We present B, V, R and I photometry for NGC 6779 (M56), a metal-poor globular cluster in the galactic halo. The observations were performed using the 1.3-m telescope at Skinakas Observatory, in Crete. The reddening of the cluster was found to be E(B−V) = 0.32 ± 0.02 [E(V−I) = 0.43 ± 0.02], significantly higher than previous estimates. The metal abundance of the cluster was derived from various parametrizations of red giant branch characteristics and it was found to be [Fe/H]ZW=−2.20 ± 0.12 dex on the Zinn–West metallicity scale, or [Fe/H]CG=−2.00 ± 0.21 dex on the Carretta–Gratton scale. The distance modulus of the cluster is estimated to be (m−M)V= 15.62 ± 0.26 (or 14.62, if we correct for the reddening to the cluster). The horizontal branch of NGC 6779 shows a clear gap at (B−V)o= 0.0. Finally, the revised value for the metallicity of NGC 6779 led to a revision of its age to 13 Gyr, using the age-index calibrations of Salaris & Weiss.

Distance modulus

Horizontal branch

Red-giant branch

10.1111/j.1365-2966.2004.07439.x

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Citations (7)

Three X-ray transients in M 31 observed with Swift

Astronomy and Astrophysics (2008)

R. Voss W. Pietsch F. Haberl H. Stiele J. Greiner

The purpose of this study is to find transient X-ray sources in M31, and to investigate and classify their nature. Three X-ray transients were observed with Swift. For each of the three X-ray transients we use the Swift X-ray and optical data together with observations from XMM-Newton and Chandra to investigate the lightcurves and the spectra of the outburst, and thereby to identify the source types. The outburst of XMMU J004215.8+411924 lasted for about one month. The source had a hard power-law spectrum with a photon index of 1.6. It was previously identified as a Be/X-ray binary based on the optical identification with a star. However, we show that with improved source coordinates it is clear that the optical source is not the counterpart to the X-ray source. The source SWIFT J004217.3+411532 had a bright outburst, after which it slowly decayed over half a year. The spectrum was soft, corresponding to a thermal accretion disk with innermost temperature of 250-600 eV. The source was not seen in the optical, and the soft spectrum indicates that the source is most likely a black hole low mass X-ray binary. M31N 2006-11a is a nova that was previously observed in the optical. We detected it both in X-rays and UV with Swift half a year after the optical maximum, after which it decayed below the Swift detection threshold within a month. The spectrum of the X-ray transient can be modelled by a black-body with a temperature of 50 eV. We use catalogues of X-ray transients in M31 to estimate their rate, and we find a lower limit of 9/yr.

Swift

Transient (computer programming)

X-ray transient

10.1051/0004-6361:20078970

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Citations (10)

Gaia Focused Product Release: Radial velocity time series of long-period variables

Astronomy and Astrophysics (2023)

Michele Trabucchi N. Mowlavï T. Lebzelter I. Lecœur-Taı̈bi M. Audard

Context. The third Gaia Data Release (DR3) provided photometric time series of more than 2 million long-period variable (LPV) candidates. Anticipating the publication of full radial-velocity data planned with Data Release 4, this Focused Product Release (FPR) provides radial-velocity time series for a selection of LPV candidates with high-quality observations. Aims. We describe the production and content of the Gaia catalog of LPV radial-velocity time series, and the methods used to compute the variability parameters published as part of the Gaia FPR. Methods. Starting from the DR3 catalog of LPV candidates, we applied several filters to construct a sample of sources with high-quality radial-velocity measurements. We modeled their radial-velocity and photometric time series to derive their periods and amplitudes, and further refined the sample by requiring compatibility between the radial-velocity period and at least one of the G , G BP , or G RP photometric periods. Results. The catalog includes radial-velocity time series and variability parameters for 9614 sources in the magnitude range 6 ≲ G /mag ≲ 14, including a flagged top-quality subsample of 6093 stars whose radial-velocity periods are fully compatible with the values derived from the G , G BP , and G RP photometric time series. The radial-velocity time series contain a mean of 24 measurements per source taken unevenly over a duration of about three years. We identify the great majority of the sources (88%) as genuine LPV candidates, with about half of them showing a pulsation period and the other half displaying a long secondary period. The remaining 12% of the catalog consists of candidate ellipsoidal binaries. Quality checks against radial velocities available in the literature show excellent agreement. We provide some illustrative examples and cautionary remarks. Conclusions. The publication of radial-velocity time series for almost ten thousand LPV candidates constitutes, by far, the largest such database available to date in the literature. The availability of simultaneous photometric measurements gives a unique added value to the Gaia catalog.

Radial velocity

10.1051/0004-6361/202347287

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Citations (6)

Calcium Abundances of Blue Horizontal Branch Stars in Omega Centauri

Noella L. D’Cruz R. J. Dickens D. Hatzidimitriou R. T. Rood R. W. O’Connell

Horizontal branch

Source

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An H I aperture synthesis mosaic of the Small Magellanic Cloud

Monthly Notices of the Royal Astronomical Society (1997)

L. Stavely-Smith Rappaport Saul D. Hatzidimitriou M. J. Kesteven D. McConnell

We present the results of a survey of neutral hydrogen emission in the Small Magellanic Cloud (SMC) with the Australia Telescope Compact Array (ATCA). The survey consists of a mosaic of 320 separate pointings of the 375-m array, resulting in a resolution of 1.6 arcmin (28 pc, for a distance of 60 kpc) over a field of 20 deg2. The rms brightness temperature sensitivity is 1.4 K, corresponding to an Hi column density sensitivity of 4 × 1018 cm−2 for each velocity channel of width 1.6km s∽1. The Hi distribution is complex and, on scales ≲1 kpc, appears to be dominated by the effects of expanding H i shells, which are probably driven by the combined effects of supernovae and stellar winds from massive stars. The picture of the SMC that arises from the current data seems to challenge the earlier belief that the SMC consists of two or more spatially separate structures with different systemic velocities. We find that the observed multiple components are, in many cases, caused by the combined effects of the numerous shells and supershells. Altogether, we identify six supershells (defined here as those with radii greater than 300 pc) and 495 giant shells. For each of these, we measure positions, radii, velocities and expansion rates, and derive ages and kinetic energy requirements. The apparent age distribution of shells is remarkably narrow, with a mean age of 5.4 Myr and an intrinsic dispersion of 2 Myr. Southern shells appear to be older, on average, by 2.5 Myr. The kinetic energy of the shells is a large fraction of the gravitational binding energy of the SMC, implying that further disintegration of the SMC will occur with time, and especially at the next close passage with the Large Magellanic Cloud (LMC) or the Galaxy, unless the SMC possesses a massive halo. Because of their interferometric nature, the images presented here are insensitive to structures of size ≳0° .6, and should not be used for deriving total H i column densities.

Large Magellanic Cloud

Small Magellanic Cloud

Local Group

10.1093/mnras/289.2.225

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Citations (153)

Spectroscopic confirmation of four novae in M31

E. A. Barsukova N. V. Borisov S. Fabrika O. Sholukhova A. F. Valeev

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Citations (0)

Carbon Stars in the SMC

Springer eBooks (1994)

D. H. Morgan D. Hatzidimitriou

Carbon star

Schmidt camera

Prism

Carbon fibers

10.1007/978-94-011-1146-1_108

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Gaia Focused Product Release: Radial velocity time series of long-period variables

arXiv (Cornell University) (2023)

Gaia Collaboration Gaia Collaboration Michele Trabucchi N. Mowlavï T. Lebzelter

The third Gaia Data Release (DR3) provided photometric time series of more than 2 million long-period variable (LPV) candidates. Anticipating the publication of full radial-velocity (RV) in DR4, this Focused Product Release (FPR) provides RV time series for a selection of LPVs with high-quality observations. We describe the production and content of the Gaia catalog of LPV RV time series, and the methods used to compute variability parameters published in the Gaia FPR. Starting from the DR3 LPVs catalog, we applied filters to construct a sample of sources with high-quality RV measurements. We modeled their RV and photometric time series to derive their periods and amplitudes, and further refined the sample by requiring compatibility between the RV period and at least one of the $G$, $G_{\rm BP}$, or $G_{\rm RP}$ photometric periods. The catalog includes RV time series and variability parameters for 9\,614 sources in the magnitude range $6\lesssim G/{\rm mag}\lesssim 14$, including a flagged top-quality subsample of 6\,093 stars whose RV periods are fully compatible with the values derived from the $G$, $G_{\rm BP}$, and $G_{\rm RP}$ photometric time series. The RV time series contain a mean of 24 measurements per source taken unevenly over a duration of about three years. We identify the great most sources (88%) as genuine LPVs, with about half of them showing a pulsation period and the other half displaying a long secondary period. The remaining 12% consists of candidate ellipsoidal binaries. Quality checks against RVs available in the literature show excellent agreement. We provide illustrative examples and cautionary remarks. The publication of RV time series for almost 10\,000 LPVs constitutes, by far, the largest such database available to date in the literature. The availability of simultaneous photometric measurements gives a unique added value to the Gaia catalog (abridged)

Radial velocity

10.48550/arxiv.2310.06051

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New Results on the Magellanic Clouds

Springer eBooks (1994)

D. Hatzidimitriou

10.1007/978-94-011-1146-1_104

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Citations (1)