arXiv subjects

M. M. Shara

Publications and source records attributed to M. M. Shara.

29 records · Page 2Linked to original sources

arXiv2011

The Spectral Energy Distributions of White Dwarfs in 47 Tucanae: The Distance to the Cluster

We present a new distance determination to the Galactic globular cluster 47 Tucanae by fitting the spectral energy distributions of its white dwarfs to pure hydrogen atmosphere white dwarf models. Our photometric dataset is obtained from a 121 orbit Hubble Space Telescope program using the Wide Field Camera 3 UVIS/IR channels, capturing F390W, F606W, F110W, and F160W images. These images cover more than 60 square arcmins and extend over a radial range of 5-13.7 arcmin (6.5-17.9 pc) within the globular cluster. Using a likelihood analysis, we obtain a best fitting unreddened distance modulus of (m - M)o=13.36+/-0.02+/-0.06 corresponding to a distance of 4.70+/-0.04+/-0.13 kpc, where the first error is random and the second is systematic. We also search the white dwarf photometry for infrared excess in the F160W filter, indicative of debris disks or low mass companions, and find no convincing cases within our sample.

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arXiv2008

Combined Chandra, XMM-Newton and Hubble Space Telescope observations of the Galactic globular cluster NGC 2808

Using new Chandra X-ray observations and existing XMM-Newton X-ray and Hubble far ultraviolet observations, we aim to detect and identify the faint X-ray sources belonging to the Galactic globular cluster NGC 2808 in order to understand their role in the evolution of globular clusters. We present a Chandra X-ray observation of the Galactic globular cluster NGC 2808. We classify the X-ray sources associated with the cluster by analysing their colours and variability. Previous observations with XMM-Newton and far ultraviolet observations with the Hubble Space Telescope are re-investigated to help identify the Chandra sources associated with the cluster. We compare our results to population synthesis models and observations of other Galactic globular clusters. We detect 113 sources, of which 16 fall inside the half-mass radius of NGC 2808 and are concentrated towards the cluster core. From statistical analysis, these 16 sources are very likely to be linked to the cluster. We detect short-term (1 day) variability in X-rays for 7 sources, of which 2 fall inside the half-mass radius, and long-term (28 months) variability for 10 further sources, of which 2 fall inside the half-mass radius. Ultraviolet counterparts are found for 8 Chandra sources in the core, of which 2 have good matching probabilities and have ultraviolet properties expected for cataclysmic variables. We find one likely neutron star-quiescent low-mass X-ray binary and 7 cataclysmic variable candidates in the core of NGC 2808. The other 8 sources are cataclysmic variable candidates, but some could possibly be active binaries or millisecond pulsars. We find a possible deficit of X-ray sources compared to 47 Tuc which could be related to the metallicity content and the complexity of the evolution of NGC 2808.

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arXiv2007

Unveiling the core of the Globular Cluster M15 in the Ultraviolet

We have obtained deep far- (FUV) and near-ultraviolet (NUV) images of the inner region of the dense globular cluster M15 with the Advanced Camera for Surveys on board the Hubble Space Telescope. The FUV-NUV colour-magnitude diagram shows a well defined track of horizontal branch stars, as well as a trail of blue stragglers and white dwarfs. The main sequence turn-off is clearly visible at FUV~23.5 mag and FUV-NUV~3 mag, and the main sequence stars form a prominent track that extends at least two magnitudes below the main sequence turn-off. As such, this is the deepest FUV-NUV colour-magnitude diagram of a globular cluster presented so far. Cataclysmic variable and blue straggler candidates are the most centrally concentrated stellar populations, which might either be an effect of mass segregation or reflect the preferred birthplace in the dense cluster core of such dynamically-formed objects. We find 41 FUV sources that exhibit significant variability. We classify the variables based on an analysis of their UV colours and variability properties. We find four previously known RR Lyrae and 13 further RR Lyrae candidates, one known Cepheid and six further candidates, six cataclysmic variables, one known and one probable SX Phoenicis star, and the well known low-mass X-ray binary AC211. Our analysis represents the first detection of SX Phoenicis pulsations in the FUV. We find that Cepheids, RR Lyraes and SX Phoenicis exhibit massive variability amplitudes in this waveband (several mags).

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arXiv2005

A Blue Straggler Binary System with at least Three Progenitors in the Core of a Globular Cluster?

We show that the X-ray source W31 in the core of the globular cluster 47 Tucanae is physically associated with the bright blue straggler BSS-7. The two sources are astrometrically matched to 0.061\arcsec, with a chance coincidence probability of less than 1%. We then analyse optical time-series photometry obtained with the {\em Hubble Space Telescope} (HST) and find that BSS-7 displays a 1.56 day periodic signal in the I band. We also construct a broad-band (far-ultraviolet through far-red) spectral energy distribution for BSS-7 and fit this with single and binary models. The binary model is a better fit to the data, and we derive the corresponding stellar parameters. All of our findings are consistent with BSS-7 being a detached binary consisting of a blue straggler primary with an X-ray-active, upper-main-sequence companion. The formation of such a system would necessarily involve at least three stars, which is consistent with recent N-body models in which blue stragglers often form via multiple encounters that can involve both single and binary stars. However, we cannot yet entirely rule out the possibility that BSS-7 descended directly from a binary system via mass transfer. The system parameters needed to distinguish definitively between these scenarios may be obtainable from time-resolved spectroscopy.

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arXiv2005

An ultracompact X-ray binary in the globular cluster M15 (NGC7078)

We have used the Advanced Camera for Surveys on board the Hubble Space Telescope to image the core of the globular cluster M15 in the far-ultraviolet (FUV) waveband. Based on these observations, we identify the FUV counterpart of the recently discovered low-mass X-ray binary M15 X-2. Our time-resolved FUV photometry shows a modulation with 0.062+/-0.004 mag semi-amplitude and we clearly detect a period of 22.5806+/-0.0002 min. We have carried out extensive Monte Carlo simulations which show that the signal is consistent with being coherent over the entire observational time range of more than 3000 cycles. This strongly suggests that it represents the orbital period of the binary system. M15 X-2 is FUV bright (approx. 17 mag) and is characterized by an extremely blue spectral energy distribution (F_lambda ~ lambda^{-2.0}). We also find evidence for an excess of flux between 1500 and 1600 Åand probably between 1600 and 2000 Å, which might be due to CIV 1550 and HeII 1640 emission lines. We also show that M15 X-2's X-ray luminosity can be powered by accretion at the rate expected for gravitational-wave-driven mass transfer at this binary period. The observed FUV emission appears to be dominated by an irradiated accretion disk around the neutron star primary, and the variability can be explained by irradiation of the low-mass white dwarf donor if the inclination of the system is approx. 34 degree. We conclude that all observational characteristics of M15 X-2 are consistent with it being an ultracompact X-ray binary, only the third confirmed such object in a globular cluster.

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arXiv2005

Far-Ultraviolet Observations of the Globular Cluster NGC 2808 Revisited: Blue Stragglers, White Dwarfs and Cataclysmic Variables

We present a reanalysis of far-ultraviolet (FUV) observations of the globular cluster NGC 2808 obtained with the Hubble Space Telescope. These data were first analyzed by Brown and coworkers, with an emphasis on the bright, blue horizontal branch (HB) stars in this cluster. Here, our focus is on the population of fainter FUV sources, which include white dwarfs (WDs), blue stragglers (BSs) and cataclysmic variables (CVs). We have therefore constructed the deepest FUV-NUV colour-magnitude diagram of NGC 2808 and also searched for variability among our FUV sources. Overall, we have found approx. 40 WD, approx. 60 BS and approx. 60 CV candidates; three of the BSs and two of the CV candidates are variable. We have also recovered a known RR Lyrae star in the core of NGC 2808, which exhibits massive (approx. 4 mag) FUV variability. We have investigated the radial distribution and found that our CV and BS candidates are more centrally concentrated than the HBs and WD candidates. This might be an effect of mass segregation, but could as well be due to the preferential formation of such dynamically-formed objects in the dense cluster core. For one of our CV candidates we found a counterpart in WFPC2 optical data published by Piotto and coworkers.

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arXiv2005

An Extended Grid of Nova Models: II. The Parameter Space of Nova Outbursts

This paper is a sequel to an earlier paper devoted to multiple, multicycle nova evolution models (Prialnik & Kovetz 1995, first paper of the series), which showed that the different characteristics of nova outbursts can be reproduced by varying the values of three basic and independent parameters: the white dwarf mass-M_{WD}, the temperature of its isothermal core-T_{WD} and the mass transfer rate-Mdot. Apart from being the largest computational classical novae parameter-space survey, we show here that the parameter space is constrained by several analytical considerations and find its limiting surfaces. Consequently, we extend the grid of multicycle nova evolution models presented in Paper I almost to its limits, adding multicycle nova outburst calculations for a considerable number of new parameter combinations. In particular, the extended parameter pace that produces nova eruptions includes low mass transfer rates down to 5e-13 Msun/yr, and more models for low T_{WD}. Resulting characteristics of these runs are added to the former parameter combination results, to provide a full grid spanning the entire parameter space for Carbon-Oxygen white dwarfs. The full grid covers the entire range of observed nova characteristics, even those of peculiar objects, which have not been numerically reproduced until now. Most remarkably, runs for very low Mdot lead to very high values for some characteristics, such as outburst amplitude A>~20, high super-Eddington luminosities at maximum, heavy element abundance of the ejecta Z_{ej}~0.63 and high ejected masses m_{ej}~7e-4 Msun.

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arXiv2004

Far-Ultraviolet Surveys of Globular Clusters: Searching for the Products of Stellar Collisions and Near Misses

Far-ultraviolet (FUV) observations with the Hubble Space Telescope are an excellent way to find and study the hot, blue stellar populations in the cores of globular clusters. These populations include dynamically-formed blue stragglers and interacting binaries (such as cataclysmic variables), i.e. the products of stellar collisions and near misses. Using the cluster 47 Tuc as an example, we show how the combination of FUV imaging and slitless spectroscopy can be used to uncover and study these populations.

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arXiv2001

A Catalog and Atlas of Cataclysmic Variables - The Living Edition

The Catalog and Atlas of Cataclysmic Variables (Edition 1 - 1993) and Edition 2 - 1997) has been a valuable source of information for the cataclysmic variable (CV) community. However, the goal of having a central location for all objects is slowly being lost as each new edition is generated. There can also be a long time delay between new information becoming available on an object and its publication in the catalog. To eliminate these concerns, as well as to make the catalog more accessible, we have created a web site which will contain a ``living'' edition of the catalog. We have also added orbital period information, as well as finding charts for novae, to the catalog.

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arXiv2000

A Far-Ultraviolet Spectroscopic Survey of the Globular Cluster 47 Tuc

We present preliminary results from a FUV spectroscopic and photometric survey of the globular cluster 47 Tuc. Our main goal is to either confirm or rule out the existence of a large population of CVs in the core of this cluster. We have so far identified approximately 425 FUV sources, most of which are probably hot, young WDs. However, we have also found approximately 30 FUV sources whose position in a FUV-optical color-magnitude diagram makes them strong CV candidates. If most or all of these objects are eventually confirmed as CVs, the large CV population predicted by tidal capture theory has finally been found. Our data has also allowed us to resolve the long-standing puzzle surrounding AKO 9, a UV-bright and highly variable 1.1 day binary system in 47 Tuc. AKO 9 is the brightest FUV source in our data and presents a blue FUV spectrum with strong CIV and HeII emission lines. Its spectrum is similar to that of the long-period, DN-type field CV GK Per. Based on this similarity and other evidence, we suggest that AKO 9 is a long-period CV in which mass-transfer is driven by the nuclear evolution of a sub-giant donor star.

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arXiv1997

Stellar Populations and Variable Stars in the Core of the Globular Cluster M5

We report the discovery of a variable blue straggler in the core of the globular cluster M5, based on a 12-hour long series of images obtained with the Planetary Camera aboard the Hubble Space Telescope. In addition, we present the light curves of 28 previously unknown or poorly studied large-amplitude variable stars (all but one are RR Lyrae) in the cluster core. A (V, U-I) color-magnitude diagram shows 24 blue stragglers within 2 core radii of the cluster center. The blue straggler population is significantly more centrally concentrated than the horizontal branch and red giant stars.

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