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E. V. Held

Publications and source records attributed to E. V. Held.

54 records · Page 3Linked to original sources
arXiv2004

Radial velocities and membership of stars in the old, distant open cluster Berkeley 29

Multi slit spectroscopy at the Telescopio Nazionale Galileo was employed to measure radial velocities for 20 stars in the direction of the old open cluster Berkeley 29, the farthest known in our Galaxy. Membership information was derived for stars along all the red giant branch, in particular near its tip, and on the red clump. The sample of bona-fide cluster members was used to revise the cluster distance to about 15 kpc, on the basis of an empirical comparison with the red clump in open clusters with known distances. A metallicity [Fe/H] = -0.74 +/- 0.18 was also estimated using the colours of spectroscopically confirmed red giant stars.

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arXiv2003

The near-IR luminosity-metallicity relationship for dwarf irregular galaxies

We briefly describe our on-going investigation of the near-IR luminosity-metallicity relationship for dwarf irregular galaxies in nearby groups of galaxies. The motivations of the project and the observational databases are introduced, and a preliminary result is presented. The 12+log(O/H) vs.H plane must be populated with more low-luminosity galaxies before a definite conclusion can be drawn.

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arXiv2003

NGC6822: short period variable stars, stellar population and distance scale

Results are presented on a study of the short period variable stars in the dwarf irregular galaxy NGC6822. We observed an almost uniformely populated classical instability strip from the Horizontal Branch up to the Classical Cepheids region. The main goal we achieved from the analysis of the faint sample is the first detection of RR Lyrae stars in this galaxy.

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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

Distance scale and variable stars in Local Group Galaxies: LMC and Fornax

We briefly review our photometric and spectroscopic study of RR Lyrae variable stars in the bar of the Large Magellanic Cloud (LMC), that allowed us to reconcile the so-called "short" and "long" distance moduli of the LMC on the value mod_LMC=18.51 +/- 0.085 mag. Then we present preliminary results from the photometric study of a 33' x 34' area in the Fornax dwarf spheroidal galaxy containing the stellar clusters Fornax 3 (NGC 1049) and 6. We identified about 1000 candidate variables in this field of Fornax, and report the first detection and measure of about 60 RR Lyrae variable stars in the globular cluster Fornax 3.

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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

RR Lyrae and short-period variable stars in the dwarf irregular galaxy NGC 6822

We report the discovery of a large number of short-period variable stars in the dwarf irregular galaxy NGC6822, based on deep time-series imaging carried out with the ESO Very Large Telescope. In particular, we found a modest population of RR Lyrae stars tracing the presence of an old stellar component in NGC6822. Measurements of the average luminosity of RR Lyrae stars provide a new independent estimate of the distance to this galaxy based on a Pop.II indicator, (m-M)o=23.36 +/-0.17. In addition, our new data show a significant population of small-amplitude, short-period variable stars filling the instability strip starting at luminosities only a few tenths of a magnitude brighter than the RR Lyrae stars.

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arXiv2003

V, J, H and K Imaging of the Metal Rich Globular Cluster NGC 6528

New near-infrared observations of NGC6528 are presented. The JHK observations complement a previous HST/NICMOS data set by Ortolani et al. (2001), in that they sample a larger area, contain a more numerous sample of red giant stars, and include the K band. Also, archival HST data sets (separated by 6.093 years) were used to proper-motion decontaminate the near-infrared sample and extract a clean VJHK catalogue. Using the present wide colour baseline, we compared the cleaned colour-magnitude diagrams of NGC6528 with those of NGC 6553 and NGC104 and derived new estimates of reddening and distance, E(B-V)=0.55 and (m-M)o=14.44 (7.7 kpc). Moreover, the morphology and location of the cleaned red giant branch were used to derive a photometric estimate of the cluster metallicity. The average of 10 metallicity indicators yields a mean value of [M/H] ~ 0.0, and [Fe/H] ~-0.20 and +0.08 on the Zinn & West (1984) and Carretta & Gratton (1997) revised metallicity scale, respectively. The best isochrone fit to the cleaned K,V-K diagram is obtained for a 12.6 Gyr and Z=0.02 isochrone, i.e. the derived metallicity of NGC6528 turns out to be very close to the mean of stars in the Baade's Window. Five AGB variable star candidates, whose membership has to be confirmed spectroscopically, are bolometrically as bright as the known long period variable stars in NGC6553. As discussed in Guarnieri et al. (1997) for NGC6553, this may indicate that an `intermediate age' population is not needed to account for the brightest stars in external galaxies such as M32.

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arXiv2003

NGC6822: detection of variable stars with ISIS2.1

Preliminary results on the detection of a conspicuous number of variable stars in the dwarf irregular galaxy NGC 6822 are presented. We stress the need for packages specifically designed to the search of variables in distant galaxies and/or when data are affected by a high level of crowding.

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arXiv2002

Cluster formation in the LMC

In this work we present preliminary results concerning the properties of about 50 clusters and associations located in a densely populated region at the Southern edge of the supershell LMC~4. The ages of the clusters and associations are derived. The cluster formation rate peaks at about 10-20 Myr ago corresponding to the age of the formation of LMC~4. Objects younger than 10 Myr are located close to CO clouds, indicating a possible trigger of the recent star formation by the interaction of the supershell with the interstellar medium. Isophotal analysis has been done for 50% of the clusters. A large number of objects present isophotal distortions commonly interpreted as sign of interaction. Cluster multiplets are coeval within 20 Myr, suggesting a common origin. This indicates that clusters possibly form in large groups. Our findings are in agreement with the unified view of multi-scale star formation, where a size-duration correlation is expected.

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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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arXiv1998

Young Galactic globular clusters II. The case of Palomar 12

We present broadband V,I CCD photometry for ca. 1700 stars towards the Galactic globular cluster Palomar 12, covering a field of 10.7' x 10.7'. From these data, a color-magnitude diagram from the red giant branch tip to ca. 2 mag below the cluster's turn-off is obtained. From a comparison with the color magnitude diagrams of 47 Tuc and M5, and using different theoretical models, we confirm that Pal 12 is younger, finding an age 68% +/- 10% that of both template clusters. Revised structural parameters are also obtained: r(c) = 37.8 +/- 0.6 and c = 1.08 +/- 0.02.

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arXiv1996

X-ray emission and internal kinematics in early-type galaxies. I. Observations

Long slit spectroscopic data for 7 early-type galaxies with X-ray emission have been analyzed to derive velocity dispersion and radial velocity profiles. Major axis rotation curves out to $R\sim R_e$ are presented. Adding these new data to those available in the literature, we have built a sample of early-type galaxies with detected X-ray emission and known kinematics (central velocity dispersion $σ_0$ and maximum rotational velocity $v_r$). Using this sample we investigate from an observational point of view the role of rotation and flattening on the X-ray emission, particularly with regard to the X-ray underluminosity of flat systems. The trend between the X-ray to optical ratio $L_X/L_B$, a measure of the hot gas content of the galaxies, and $v_r/σ_0$ is L-shaped, with the X-ray brightest objects confined at $v_r/σ_0 <0.4$, both for Es and S0s. Neither for low or intermediate, nor for high $L_X/L_B$, there is any clear correlation between X-ray emission and rotational properties. The trend between $L_X/L_B$ and the ellipticity $ε$ is also L-shaped: there are no high $L_X/L_B$ objects with high $ε$. The existence of a relation between $v_r/σ_0$ and $ε$ makes it difficult to assess whether rotation or flattening is at the basis of the trends found. Our observational findings are finally discussed in connection with the effects that rotation and flattening are predicted to have in the cooling flow and in the wind/outflow/inflow scenarios.

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arXiv1993

Deep Kinematics and Dynamics of Edge-on S0 Galaxies. I. NGC 3115

As a first step of a program aimed to the detection of dark matter (or radial variations of M/L) in early-type galaxies, we report deep spectroscopic observations of the bulge-dominated edge-on S0 galaxy NGC 3115, made at ESO, La Silla, using EFOSC at the 3.6m telescope and EMMI at NTT. Such observations allow measurements of the rotational velocity out to 1.8 a_e (effective radii) from the galaxy center, where the surface brightness is μ_B ~ 24 mag arcsec**-2. The rotation curve quickly reaches an asymptotic value, v_f ~ 260 km/s, with only marginal indication of systematic decline within the range of our observations. The line-of-sight velocity dispersion has also been measured; it decreases steeply from a rather high central value and flattens out (<σ> ~ 100 km/s) within our observing range (a ~< 1.3 a_e). Models built on these data and simple dynamical arguments show that the M/L of NGC 3115 must thus be increasing from M/L = 6 (in solar units) in the inner regions (~ 1 a_e) to at least M/L >= 10 in the outermost regions (~ 2 a_e).

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