBVIJHKs+IRAC+MIPS24 bands. We compare the color magnitude diagrams and color-color diagrams to those of the Large Magellanic Cloud (LMC), finding that the brightest infrared sources in the SMC are also the red supergiants, supergiant B[e] (sgB[e]) stars, luminous blue variables, and Wolf-Rayet stars, with the latter exhibiting less infrared excess, the red supergiants being less dusty and the sgB[e] stars being on average less luminous. Among the objects detected at 24 um are a few very luminous hypergiants, 4 B-type stars with peculiar, flat spectral energy distributions, and all 3 known luminous blue variables. We detect a distinct Be star sequence, displaced to the red, and suggest a novel method of confirming Be star candidates photometrically. We find a higher fraction of Oe and Be stars among O and early-B stars in the SMC, respectively, when compared to the LMC, and that the SMC Be stars occur at higher luminosities. We estimate mass-loss rates for the red supergiants, confirming the correlation with luminosity even at the metallicity of the SMC. Finally, we confirm the new class of stars displaying composite A & F type spectra, the sgB[e] nature of 2dFS1804 and find the F0 supergiant 2dFS3528 to be a candidate luminous blue variable with cold dust.

astro-ph.SR↗

arXiv2010

The end of the white dwarf cooling sequence in M 67

In this paper, we present for the first time a proper-motion-selected white dwarf (WD) sample of the old Galactic open cluster M 67, down to the bottom of the WD cooling sequence (CS). The color-magnitude diagram is based on data collected with the LBC-Blue camera at the prime focus of LBT. As first epoch data, we used CFHT-archive images collected 10 years before LBC data. We measured proper motions of all the identified sources. Proper motions are then used to separate foreground and background objects from the cluster stars, including WDs. Finally, the field-object cleaned WD CS in the V vs. B-I color-magnitude diagram is compared with the models. We confirm that the age derived from the location of the bottom of the WD CS is consistent with the turn off age.

astro-ph.SR↗

arXiv2009

Spitzer SAGE Infrared Photometry of Massive Stars in the Large Magellanic Cloud

We present a catalog of 1750 massive stars in the Large Magellanic Cloud, with accurate spectral types compiled from the literature, and a photometric catalog for a subset of 1268 of these stars, with the goal of exploring their infrared properties. The photometric catalog consists of stars with infrared counterparts in the Spitzer SAGE survey database, for which we present uniform photometry from 0.3-24 microns in the UBVIJHKs+IRAC+MIPS24 bands. The resulting infrared color-magnitude diagrams illustrate that the supergiant B[e], red supergiant and luminous blue variable (LBV) stars are among the brightest infrared point sources in the Large Magellanic Cloud, due to their intrinsic brightness, and at longer wavelengths, due to dust. We detect infrared excesses due to free-free emission among ~900 OB stars, which correlate with luminosity class. We confirm the presence of dust around 10 supergiant B[e] stars, finding the shape of their spectral energy distributions (SEDs) to be very similar, in contrast to the variety of SED shapes among the spectrally variable LBVs. The similar luminosities of B[e] supergiants (log L/Lo>=4) and the rare, dusty progenitors of the new class of optical transients (e.g. SN 2008S and NGC 300 OT), plus the fact that dust is present in both types of objects, suggests a common origin for them. We find the infrared colors for Wolf-Rayet stars to be independent of spectral type and their SEDs to be flatter than what models predict. The results of this study provide the first comprehensive roadmap for interpreting luminous, massive, resolved stellar populations in nearby galaxies at infrared wavelengths.

astro-ph.SR↗

arXiv2008

Photometric variability of WC9 stars

Do some Wolf-Rayet stars owe their strong winds to something else besides radiation pressure? The answer to this question is still not entirely obvious, especially in certain Wolf-Rayet subclasses, mainly WN8 and WC9. Both of these types of Wolf-Rayet stars are thought to be highly variable, as suggested by observations, possibly due to pulsations. However, only the WN8 stars have so far been vigorously and systematically investigated for variability. We present here the results of a systematic survey during 3 consecutive weeks of 19 Galactic WC9 stars and 1 WC8 star for photometric variability in two optical bands, V and I. Of particular interest are the correlated variations in brightness and colour index in the context of carbon-dust formation, which occurs frequently in WC9 and some WC8 stars. In the most variable case, WR76, we used this information to derive a typical dust grain size of ~ 0.1 um. However, most photometric variations occur at surprisingly low levels and in fact almost half of our sample shows no significant variability at all above the instrumental level (sigma ~ 0.005 - 0.01 mag).

astro-ph↗

arXiv2007

The initial-final mass relationship from white dwarfs in common proper motion pairs

The initial-final mass relationship of white dwarfs, which is poorly constrained, is of paramount importance for different aspects in modern astrophysics. From an observational perspective, most of the studies up to now have been done using white dwarfs in open clusters. In order to improve the initial-final mass relationship we explore the possibility of deriving a semi-empirical relation studying white dwarfs in common proper motion pairs. We have acquired long-slit spectra of the white dwarf members of the selected common proper motion pairs, as well as high resolution spectra of their companions. From these observations, a full analysis of the two members of each common proper motion pair has lead to the initial and final masses of the white dwarfs. These observations have allowed us to provide updated information for the white dwarfs, since some of them were misclassified. This work is the first one in using common proper motion pairs to improve the initial-final mass relationship, and has also allowed to cover the poorly explored low-mass domain. As in the case of studies based on white dwarfs in open clusters, the distribution of the semi-empirical data presents a large scatter, which is higher than the expected uncertainties in the derived values. This suggests that the initial-final mass relationship may not be a single-valued function.

astro-ph↗

arXiv2006

The Nature of the Variable Galactic Center Source GCIRS 16SW Revisited: A Massive Eclipsing Binary

We present a re-analysis of our H- and K-band photometry and light-curves for GCIRS 16SW, a regular periodic source near the Galactic center. These data include those presented by DePoy et al. (2004); we correct a sign error in their reduction, finding GCIRS 16SW to be an eclipsing binary with no color variations. We find the system to be an equal mass overcontact binary (both stars overfilling their Roche lobes) in a circular orbit with a period P=19.4513 days, an inclination angle i=71 degrees. This confirms and strengthens the findings of Martins et al. (2006) that GCIRS 16SW is an eclipsing binary composed of two ~50Msun stars, further supporting evidence of recent star formation very close to the Galactic center. Finally, the calculated luminosity of each component is close to the Eddington luminosity, implying that the temperature of 24400 K given by Najarro et al. (1997) might be overestimated for these evolved stars.

astro-ph↗

arXiv2006

Eclipsing Binaries: Tools for Calibrating the Extragalactic Distance Scale

In the last decade, over 7000 eclipsing binaries have been discovered in the Local Group through various variability surveys. Measuring fundamental parameters of these eclipsing binaries has become feasible with 8 meter class telescopes, making it possible to use eclipsing binaries as distance indicators. Distances with eclipsing binaries provide an independent method for calibrating the extragalactic distance scale and thus determining the Hubble constant. This method has been used for determining distances to eclipsing binaries in the Magellanic Clouds and the Andromeda Galaxy and most recently to a detached eclipsing binary in the Triangulum Galaxy by the DIRECT Project. The increasing number of eclipsing binaries found by microlensing and variability surveys also provide a rich database for advancing our understanding of star formation and evolution.

astro-ph↗

arXiv2006

The First DIRECT Distance Determination to a Detached Eclipsing Binary in M33

We present the first direct distance determination to a detached eclipsing binary in M33, which was found by the DIRECT Project. Located in the OB 66 association at coordinates (alpha, delta)=(01:33:46.17,+30:44:39.9) for J2000.0, it was one of the most suitable detached eclipsing binaries found by DIRECT for distance determination, given its apparent magnitude and orbital period. We obtained follow-up BV time series photometry, JHKs photometry and optical spectroscopy from which we determined the parameters of the system. It contains two O7 main sequence stars with masses of 33.4+/-3.5 Mo and 30.0+/-3.3 Mo and radii of 12.3+/-0.4 Ro and 8.8+/-0.3 Ro, respectively. We derive temperatures of 37000+/-1500 K and 35600+/-1500 K. Using BVRJHKs photometry for the flux calibration, we obtain a distance modulus of 24.92+/-0.12 mag (964+/-54 kpc), which is ~0.3 mag longer than the Key Project distance to M33. We discuss the implications of our result and the importance of establishing M33 as an independent rung on the cosmological distance ladder.

astro-ph↗

arXiv2006

Detection of a Variable Infrared Excess Around SDSS 121209.31+013627.7

We present near-infrared $JHK_s$ 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 $K_s-$band. The observed near-infrared excess can be explained by the presence of an L7 brown dwarf and an extra emission source. The $J$ and $H$ 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 $K_s$ lightcurve, we get a refined period for the system of 88$\pm$1 minutes. We show that the observed variability in $K_s-$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.

astro-ph↗

arXiv2005

Deep Photometry of GRB 041006 Afterglow: Hypernova Bump at Redshift z=0.716

We present deep optical photometry of the afterglow of gamma-ray burst (GRB) 041006 and its associated hypernova obtained over 65 days after detection (55 R-band epochs on 10 different nights). Our early data (t<4 days) joined with published GCN data indicates a steepening decay, approaching F_nu ~t^{-0.6} at early times (<<1 day) and F_nu ~t^{-1.3} at late times. The break at t_b=0.16+-0.04 days is the earliest reported jet break among all GRB afterglows. During our first night, we obtained 39 exposures spanning 2.15 hours from 0.62 to 0.71 days after the burst that reveal a smooth afterglow, with an rms deviation of 0.024 mag from the local power-law fit, consistent with photometric errors. After t~4 days, the decay slows considerably, and the light curve remains approximately flat at R~24 mag for a month before decaying by another magnitude to reach R~25 mag two months after the burst. This ``bump'' is well-fitted by a k-corrected light curve of SN1998bw, but only if stretched by a factor of 1.38 in time. In comparison with the other GRB-related SNe bumps, GRB 041006 stakes out new parameter space for GRB/SNe, with a very bright and significantly stretched late-time SN light curve. Within a small sample of fairly well observed GRB/SN bumps, we see a hint of a possible correlation between their peak luminosity and their ``stretch factor'', broadly similar to the well-studied Phillips relation for the type Ia supernovae.

astro-ph↗

arXiv2004

WR 20a is an Eclipsing Binary: Accurate Determination of Parameters for an Extremely Massive Wolf-Rayet System

We present a high-precision I-band light curve for the Wolf-Rayet binary WR 20a, obtained as a sub-project of the Optical Gravitational Lensing Experiment. Rauw et al. have recently presented spectroscopy for this system, strongly suggesting extremely large minimum masses of 70.7 +/- 4.0 Mo and 68.8 +/- 3.8 Mo for the component stars of the system, with the exact values depending strongly on the period of the system. We detect deep eclipses of about 0.4 mag in the light curve of WR 20a, confirming and refining the suspected period of P=3.686 days and deriving an inclination angle of i=74.5 +/- 2 deg. Using these photometric data and the radial velocity data of Rauw et al., we derive the masses for the two components of WR 20a to be 83.0 +/- 5.0 Mo and 82.0 +/- 5.0 Mo. Therefore, WR 20a is confirmed to consist of two extremely massive stars and to be the most massive binary known with an accurate mass determination.

astro-ph↗

arXiv2003

The RR Lyrae Distance to the Draco Dwarf Spheroidal Galaxy

We present the first CCD variability study of the Draco dwarf spheroidal galaxy. The data were obtained with the FLWO 1.2 m telescope on 22 nights, over a period of 10 months, covering a 22 x 22 arcmin field centered at RA=17:19:57.5, Dec=57:50:05, J2000.0. The analysis of the BVI images produced 163 variable stars, 146 of which were RR Lyrae: 123 RRab, 16 RRc, 6 RRd and one RR12. The other variables include a SX Phe star, four anomalous Cepheids and a field eclipsing binary. Using the short distance scale statistical parallax calibration of Gould & Popowski and 94 RRab stars from our field, we obtain a distance modulus of (m-M)_0=19.40 +/- 0.02 (stat) +/- 0.15 (syst) mag for Draco, corresponding to a distance of 75.7 kpc +/- 0.7 (stat) +/- 5.4 (syst) kpc. By comparing the spread in magnitudes of RRab stars in B,V and I, we find no evidence for internal dust in the Draco dwarf spheroidal galaxy. The catalog of all variables, as well as their photometry and finding charts, is available electronically via anonymous ftp and the World Wide Web. The complete set of the CCD frames is available upon request.

astro-ph↗

arXiv2003

Re-analysis of VLT Data for M83 with Image Subtraction -- Nine-fold Increase in Number of Cepheids

We apply the image subtraction method to re-analyze the ESO Very Large Telescope data on M83 (NGC 5236), obtained and analyzed by Thim et al. Whereas Thim et al. found 12 Cepheids with periods between 12-55 days, we find 112 Cepheids with periods ranging from 7-91 days, as well as ~60 other variables. These include 2 candidate eclipsing binaries, which, if confirmed, would be the first optically discovered outside the Local Group. We thus demonstrate that the image subtraction method is much more powerful for detecting variability, especially in crowded fields. However, HST observations are necessary to obtain a Cepheid period-luminosity distance not dominated by blending and crowding. We propose a ``hybrid'' approach, where numerous Cepheids are discovered and characterized using large ground-based telescopes and then followed-up with the HST to obtain precise distances.

astro-ph↗

arXiv2003

DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries and Cepheids. IX. Variables in the Field M31Y Discovered with Image Subtraction

The DIRECT Project aims to obtain direct distances to two Local Group galaxies, M31 and M33, which occupy a crucial position near the bottom of the cosmological distance ladder. The first step is to search for detached eclipsing binaries (DEBs) and Cepheids using 1 m class telescopes to select good candidates, which will be followed up spectroscopically on 6.5-10 m class telescopes. In this ninth paper we present a catalog of variable stars discovered with image subtraction in field M31Y [RA=10.97 deg, Dec=41.69 deg, J2000.0]. The data were obtained with the FLWO 1.2 m telescope on 25 nights, over a period of 6 months. In our search covering 22x22 arcmin we discovered 41 eclipsing binaries, 126 Cepheids, and 97 other periodic or nonperiodic variables, including a LBV candidate, a nova and a galactic CV. The catalog of variables, as well as their photometry and finding charts, is available electronically via anonymous ftp and the World Wide Web. The complete set of the CCD frames is available upon request.

astro-ph↗