arXiv subjects

I. Saviane

Publications and source records attributed to I. Saviane.

At least 73 records · Page 4Linked to original sources

arXiv2006

Early spectral evolution of Nova Sgr 2004 (V5114 Sgr)

We present optical and near-infrared spectral evolution of the Galactic nova V5114 Sgr (2004) during few months after the outburst. We use multi-band photometry and line intensities derived from spectroscopy to put constrains on the distance and the physical conditions of the ejecta of V5114 Sgr. The nova showed a fast decline (t_2 \simeq 11 days) and spectral features of FeII spectroscopic class. It reached M_V = -8.7 \pm 0.2 mag at maximum light, from which we derive a distance of 7700 \pm 700 kpc and a distance from the galactic plane of about 800 pc. Hydrogen and Oxygen mass of the ejecta are measured from emission lines, leading to 10^{-6} and 10^{-7} M_\odot, respectively. We compute the filling factor of the ejecta to be in the range 0.1 -- 10^{-3} . We found the value of the filling factor to decrease with time. The same is also observed in other novae, then giving support to the idea that nova shells are not homogeneously filled in, rather being the material clumped in relatively higher density blobs less affected by the general expanding motion of the ejecta.

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arXiv2005

Photometric monitoring of the doubly imaged quasar UM673: possible evidence for chromatic microlensing

We present the results of two-band CCD photometric monitoring of the gravitationally lensed quasar Q 0142-100 (UM 673).The data, obtained at ESO-La Silla with the 1.54 m Danish telescope in the Gunn i-band (October 1998 - September 1999) and in the Johnson V-band (October 1998 to December 2001), were analyzed using three different photometric methods. The light-curves obtained with all methods show variations, with a peak-to-peak amplitude of 0.14 magnitude in $V$. Although it was not possible to measure the time delay between the two lensed QSO images, the brighter component displays possible evidence for microlensing: it becomes bluer as it gets brighter, as expected under the assumption of differential magnification of a quasar accretion disk

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arXiv2005

HST/ACS observations of the old and metal-poor Sagittarius dwarf irregular galaxy

We have obtained deep color-magnitude diagrams of the Sagittarius dwarf irregular galaxy with the HST/ACS. The new diagrams unveil for the first time the star formation history of SagDIG. We identify a small population of red horizontal branch stars, which sets the first epoch of star formation in SagDIG back to ~9-10 Gyr ago. This shows that SagDIG has been able to maintain a very low mean metallicity (-2.2<[Fe/H]<-1.9) over a 10 Gyr life time, and most importantly, that all Local Group dwarf galaxies share an ancient (~10 Gyr) initial episode of star formation. Thus, at the moment, IZw18 remains the only exception to this general trend, favoring the so-called ``young galaxy'' scenario. However, a re-analysis of ACS observations of IZw18 suggests that an older RGB population may be present also in IZw18. We also compare the distribution of atomic hydrogen with that of stellar populations of various ages. We find little correlation between stellar populations older than ~1 Gyr and the HI. In particular we fail to find any generation of stars that preferentially lie within the large HI hole. We therefore exclude the possibility that the SagDIG ring-like HI distribution is the result of multiple supernova events. Alternative scenarios have to be explored.

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arXiv2003

The Stellar Content of the Southern Tail of NGC 4038/9 and a Revised Distance

We have used the Hubble Space Telescope and Wide Field Planetary Camera 2 to image the putative tidal dwarf galaxy located at the tip of the Southern tidal tail of NGC 4038/9, the Antennae. We resolve individual stars, and identify two stellar populations. Hundreds of massive stars are present, concentrated into tight OB associations on scales of 200 pc, with ages ranging from 2-100 Myr. An older stellar population is distributed roughly following the outer contours of the neutral hydrogen in the tidal tail; we associate these stars with material ejected from the outer disks of the two spirals. The older stellar population has a red giant branch tip at I=26.5+/-0.2 from which we derive a distance modulus (m-M)_0=30.7+/-0.25. The implied distance of 13.8+/-1.7 Mpc is significantly smaller than commonly quoted distances for NGC 4038/9. In contrast to the previously studied core of the merger, we find no super star clusters. One might conclude that SSCs require the higher pressures found in the central regions in order to form, while spontaneous star formation in the tail produces the kind of O-B star associations seen in dwarf irregular galaxies. The youngest population in the putative tidal dwarf has a total stellar mass of ~2x10^5 Msun, while the old population has a stellar mass of \~7x10^7 Msun. If our smaller distance modulus is correct, it has far-reaching consequences for this proto-typical merger. Specifically, the luminous to dynamical mass limits for the tidal dwarf candidates are significantly less than 1, the central super star clusters have sizes typical of galactic globular clusters rather than being 1.5 times as large, and the unusually luminous X-ray population becomes both less luminous and less populous.

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arXiv2003

Discovery of New Milky Way Star Cluster Candidates in the 2MASS Point Source Catalog II. Physical Properties of the Star Cluster CC01

Three new obscured Milky Way clusters were detected as surface density peaks in the 2MASS point source catalog during our on-going search for hidden globular clusters and massive Arches-like star clusters. One more cluster was discovered serendipitously during a visual inspection of the candidates. The first deep J, H, and Ks imaging of the cluster [IBP2002] CC01 is presented. We estimated a cluster age of ~1-3 Myr, distance modulus of (m-M)0=12.56+-0.08 mag (D=3.5 Kpc), and extinction of AV~7.7 mag. We also derived the initial mass function slope for the cluster: Gamma=-2.23+-0.16. The integration over the initial mass function yielded a total cluster mass M_{total}<=1800+-200Msol. CC01 appears to be a regular, not very massive star cluster, whose formation has probably been induced by the shock front from the nearby HII region Sh2-228.

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arXiv2003

Variable stars as tracers of stellar populations in Local Group galaxies: Leo I and NGC 6822

Results are presented of a study of the variable star populations in the dwarf spheroidal galaxy Leo I and in the dwarf irregular galaxy NGC 6822, based on time series photometry obtained with the Wide Field Imager of the 2.2 m ESO/MPI telescope (Leo I) and the Very Large Telescope (NGC 6822). We found about 250 (lower limit) variables in Leo I most of which are RR Lyrae stars. In NGC 6822 we identified 450 candidate variables among which about 20 are RR Lyrae stars, and many are low-luminosity, small-amplitude Cepheids.

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arXiv2003

Clues on the evolution of the Carina dwarf spheroidal galaxy from the color distribution of its red giant stars

The thin red giant branch (RGB) of the Carina dwarf spheroidal galaxy appears at first sight quite puzzling and seemingly in contrast with the presence of several distinct bursts of star formation. In this Letter, we provide a measurement of the color spread of red giant stars in Carina based on new BVI wide-field observations, and model the width of the RGB by means of synthetic color-magnitude diagrams. The measured color spread, Sigma{V-I}=0.021 +/- 0.005, is quite naturally accounted for by the star-formation history of the galaxy. The thin RGB appears to be essentially related to the limited age range of its dominant stellar populations, with no need for a metallicity dispersion at a given age. This result is relatively robust with respect to changes in the assumed age-metallicity relation, as long as the mean metallicity over the galaxy lifetime matches the observed value ([Fe/H] = -1.91 +/- 0.12 after correction for the age effects). This analysis of photometric data also sets some constraints on the chemical evolution of Carina by indicating that the chemical abundance of the interstellar medium in Carina remained low throughout each episode of star formation even though these episodes occurred over many Gyr.

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arXiv2003

The emission distribution of RR Pictoris

We present time-resolved optical spectroscopy of the old nova RR Pic. Two emission lines (Halpha and He I) are present in the observed part of the spectrum and both show strong variability. Halpha has been used for Doppler tomography in order to map the emission distribution in this system for the first time. The resulting map shows the emission from the disc as well as two additional emission sources on the leading and trailing side of the disc. Furthermore we find evidence for the presence of either a disc-overflow or an asymmetric outflow from the binary with velocities up to +- 1200km/s. The origin of the outflow would be the emission source on the leading side of the accretion disc.

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arXiv2002

Age and Metallicity Distribution of the Galactic Bulge from Extensive Optical and Near-IR Stellar Photometry

We present a new determination of the metallicity distribution, age, and luminosity function of the Galactic bulge stellar population. By combining near-IR data from the 2MASS survey, from the SOFI imager at ESO NTT and the NICMOS camera on board HST we were able to construct color-magnitude diagrams (CMD) and luminosity functions (LF) with large statistics and small photometric errors from the Asymptotic Giant Branch (AGB) and Red Giant Branch (RGB) tip down to $\sim 0.15 M_\odot$. This is the most extended and complete LF so far obtained for the galactic bulge. Similar near-IR data for a disk control field were used to decontaminate the bulge CMDs from foreground disk stars, and hence to set stronger constraint on the bulge age, which we found to be as large as that of Galactic globular clusters, or $\gsim 10$ Gyr. No trace is found for any younger stellar population. Synthetic CMDs have been constructed to simulate the effect of photometric errors, blending, differential reddening, metallicity dispersion and depth effect in the comparison with the observational data. By combining the near-IR data with optical ones, from the Wide Field Imager at the ESO/MPG 2.2m telescope, a disk-decontaminated $(M_K,V-K)$ CMD has been constructed and used to derive the bulge metallicity distribution, by comparison with empirical RGB templates. The bulge metallicity is found to peak at near solar value, with a sharp cutoff just above solar, and a tail towards lower metallicity that does not appreciably extend below [M/H]$\sim-1.5$.

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arXiv2002

HST color-magnitude diagrams of 74 galactic globular clusters in the HST F439W and F555W bands

We present the complete photometric database and the color-magnitude diagrams for 74 Galactic globular clusters observed with the HST/WFPC2 camera in the F439W and F555W bands. A detailed discussion of the various reduction steps is also presented, and of the procedures to transform instrumental magnitudes into both the HST F439W and F555W flight system and the standard Johnson B and V systems. We also describe the artificial star experiments which have been performed to derive the star count completeness in all the relevant branches of the color magnitude diagram. The entire photometric database and the completeness function will be made available on the Web immediately after the publication of the present paper.

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arXiv2002

New abundance measurements in UKS 1927-177, a very metal-poor galaxy in the Local Group

We present new results from optical spectroscopy of the brightest Hii region in the dwarf irregular galaxy UKS 1927-177 in Sagittarius (SagDIG). From high signal-to-noise spectra, reddening-corrected line flux ratios have been measured with typical uncertainties of a few percent, from which the oxygen abundance is rediscussed, and new abundance estimates are derived for N and Ne. The O abundance in SagDIG, estimated with the empirical abundance indicator R23 and other methods, is in the range 12+log(O/H)=7.26 to 7.50. The fact that SagDIG is ~10 times closer than IZw18 makes it an ideal target to test the hypothesis of the existence of young galaxies in the present-day universe. Indeed, stellar photometry suggests that this galaxy may harbor a stellar population older than a few Gyr, and possibly an old stellar component as well. The case of SagDIG therefore supports the view that very low chemical abundances can be maintained throughout the life of a dwarf stellar system, even in the presence of multiple star formation episodes.

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arXiv2002

The Sagittarius dwarf irregular galaxy: metallicity and stellar populations

We present deep $BVI$ observations of the dwarf irregular galaxy UKS1927-177 in Sagittarius. Statistically cleaned $V$, $(B-I)$ CMDs clearly display the key evolutionary features in this galaxy. Previously detected C stars are located in the CMDs and shown to be variable, thus confirming the presence of a significant upper-AGB intermediate age population. A group of likely red supergiants is also identified, whose magnitude and color is consistent with a 30 Myr old burst of star formation. The observed colors of both blue and red stars in SagDIG are best explained by introducing a differential reddening scenario in which internal dust extinction affects the star forming regions. Adopting a low reddening for the red giants, $E(B-V) = 0.07 \pm 0.02$, gives [Fe/H]=$-2.1 \pm 0.2$ for the mean stellar metallicity, a value consistent with the [O/H] abundance measured in the HII regions. This revised metallicity, which is in accord with the trend of metallicity against luminosity for dwarf irregular galaxies, is indicative of a ``normal'', although metal-poor, dIrr galaxy. A quantitative description is given of the spatial distribution of stars in different age intervals, in comparison with the distribution of the neutral hydrogen. We find that the youngest stars are located near the major peaks of emission on the HI shell, whereas the red giants and intermediate-age C stars define an extended halo or disk with scale length comparable to the size of the hydrogen cloud. The relationship between the distribution of ISM and star formation is briefly discussed.

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arXiv2001

RR Lyrae variables in the dwarf spheroidal galaxy Leo I

We report the discovery of a significant population of RR Lyrae variables in the dwarf spheroidal galaxy Leo I. Based on 40 V and 22 B images of the galaxy taken using the ESO Wide Field Imager we have identified so far 74 candidate RR Lyrae's in two CCD's hosting the main body of the galaxy. Full coverage of the light variations and pulsation periods have been obtained for 54 of them, 47 of which are Bailey {\it ab}-type RR Lyrae's (RRab's) and 7 are {\it c}-type (RRc's). The period distribution of the presently confirmed sample of RRab's peaks at P=0\fd60, with a minimum period of 0\fd54. The pulsational properties indicate for Leo I an intermediate Oosterhoff type, similar to other dwarf galaxies in the Local Group and the LMC. However, the rather long minimum period of the {\it ab}-type variables, and the significant number of RRab's with long period and large amplitude, suggest that the bulk of the old population in Leo I is more like the Oosterhoff type II globular clusters. The most straightforward interpretation is that a range in metallicity is present among the RR Lyrae's of Leo I, with a significant population of very metal-poor stars. Alternatively, these OoII variables could be more evolved. The average apparent magnitude of the RR Lyrae's across the full cycle is $ = 22.60 \pm 0.12$ mag, yielding a distance modulus $(m-M)_{V,0}= 22.04\pm 0.14$ mag for Leo I on the ``long'' distance scale.

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arXiv2001

Wide Field HR diagrams of Local Group Dwarf Galaxies

We present new observations of Sextans and Leo I dwarf galaxies, that provide new insight on their stellar populations. The large sample of stars measured in Sextans allowed us to obtain a reliable estimate of the metal content of this galaxy, to study the population spatial gradients, and to investigate the nature of the large number of blue stragglers. In Leo I, we discovered a significant population of RR Lyrae variables. The analysis of the pulsational properties of the detected variables suggests that this galaxy is an Oosterhoff intermediate object, similar to the LMC and to other dwarf galaxies in the Local Group.

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arXiv2001

Luminosity and mass function of galactic open clusters: I. NGC 4815

We present deep V and I photometry for the open cluster NGC 4815 and four surrounding Galactic fields down to a limiting magnitude $V\sim25$. These data are used to study cluster spatial extension by means of star counts, and to derive the luminosity (LF) and mass function (MF). The radius turns out to be $3.6\pm0.3 arcmin$ at V=19.0 level, whereas the mass amounts at $880\pm230 m_{\odot}$ down to V=20.8. From the color-magnitude diagram, we obtain the LFs in the V and I bands, using both the standard histogram and an adaptive kernel. After correction for incompleteness and field star contamination, the LFs have been transformed into the present day mass functions (PDMF). The PDMFs from the V and I photometry can be represented as a power-law with a slope $α= 3.1\pm0.3$ and $α= 2.9\pm0.3$ (the \cite{salp55} MF in this notation has a slope $α= 2.35$) respectively, in the mass range $2.5 \leq \frac{m}{m_{\odot}} \leq 0.8$. Below this mass, the MF cannot be considered as representative of the cluster IMF, as it is the result of the combined effect of strong irregularities in the stellar background, probable internal dynamical evolution of the cluster and/or interaction of the cluster with the dense Galactic field. Unresolved binaries and mass segregation can only flatten the apparent derived IMF, so we expect that the real IMF must be steeper than the quoted slope by an unknown amount.

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arXiv2000

The anomalous Galactic globular cluster NGC 2808

We present UBVI photometry for about 60,000 stars in a region of 44'x33' centered on the Galactic globular cluster (GGC) NGC 2808. The central region has been mapped with the HST WFPC2 camera. We cover a field which extends from the center to about 1.7 tidal radii. We confirm the anomalously elongated HB with gaps along it, and we show that the gaps are statistically significant. We find that: 1) the extended blue tail of HB (EBT) is present beyond 400''(=9r_h), and extends to V=21.2 also in these external regions; 2) also the gaps on the EBT are present at least out to 400'' from the cluster center, and possibly beyond that. The location of the gaps in the HB seems to be the same all over the cluster. 3) there are no significant radial gradients in the distribution of the stars in the HB. The observational facts presented in this paper seem to exclude the possibility that the EBTs originate from tidal stripping of the envelope of the RGB stars due to close encounters in high density environments or to mass transfer in close binaries. Also, it is not clear whether the presence of EBTs and gaps on them are the manifestation of the same physical phenomenon. We have also shown that the jump recently identified in the HB using Str"omgren-u photometry is clearly visible also in the U, U-B plane, at T_e~11,600K. Finally, we present the luminosity function (LF) of the RGB. The LF clearly shows the presence of the usual RGB bump, but also a second feature, 1.4 magnitudes brighter in V, that we have named RGB heap. The RGB heap, visible also in other GGCs, is a new feature that, because of its position on the RGB, we have tentatively associated with the recently discovered K giant variables.

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arXiv2000

Photometric catalog of nearby globular clusters (II)

In this paper we present the second and final part of a large and photometrically homogeneous CCD color-magnitude diagram (CMD) data base, comprising 52 nearby Galactic globular clusters (GGC) imaged in the V and I bands. The catalog has been collected using only two telescopes (one for each hemisphere). The observed clusters represent 75% of the known Galactic globulars with (m-M)_v <= 16.15 mag, cover most of the globular cluster metallicity range (-2.2 <= [Fe/H] <= -0.4), and span Galactocentric distances from ~1.2 to ~18.5 kpc. In particular, here we present the CMDs for the 13 GGCs observed in the Northern hemisphere. The remaining 39 Southern hemisphere clusters of the catalog have been presented in a companion paper (Rosenberg et al., 2000, A&AS 144, 5). We present the first CCD color magnitude diagram for NGC6779 (M56). All the CMDs extend from at least ~2 magnitudes below the turn-off (i.e. V_lim ~22) to the tip of the red giant branch. The calibration has been done using a large number of standard stars, and the absolute calibration is reliable to a ~0.02 mag level in both filters. This catalog, because of its homogeneity, is expected to represent a useful data base for the measurement of the main absolute and relative parameters characterizing the CMD of GGCs.

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arXiv2000

Photometric catalog of nearby globular clusters (I)

We present the first part of the first large and homogeneous CCD color-magnitude diagram (CMD) data base, comprising 52 nearby Galactic globular clusters (GGC) imaged in the V and I bands using only two telescopes (one for each hemisphere). The observed clusters represent 75% of the known Galactic globulars with (m-M)v<=16.15 mag, cover most of the globular cluster metallicity range (-2.2 <= [Fe/H] <= -0.4), and span Galactocentric distances from ~1.2 to ~18.5 kpc. In this paper, the CMDs for the 39 GGCs observed in the southern hemisphere are presented. The remaining 13 northern hemisphere clusters of the catalog are presented in a companion paper. For four clusters (NGC 4833, NGC 5986, NGC 6543, and NGC 6638) we present for the first time a CMD from CCD data. The typical CMD spans from the 22nd V magnitude to the tip of the red giant branch. Based on a large number of standard stars, the absolute photometric calibration is reliable to the ~0.02 mag level in both filters. This catalog, because of its homogeneity, is expected to represent a useful data base for the measurement of the main absolute and relative parameters characterizing the CMD of GGCs.

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