Papers by Despina Hatzidimitriou
VizieR Online Data Catalog: Variable AGN candidates catalog (Pouliasis+, 2019)
The $\textit{Gaia}$-ESO Survey: the inner disk, intermediate-age open cluster Trumpler 23
Based on data products from observations made with ESO Telescopes at the La Silla Paranal Observa... more Based on data products from observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 188.B-3002. These data products have been processed by the Cambridge Astronomy Survey Unit (CASU) at the Institute of Astronomy, University of Cambridge, and by the FLAMES/UVES reduction team at INAF/Osservatorio Astrofisico di Arcetri. These data have been obtained from the Gaia-ESO Survey Data Archive, prepared and hosted by the Wide Field Astronomy Unit, Institute for Astronomy, University of Edinburgh, which is funded by the UK Science and Technology Facilities Council. This work was partly supported by the European Union FP7 programme through ERC grant number 320360 and by the Leverhulme Trust through grant RPG-2012-541. We acknowledge the support from INAF and Ministero dell’ Istruzione, dell’ Universita’ e della Ricerca (MIUR) in the form of the grant “Premiale VLT 2012”. The results presented here benefit from discussions held during the Gaia-ESO workshops...
VizieR Online Data Catalog: Gaia-ESO Survey: NGC6705 (Cantat-Gaudin+, 2014)
VizieR Online Data Catalog: 46 open clusters GaiaDR2 HR diagrams (Gaia Collaboration, 2018)
New and recurrent X-ray transients in M31 observed with XMM-Newton in January 2016 - part 2
Astronomy & Astrophysics
Aims. We produce a clean and well-characterised catalogue of objects within 100 pc of the Sun fro... more Aims. We produce a clean and well-characterised catalogue of objects within 100 pc of the Sun from the Gaia Early Data Release 3. We characterise the catalogue through comparisons to the full data release, external catalogues, and simulations. We carry out a first analysis of the science that is possible with this sample to demonstrate its potential and best practices for its use. Methods. Theselection of objects within 100 pc from the full catalogue used selected training sets, machine-learning procedures, astrometric quantities, and solution quality indicators to determine a probability that the astrometric solution is reliable. The training set construction exploited the astrometric data, quality flags, and external photometry. For all candidates we calculated distance posterior probability densities using Bayesian procedures and mock catalogues to define priors. Any object with reliable astrometry and a non-zero probability of being within 100 pc is included in the catalogue. Re...
Bright Sources in a Chandra Survey of the Bar Region in the Small Magellanic Cloud Galaxy
ABSTRACT
This paper describes the first results from a 20 deg 2 mosaic of the Small Magellanic Cloud (SMC)... more This paper describes the first results from a 20 deg 2 mosaic of the Small Magellanic Cloud (SMC) in the A21-cm line of neutral hydrogen. The mosaic consists of 320 separate pointings with the 375-m array of the Australia Telescope Compact Array. The angular resolution is 1' • 5 (26 pc, for a distance of 60 kpc) and the velocity resolution is l -6 k m s _ 1 . The images reveal a structure of remarkable complexity, with much of the spatial power contained in high-brightness temperature compact knots and filaments. Numerous wind-blown 'bubbles' and 'supershells' are evident in the data, both inside and outside the stellar confines of the SMC. Some high-density Hi regions are seen to correlate with Ha regions, indicating sites of current star formation. However, many high-column-density HI regions are devoid of optical emission and may represent regions of future star formation. These regions may be under-abundant in diffuse molecular gas due to the high radiation field and low metallicity of the SMC.
Calcium abundances of blue horizontal branch stars in omega Centauri
Spectra of 480 blue horizontal branch (BHB) stars in omega Centauri have been obtained using the ... more Spectra of 480 blue horizontal branch (BHB) stars in omega Centauri have been obtained using the multiobject fiber spectroscopic facility, Autofib, at the 3.9 m AAT. Preliminary analysis in the temperature range 7500K - 8200K indicates that a large fraction of BHB stars are at lower metallicities compared to the RR Lyrae stars and the giants. This could possibly be
The NGC 6426 RR Lyrae Variables and Horizontal-Branch Morphology
The Astronomical Journal, 2000
ABSTRACT
Monthly Notices of the Royal Astronomical Society, 1997
We present the results of a survey of neutral hydrogen emission in the Small Magellanic Cloud (SM... more 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 deg 2 • The rms brightness temperature sensitivity is 1.4 K, corresponding to an H I column density sensitivity of 4 x 10 18 cm -2 for each velocity channel of width 1.6kms-l . The HI distribution is complex and, on scales :51 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 ~O~ 6, and should not be used for deriving total H I column densities.
Confining the time of outburst of an optical nova candidate in M 31
We confirm a nova candidate in M 31 reported to us by K. Nishiyama and F. Kabashima and present a... more We confirm a nova candidate in M 31 reported to us by K. Nishiyama and F. Kabashima and present a pre-discovery detection and upper limit that allows us to tightly constrain the time of outburst for this object. New observations show that the source is still increasing in brightness. The nova candidate was discovered by Nishiyama and Kabashima (Miyaki-Argenteus observatory, Japan) on 2010 September 30.566 UT, with no object visible at its position on September 28.606 UT (private communication).
Atmospheric Chemistry and Physics, 2004
Downwelling longwave fluxes, DLFs, have been derived for each month over a ten year period (1984-... more Downwelling longwave fluxes, DLFs, have been derived for each month over a ten year period (1984-1993), on a global scale with a resolution of 2.5 • × 2.5 •. The fluxes were computed using a deterministic model for atmospheric radiation transfer, along with satellite and reanalysis data for the key atmospheric input parameters, i.e. cloud properties, and specific humidity and temperature profiles. The cloud climatologies were taken from the latest released and improved International Satellite Climatology Project D2 series. Specific humidity and temperature vertical profiles were taken from three different reanalysis datasets; NCEP/NCAR, GEOS, and ECMWF (acronyms explained in main text). DLFs were computed for each reanalysis dataset, with differences reaching values as high as 30 Wm −2 in specific regions, particularly over high altitude areas and deserts. However, globally, the agreement is good, with the rms of the difference between the DLFs derived from the different reanalysis datasets ranging from 5 to 7 Wm −2. The results are presented as geographical distributions and as time series of hemispheric and global averages. The DLF time series based on the different reanalysis datasets show similar seasonal and inter-annual variations, and similar anomalies related to the 86/87 El Niño and 89/90 La Niña events. The global ten-year average of the DLF was found to be between 342.2 Wm −2 and 344.3 Wm −2 , depending on the dataset. We also conducted a detailed sensitivity analysis of the calculated DLFs to the key input data. Plots are given that can be used to obtain a quick assessment of the sensitivity of the DLF to each of the three key climatic quantities, for specific climatic conditions corresponding to different regions of the globe. Our model downwelling fluxes are validated against available data from ground-based stations distributed over the globe, as given by the Baseline Surface Radiation Network. There is a negative bias of the model fluxes when compared against BSRN fluxes, ranging from −7 to −9 Wm −2 , mostly caused by low cloud amount differences between the station and satellite measurements, particularly in cold climates. Finally, we compare our model results with those of other deterministic models and general circulation models.
Atmospheric Chemistry and Physics, 2005
The monthly mean shortwave (SW) radiation budget at the Earth's surface (SRB) was computed on 2.5... more The monthly mean shortwave (SW) radiation budget at the Earth's surface (SRB) was computed on 2.5degree longitude-latitude resolution for the 17-year period from 1984 to 2000, using a radiative transfer model accounting for the key physical parameters that determine the surface SRB, and long-term climatological data from the International Satellite Cloud Climatology Project (ISCCP-D2). The model input data were supplemented by data from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) and European Center for Medium Range Weather Forecasts (ECMWF) Global Reanalysis projects, and other global data bases such as TIROS Operational Vertical Sounder (TOVS) and Global Aerosol Data Set (GADS). The model surface radiative fluxes were validated against surface measurements from 22 stations of the Baseline Surface Radiation Network (BSRN) covering the years 1992-2000, and from 700 stations of the Global Energy Balance Archive (GEBA), covering the period 1984-2000. The model is in good agreement with BSRN and GEBA, with a negative bias of 14 and 6.5 Wm −2 , respectively. The model is able to reproduce interesting features of the seasonal and geographical variation of the surface SW fluxes at global scale. Based on the 17year average model results, the global mean SW downward surface radiation (DSR) is equal to 171.6 Wm −2 , whereas the net downward (or absorbed) surface SW radiation is equal to 149.4 Wm −2 , values that correspond to 50.2 and 43.7% of the incoming SW radiation at the top of the Earth's atmosphere. These values involve a long-term surface albedo equal to 12.9%. Significant increasing trends in DSR and net DSR fluxes were found, equal to 4.1 and 3.7 Wm −2 , respectively, over the 1984-2000 period (equivalent to 2.4 and
Atmospheric Chemistry and Physics, 2004
In the present paper, we have calculated the outgoing longwave radiation at the top of the atmosp... more In the present paper, we have calculated the outgoing longwave radiation at the top of the atmosphere (OLR at TOA) using a deterministic radiation transfer model, cloud data from ISCCP-D, and atmospheric temperature and humidity data from NCEP/NCAR reanalysis, for the seventeen-year period 1984-2000. We constructed anomaly timeseries of the OLR at TOA, as well as of all of the key input climatological data, averaged in the tropical region between 20 • N and 20 • S. We compared the anomaly time-series of the model calculated OLR at TOA with that obtained from the ERBE S-10N (WFOV NF edition 2) non-scanner measurements. The model results display very similar seasonal and inter-annual variability as the ERBS data, and indicate a decadal increase of OLR at TOA of 1.9±0.2 Wm −2 /decade, which is lower than that displayed by the ERBS time-series (3.5±0.3 Wm −2). Analysis of the inter-annual and long-term variability of the various parameters determining the OLR at TOA, showed that the most important contribution to the observed trend comes from a decrease in high-level cloud cover over the period 1984-2000, followed by an apparent drying of the upper troposphere and a decrease in low-level cloudiness. Opposite but small trends are introduced by a decrease in low-level cloud top pressure, an apparent cooling of the lower stratosphere (at the 50 mbar level) and a small decadal increase in mid-level cloud cover.
Atmospheric Chemistry and Physics, 2006
A survey of carbon stars in the SMC (Morgan+, 1995)
Astronomy and Astrophysics Supplement Series, 1997
We present a radial velocity survey of a sample of the field population of carbon stars in the ou... more We present a radial velocity survey of a sample of the field population of carbon stars in the outer parts of the Small Magellanic Cloud (SMC). This first set of results includes radial velocities for 71 carbon stars, with an individual precision of ±2 − 5 km/s. The mean heliocentric velocity of the stars (excluding one very high velocity star) is 149.3±3.0 km/s with a velocity dispersion of 25.2 ± 2.1 km/s. These values drop to 145.5 ± 2.7 km/s and 20.6 ± 1.9 km/s respectively, if we exclude the stars belonging to the Outer Wing. The velocity distribution does not show the multiple peaks seen in some samples of Population I objects. The mass of the SMC as inferred from the above velocity dispersion (without the outer Wing stars) is 1.2 10 9 M .
Proceedings of the International Astronomical Union
Almost all confirmed optical counterparts of HMXBs in the SMC are OB stars with equatorial decret... more Almost all confirmed optical counterparts of HMXBs in the SMC are OB stars with equatorial decretion disks (OBe). These sources emit strongly in Balmer lines and standout when imaged through narrow-band Hα imaging. The lack of secure counterparts for a significant fraction of the HMXBs motivated us to search for more. Using the catalogs for OB/OBe stars (Maravelias et al.2017) and for HMXBs (Haberl & Sturm 2016) we detect 70 optical counterparts (out of 104 covered by our survey). We provide the first identification of the optical counterpart to the source XTEJ0050-731. We verify that 17 previously uncertain optical counterparts are indeed the proper matches. Regarding 52 confirmed HMXBs (known optical counterparts with Hα emission), we detect 39 as OBe and another 13 as OB stars. This allows a direct estimation of the fraction of active OBe stars in HMXBs that show Hα emission at a given epoch to be at least ∼75% of their total HMXB population.
Astronomy & Astrophysics
Context. We carried out new observations of two fields in the star-forming northern ring of M 31 ... more Context. We carried out new observations of two fields in the star-forming northern ring of M 31 with XMM-Newton with each one of them consisting of two exposures of about 100 ks each. A previous XMM-Newton survey of the entire M 31 galaxy revealed extended diffuse X-ray emission in these regions. Aims. We study the population of X-ray sources in the northern disc of M 31 by compiling a complete list of X-ray sources down to a sensitivity limit of ∼7 × 1034 erg s−1 (0.5–2.0 keV) and improve the identification of the X-ray sources. The major objective of the observing programme was the study of the hot phase of the interstellar medium (ISM) in M 31. The analysis of the diffuse emission and the study of the ISM is presented in a separate paper. Methods. We analysed the spectral properties of all detected sources using hardness ratios and spectra if the statistics were high enough. We also checked for variability. In order to classify the sources detected in the new deep XMM-Newton obs...
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Papers by Despina Hatzidimitriou