arXiv subjects

E. Davoust

Publications and source records attributed to E. Davoust.

At least 19 recordsLinked to original sources

arXiv2015

Stellar populations in $ω$ Centauri: a multivariate analysis

We have performed multivariate statistical analyses of photometric and chemical abundance parameters of three large samples of stars in the globular cluster $ω$ Centauri. The statistical analysis of a sample of 735 stars based on seven chemical abundances with the method of Maximum Parsimony (cladistics) yields the most promising results: seven groups are found, distributed along three branches with distinct chemical, spatial and kinematical properties. A progressive chemical evolution can be traced from one group to the next, but also within groups, suggestive of an inhomogeneous chemical enrichment of the initial interstellar matter. The adjustment of stellar evolution models shows that the groups with metallicities [Fe/H]\textgreater{}-1.5 are Helium-enriched, thus presumably of second generation. The spatial concentration of the groups increases with chemical evolution, except for two groups, which stand out in their other properties as well. The amplitude of rotation decreases with chemical evolution, except for two of the three metal-rich groups, which rotate fastest, as predicted by recent hydrodynamical simulations. The properties of the groups are interpreted in terms of star formation in gas clouds of different origins. In conclusion, our multivariate analysis has shown that metallicity alone cannot segregate the different populations of $ω$ Centauri.

astro-ph.GA↗

arXiv2015

Medium resolution spectroscopy and chemical composition of Galactic globular clusters

We used integrated-light medium-resolution spectra of six Galactic globular clusters and model stellar atmospheres to carry out population synthesis and to derive chemical composition and age of the clusters. We used medium-resolution spectra of globular clusters published by Schiavon et al. (2005), as well as our long-slit observations with the 1.93 m telescope of the Haute Provence Observatory. The observed spectra were fitted to the theoretical ones interactively. As an initial approach, we used masses, radii and log g of stars in the clusters corresponding to the best fitting isochrones in the observed color-magnitude diagrams. The computed synthetic blanketed spectra of stars were summed according to the Chabrier mass function. To improve the determination of age and helium content, the shape and depth of the Balmer absorption lines was analysed. The abundances of Mg, Ca, C and several other elements were derived. A reasonable agreement with the literature data both in chemical composition and in age of the clusters is found. Our method might be useful for the development of stellar population models and for a better understanding of extragalactic star clusters.

astro-ph.SR↗

arXiv2014

Gemini spectroscopy of the outer disk star cluster BH176

BH176 is an old metal-rich star cluster. It is spatially and kinematically consistent with belonging to the Monoceros Ring. It is larger in size and more distant from the Galactic plane than typical open clusters, and it does not belong to the Galactic bulge. Our aim is to determine the origin of this unique object by accurately determining its distance, metallicity, and age. The best way to reach this goal is to combine spectroscopic and photometric methods. We present medium-resolution observations of red clump and red giant branch stars in BH176 obtained with the Gemini South Multi-Object Spectrograph.We derive radial velocities, metallicities, effective temperatures, and surface gravities of the observed stars and use these parameters to distinguish member stars from field objects. We determine the following parameters for BH176: $V_h= 0\pm 15$ km/s, $[Fe/H]=-0.1\pm 0.1$, age $7\pm 0.5$ Gyr, $E(V-I)=0.79\pm 0.03$, distance $ 15.2\pm 0.2$ kpc, $α$-element abundance $[α/Fe] \sim 0.25$ dex (the mean of [Mg/Fe], and [Ca/Fe]). BH176 is a member of old Galactic open clusters that presumably belong to the thick disk. It may have originated as a massive star cluster after the encounter of the forming thin disk with a high-velocity gas cloud or as a satellite dwarf galaxy.

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arXiv2012

Modeling and Analysis of a Spectrum of the Globular Cluster NGC 2419

NGC 2419 is the most distant massive globular cluster in the outer Galactic halo. It is unusual also due to the chemical peculiarities found in its red giant stars in recent years. We study the stellar population of this unusual object using spectra obtained at the 1.93-m telescope of the Haute-Provence Observatory. At variance with commonly used methods of high-resolution spectroscopy applicable only to bright stars, we employ spectroscopic information on the integrated light of the cluster. We carry out population synthesis modeling of medium-resolution spectra using synthetic stellar atmosphere models based on a theoretical isochrone corresponding accurately to the observed color-magnitude diagram. We study the influence of non-Local Thermodynamic Equilibrium for some chemical elements on our results. The derived age (12.6 Gyr), [Fe/H]=-2.25 dex, helium content Y=0.25, and abundances of 12 other chemical elements are in a good qualitative agreement with published high-resolution spectroscopy estimates for red giant members in the cluster. On the other hand, the derived element abundance, [alpha/Fe]=0.13 dex (the mean of [O/Fe], [Mg/Fe] and [Ca/Fe]), differs from the published one ([alpha/Fe] =0.4 dex) for selected red giants in the cluster and may be explained by a large dispersion in the alpha-element abundances recently discovered in NGC2419. We suggest that studies of the {\it integrated} light in the cluster using high-resolution spectrographs in different wavelength regions will help to understand the nature of these chemical anomalies.

astro-ph.GA↗

arXiv2012

A CH star in the globular cluster NGC 6426

We report on the serendipitous discovery of a carbon star near the centre of the low-metallicity globular cluster NGC 6426. We determined its membership and chemical properties using medium-resolution spectra. The radial velocity of -159 km/s makes it a member of the cluster. We used photometric data from the literature and the COMARCS stellar atmospheric models to derive its luminosity, effective temperature, surface gravity, metallicity, and approximate C, N, and O abundance ratios. According to these properties, we suggest that this star is a genuine carbon rich low-metallicity AGB star.

astro-ph.GA↗

arXiv2011

One blind and three targeted searches for (sub)millisecond pulsars

We conducted one blind and three targeted searches for millisecond and submillisecond pulsars. The blind search was conducted within 3deg of the Galactic plane and at longitudes between 20 and 110deg. It takes 22073 pointings to cover this region, and 5487 different positions in the sky. The first targeted search was aimed at Galactic globular clusters, the second one at 24 bright polarized and pointlike radiosources with steep spectra, and the third at 65 faint polarized and pointlike radiosources. The observations were conducted at the large radiotelescope of Nancay Observatory, at a frequency near 1400 MHz. Two successive backends were used, first a VLBI S2 system, second a digital acquisition board and a PC with large storage capacity sampling the signal at 50 Mb/s on one bit, over a 24-MHz band and in one polarization. The bandwidth of acquisition of the second backend was later increased to 48 MHz and the sampling rate to 100 Mb/s. The survey used the three successive setups, with respective sensitivities of 3.5, 2.2, and 1.7 mJy. The targeted-search data were obtained with the third setup and reduced with a method based on the Hough transform, yielding a sensitivity of 0.9 mJy. The processing of the data was done in slightly differed time by soft-correlation in all cases. No new short-period millisecond pulsars were discovered in the different searches. To better understand the null result of the blind survey, we estimate the probability of detecting one or more short-period pulsars among a given Galactic population of synthetic pulsars with our setup: 25% for the actual incomplete survey and 79% if we had completed the whole survey with a uniform nominal sensitivity of 1.7 mJy. The alternative of surveying a smaller, presumably more densely populated, region with a higher sensitivity would have a low return and would be impractical at a transit instrument. (abridged)

astro-ph.SR↗

arXiv2010

Looking for a link between Gas Content of Dwarf Galaxies and Properties of their Globular Cluster systems

Star clusters are fundamental building blocks of galaxies. Their formation is related to the density and pressure in progenitor molecular clouds and their environmental conditions. To understand better the dynamical processes driving star formation and chemical evolution, we compare ages, metallicities, and alpha-element abundance ratios of globular clusters in nearby dwarf galaxies of different luminosities and morphological types, and situated in different environments. The data are based on our 6m telescope medium-resolution spectroscopic observations. We find that a mean metallicity of GCs in a galaxy at a given age is higher for early-type dwarfs, than for late-type dwarf irregulars and spirals.

astro-ph.CO↗

arXiv2005

The precision of large radio continuum source catalogues- An application of the SPECFIND tool

The accuracy in position and flux density of 19 large radio continuum source catalogues has been determined using SPECFIND, a new tool recently made available through the CDS. The ~67000 radio continuum spectra with three or more frequencies produced by SPECFIND were used to cross-correlate sources from different catalogues and to calculate offsets in right ascension and declination in the various catalogues with respect to the positions given in the NRAO VLA Sky Survey (NVSS) catalogue, which was adopted as a reference. The flux densities reported in the catalogues were compared to those predicted by the composite spectra, enabling us to assess the quality of the flux density calibration of the different catalogues.

astro-ph↗

arXiv2004

A method for determining radio continuum spectra, and its application to large surveys

A new tool to extract cross-identifications and radio continuum spectra from radio catalogues contained in the VIZIER database of the CDS is presented. The code can handle radio surveys at different frequencies with different resolutions. It has been applied to 22 survey catalogues at 11 different frequencies containing a total of 3.5 million sources, which resulted in over 700000 independent radio cross-identifications and ~67000 independent radio spectra with more than two frequency points. A validation of the code has been performed using independent radio cross-correlations from the literature. The mean error of the determined spectral index is +-0.3. The code produces an output of variable format that can easily be adapted to the purpose of the user.

astro-ph↗

arXiv2003

A Molecular Gas Bridge between the Taffy Galaxies

The Taffy Galaxies system, UGC 12914/5, contains huge amounts of molecular gas in the bridge region between the receding spirals after a direct collision. $2 - 9 \times 10^9$M$_\odot$ of molecular gas is present between the galaxies, more than the CO emission from the entire Milky Way! Such dense gas can only be torn off by collisions between dense clouds, in this case with relative velocities of about 800 $\kms$, such that the remnant cloud acquires an intermediate velocity and is left in the bridge after separation of the colliding galaxies. We suggest that after ionization in the collision front, the gas cooled and recombined very quickly such that the density remained high and the gas left the colliding disks in molecular form.

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arXiv2000

Starbursts in barred spiral galaxies IV: On young bars and the formation of abundance gradients

The oxygen (O/H) and N/O abundance ratios along the bar of 16 barred spiral starburst galaxies are determined using long-slit spectroscopy. The abundance gradients and the spatial distribution of the ionized gas along the bar are used to understand the role of bars in starburst galaxies. The oxygen abundance gradients are steeper than in normal barred galaxies, while the intersects are low. This excludes the possibility that these are chemically evolved galaxies rejuvenated by the effect of a bar. The nitrogen-to-oxygen abundance gradients are flatter than the oxygen ones. But N/O intersects are high, which rules out the possibility that a large quantity of gas was recently funneled by a bar toward the center of a young galaxy. Bars, therefore, cannot be at the origin of the bursts in the nuclei of our sample galaxies. The oxygen and N/O abundance gradients are generally stronger in the bar than in the disk and are linked together by a linear relation. This can be fully explained in terms of star formation history: the gradients build up from the inside out, becoming stronger as the oxygen and N/O abundances increase in the bulge while staying low in the disk. In many of the sample galaxies, star formation occurs at one or both ends of the bar. The low level of chemical enrichment in these regions suggests that they recently experienced bar-triggered star formation: this is the only visible effect of bars. Our analysis shows that bars probably appeared very recently (a few 10^7 years) in the starburst galaxies, which are relatively "young" galaxies still in the process of formation.(Abridged)

astro-ph↗

arXiv2000

The formation of bars and disks in Markarian starburst galaxies

We have proposed in a companion paper (Considere et al. 2000) that bars appeared recently in massive starburst nucleus galaxies. We now test this hypothesis on an extended sample of barred and unbarred Markarian starburst galaxies, using several samples of normal galaxies as control samples. In support of this hypothesis, we show that the proportion of barred galaxies is much lower in Markarian starburst galaxies than in normal galaxies. In addition to this deficiency of bars, we find that Markarian starburst galaxies have smaller disks than normal galaxies, and that the disks of unbarred starburst galaxies are smaller, on average, than barred ones. Finally, we show that the Markarian starburst galaxies do not seem to follow the local Tully--Fisher relation. Various alternatives are examined to explain the deficiency of bars and the small disk dimensions in Markarian starburst galaxies. One possibility, which is in agreement with the young bar hypothesis, is that the formation of disks happens after the formation of bulges and that bars appear only later, when enough gas has been accreted in the disk.

astro-ph↗

arXiv1999

Extensive near-infrared (H-band) photometry in Coma

We present extensive and accurate photometry in the near-infrared H band of a complete sample of objects in an area of about 400 arcmin2 toward the Coma cluster of galaxies. The sample, including about 300 objects, is complete down to H~17 mag, the exact value depending on the type of magnitude (isophotal, aperture, Kron) and the particular region studied. This is six magnitudes below the characteristic magnitude of galaxies, well into the dwarfs' regime at the distance of the Coma cluster. For each object (star or galaxy) we provide aperture magnitudes computed within five different apertures, the magnitude within the 22 mag arcsec2 isophote, the Kron magnitude and radius, magnitude errors, as well as the coordinates, the isophotal area, and a stellarity index. Photometric errors are 0.2 mag at the completness limit. This sample is meant to be the zero-redshift reference for evolutionary studies of galaxies.

astro-ph↗

arXiv1999

The Chemical Evolution of Galaxies by Successive Starbursts

We propose an evolutionary scenario by successive bursts of star formation to reproduce the chemical properties of massive nearby Starburst Nucleus Galaxies (SBNGs). The N/O abundance ratios in SBNGs are 0.2 dex higher than in normal HII regions observed in the disks of late-type spirals. The variation of the N/O ratio as a function of metallicity follows a "primary + secondary" relation, but the increase of nitrogen does not appear as a continuous process. Assuming that nitrogen is produced by intermediate-mass stars, we show that our observations are consistent with a model where the bulk of nitrogen in SBNGs was formed during past sequences of bursts of star formation which probably started 2 or 3 Gyrs in the past.

astro-ph↗

arXiv1999

The abundance of nitrogen in starburst nucleus galaxies

We show that the excess of nitrogen emission observed in a large sample of Starburst Nucleus Galaxies (SBNGs) can only be explained at a given metallicity by an overabundance of nitrogen with respect to normal HII regions in the disks of late-type spirals. The N/O ratios in the SBNGs are comparable to the values found in the bulges of normal early-type spirals, which suggests that what we observe could be the main production of nitrogen in the bulges of these galaxies. The variation of the N/O ratio as a function of metallicity in SBNGs follows a primary + secondary relation, but the increase of nitrogen does not appear as a continuous process. In SBNGs, nitrogen is probably produced by different populations of intermediate-mass stars, which were formed during past sequences of bursts of star formation. This assumption pushes the origin of the main bursts 2-3 Gyrs back in the past. On a cosmological scale, this time interval corresponds to redshifts z = 0.2-0.3, where a significant increase of star formation activity occurred. The origin of the SBNG phenomenon would thus have cosmological implications, it would be related to a more active phase of star formation in the Universe sometime in its recent past.

astro-ph↗

arXiv1999

Starbursts in barred spiral galaxies. V. Morphological analysis of bars

We have measured the bar lengths and widths of 125 barred galaxies observed with CCDs. The dependence of bar strength (identified with bar axis ratio) on morphological type, nuclear activity, central and mid-bar surface brightness is investigated. The properties of the bars are best explained if the sample is divided into early- (< SBbc) and late-type galaxies, and into active (starburst, Seyfert or LINER) and normal galaxies. We find that galaxies with very long bars are mostly active and that normal late-type galaxies have a distinct behavior from the three other groups of galaxies. We confirm earlier findings that active late-type galaxies tend to have both stronger and longer bars than normal ones. An important result of this paper is that early-type galaxies do not share this behavior : they all tend to have strong bars, whether they are active or not. We also find correlations between bar strength and relative surface brightness in the middle and at the edge of the bar, which are not followed by normal late-type galaxies. These results are interpreted in the light of recent numerical simulations and paradigms about galaxy evolution. They suggest that normal late-type galaxies represent the first stage of galaxy evolution, and that bars in early- and late-type galaxies do not have the same properties because they have a different origin.

astro-ph↗

arXiv1998

The PDS starburst galaxies

(Abridged) We discuss the nature of the galaxies found in the Pico dos Dias Survey (PDS) for young stellar objects. The PDS galaxies were selected from the IRAS Point Source catalog. They have flux density of moderate or high quality at 12, 25 and 60 $μ$m and spectral indices in the ranges $-3.00 \leq α(25,12) \leq +0.35$ and $-2.50 \leq α(60,25) \leq +0.85$. These criteria allowed the detection of 382 galaxies, which are a mixture of starburst and Seyfert galaxies. The starburst galaxies show an excess of FIR luminosity and their IRAS colors are significantly different from those of Seyfert galaxies -- 99% of the starburst galaxies in our sample have a spectral index $α(60,25) < -1.9$. As opposed to Seyfert galaxies, very few PDS starbursts are detected in X-rays. In the infrared, the starburst galaxies form a continuous sequence with normal galaxies. But they generally can be distinguished from normal galaxies by their spectral index $α(60,25) > -2.5$. This color cut--off also marks a change in the dominant morphologies of the galaxies: the normal IRAS galaxies are preferentially late--type spirals (Sb and later), while the starbursts are more numerous among early--type spirals (earlier than Sbc). No difference is found between the starbursts detected in the FIR and those detected on the basis of UV excess. The PDS starburst galaxies represent the FIR luminous branch of the UV-bright starburst nucleus galaxies, with mean FIR luminosity $\log({\rm L}_{\rm IR}/{\rm L}_\odot) = 10.3 \pm 0.5$ and redshifts smaller than 0.1. They form a complete sample limited in flux in the FIR at $2\times10^{-10}$ erg cm$^{-2}$ s$^{-1}$.

astro-ph↗

arXiv1997

Starbursts in Barred Spiral Galaxies. III. Definition of a homogeneous sample of Starburst Nucleus Galaxies

This paper presents optical long-slit spectroscopic observations of 105 barred Markarian IRAS galaxies. These observations are used to determine the spectral type of emission-line regions in the nucleus and along the bar of the galaxies, in order to define a homogeneous sample of Starburst Nucleus Galaxies (SBNGs). Our selection criteria have been very efficient for selecting star-forming galaxies, since our sample of 221 emission-line regions includes 82% nuclear or extranuclear starbursts. The contamination by Seyferts is low (9%). The remaining galaxies (9%) are objects with ambiguous classification (HII or LINER). The dust content and Halpha luminosity increase towards the nuclei of the galaxies. No significant variation of the electron density is found between nuclear and bar HII regions. However, the mean Halpha luminosity and electron density in the bar are higher than in typical disk HII regions. We investigate different mechanisms for explaining the excess of nitrogen emission observed in our starburst nuclei. There is no evidence for the presence of a weak hidden active galactic nucleus in our starburst galaxies. The cause of this excess is probably a selective enrichment of nitrogen in the nuclei of the galaxies, following a succession of short and intense bursts of star formation. Our sample of SBNGs, located at a mean redshift of 0.015, has moderate Halpha (10^41 erg/s) and far infrared (10^10 Lsun) luminosities. The types are distributed equally among early- and late-type giant spirals with a slight preference for Sbc/Sc types because of their barred morphology. The majority (62%) of SBNGs are isolated with no sign of gravitational interaction. In terms of distance, luminosity and level of interaction, SBNGs are intermediate between HII galaxies and luminous infrared galaxies.

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