arXiv subjects

Livia Origlia

Publications and source records attributed to Livia Origlia.

At least 37 records · Page 2Linked to original sources

arXiv2011

The Bulge Radial Velocity Assay (BRAVA): II. Complete Sample and Data Release

We present new radial velocity measurements from the Bulge Radial Velocity Assay (BRAVA), a large scale spectroscopic survey of M-type giants in the Galactic bulge/bar region. The sample of ~4500 new radial velocities, mostly in the region -10 deg < l < +10 deg and b ~ -6 deg more than doubles the existent published data set. Our new data extend our rotation curve and velocity dispersion profile to +20 deg, which is ~2.8 kpc from the Galactic Center. The new data confirm the cylindrical rotation observed at -6 deg and -8 deg, and are an excellent fit to the Shen et al. (2010) N-body bar model. We measure the strength of the TiO molecular band as a first step towards a metallicity ranking of the stellar sample, from which we confirm the presence of a vertical abundance gradient. Our survey finds no strong evidence of previously unknown kinematic streams. We also publish our complete catalog of radial velocities, photometry, TiO band strengths, and spectra, which is available at the IRSA archive: http://irsa.ipac.caltech.edu/ as well as at UCLA: http://brava.astro.ucla.edu/.

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arXiv2011

Detailed abundances for M giants in two inner bulge fields from Infrared Spectroscopy

We report abundance analysis for 30 M giant stars in two inner Galactic bulge fields at (l,b)=(0,-1.75) deg and at (l,b)=(1,-2.65) deg, based on R=25,000 infrared spectroscopy from 1.5-1.8um using NIRSPEC at the Keck II telescope. We find iron abundances of <[Fe/H]>=-0.16 +/- 0.03 dex with a 1-sigma dispersion of 0.12 +/- 0.02 and <[Fe/H]>=-0.21 +/- 0.02 dex, with a 1-sigma dispersion of 0.09+/- 0.016 for the (l,b)=(0,-1.75) and (l,b)=(1,-2.65) deg fields, respectively. In agreement with all prior studies, we find enhanced [alpha/Fe] of +0.3 dex. We confirm the lack of any major vertical abundance or composition gradient in the innermost ~600 pc between Baade's window and 150 pc from the Galactic plane. We also confirm that the known enhancement of alpha elements observed between 500 and 1000 pc from the nucleus is also present over the volume of the inner bulge and may therefore be presumed to be a general characteristic of bulge/bar stars within 1 kpc of the Galactic Center.

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arXiv2010

Dust is forming along the red giant branch of 47 TUC

We present additional evidence that dust is really forming along the red giant branch (RGB) of 47 Tuc at luminosities ranging from above the horizontal branch to the RGB-tip (Origlia et al. 2007). The presence of dust had been inferred from an infrared excess in the (K-8) color, with K measured from high spatial resolution ground based near-IR photometry and "8" referring to Spitzer-IRAC 8 micron photometry. We show how (K-8) is a far more sensitive diagnostic for detecting tiny circumstellar envelopes around warm giants than colors using only the Spitzer-IRAC bands, for example the (3.6-8) color used by Boyer et al. (2010). In addition, we also show high resolution HST-ACS I band images of the giant stars which have (K-8) color excess. These images clearly demonstrate that Boyer et al (2010) statement that our detections of color excess associated with stars below the RGB-tip arise from blends and artefacts is simply not valid.

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arXiv2010

X-ray gaseous emission in the galaxy M82

The main results from a deep X-ray observation of M82 are summarised: spatially-dependent chemical abundances, temperature structure of the gas, charge-exchange emission lines in the spectrum. We also present an update of the chemical bundances, based on a more refined extraction of spectra.

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arXiv2009

Chemical abundance patterns in the inner Galaxy: the Scutum Red Supergiant Clusters

The location of the Scutum Red-Supergiant (RSG) clusters at the end of the Galactic Bar makes them an excellent probe of the Galaxy's secular evolution; while the clusters themselves are ideal testbeds in which to study the predictions of stellar evolutionary theory. To this end, we present a study of the RSGs' surface abundances using a combination of high-resolution H-band spectroscopy and spectral synthesis analysis. We provide abundance measurements for elements C, O, Si, Mg, Ti, and Fe. We find that the surface abundances of the stars studied are consistent with CNO burning and deep, rotationally enhanced mixing. The average a/Fe ratios of the clusters are solar, consistent with a thin-disk population. However, we find significantly sub-solar Fe/H ratios for each cluster, a result which strongly contradicts a simple extrapolation of the Galactic metallicity gradient to lower Galacto-centric distances. We suggest that a simple one-dimensional parameterization of the Galaxy's abundance patterns is insufficient at low Galactocentric distances, as large azimuthal variations may be present. Indeed, we show that the abundances of O, Si and Mg are consistent with independent measurements of objects in similar locations in the Galaxy. In combining our results with other data in the literature, we present evidence for large-scale (~kpc) azimuthal variations in abundances at Galacto-centric distances of 3-5kpc. While we cannot rule-out that this observed behaviour is due to systematic offsets between different measurement techniques, we do find evidence for similar behaviour in a study of the barred-spiral galaxy NGC4736 which uses homogeneous methodology. We suggest that these azimuthal abundance variations could result from the intense but patchy star formation driven by the potential of the central bar.

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arXiv2008

The chemical abundances in the Galactic Centre from the atmospheres of Red Supergiants

The Galactic Centre (GC) has experienced a high degree of recent star-forming activity, as evidenced by the large number of massive stars currently residing there. The relative abundances of chemical elements in the GC may provide insights into the origins of this activity. Here, we present high-resolution $H$-band spectra of two Red Supergiants in the GC (IRS~7 and VR~5-7), and in combination with spectral synthesis we derive abundances for Fe and C, as well as other $α$-elements Ca, Si, Mg Ti and O. We find that the C-depletion in VR~5-7 is consistent with the predictions of evolutionary models of RSGs, while the heavy depletion of C and O in IRS~7's atmosphere is indicative of deep mixing, possibly due to fast initial rotation and/or enhanced mass-loss. Our results indicate that the {\it current} surface Fe/H content of each star is slightly above Solar. However, comparisons to evolutionary models indicate that the {\it initial} Fe/H ratio was likely closer to Solar, and has been driven higher by H-depletion at the stars' surface. Overall, we find $α$/Fe ratios for both stars which are consistent with the thin Galactic disk. These results are consistent with other chemical studies of the GC, given the precision to which abundances can currently be determined. We argue that the GC abundances are consistent with a scenario in which the recent star-forming activity in the GC was fuelled by either material travelling down the Bar from the inner disk, or from the winds of stars in the inner Bulge -- with no need to invoke top-heavy stellar Initial Mass Functions to explain anomalous abundance ratios.

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arXiv2008

Near-Infrared Photometry of Four Stellar Clusters in the Small Magellanic Cloud

We present high-quality J, H and K photometry of four Small Magellanic Cloud stellar clusters with intermediate ages in the 1-7 Gyr range (namely NGC 339, 361, 416 and 419) . We obtained deep Color-Magnitude Diagrams to study the evolved sequences and providing a detailed census of the Red Giant Branch (RGB), Asymptotic Giant Branch (AGB) and Carbon star populations in each cluster and their contribution to the total cluster light. We find that in the 5-7 Gyr old clusters AGB stars account for ~6 % of the total light in K-band, Carbon stars are lacking and RGB stars account for ~45 % of the total bolometric luminosity. These empirical findings are in good agreement with the theoretical predictions. Finally, we derived photometric metallicities computed by using the properties of the RGB and finding an iron content of [Fe/H]=-1.18, -1.08, -0.99 and -0.96 dex for NGC 339, 361, 416 and 419 respectively.

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arXiv2008

A deep X-ray observation of M82 with XMM-Newton

We report on the analysis of a deep (100 ks) observation of the starburst galaxy M82 with the EPIC and RGS instruments on board the X-ray telescope XMM-Newton. The broad-band (0.5-10 keV) emission is due to at least three spectral components: i) continuum emission from point sources; ii) thermal plasma emission from hot gas; iii) charge exchange emission from neutral metals (Mg and Si). The plasma emission has a double-peaked differential emission measure, with the peaks at ~0.5 keV and ~7 keV. Spatially resolved spectroscopy has shown that the chemical absolute abundances are not uniformly distributed in the outflow, but are larger in the outskirts and smaller close to the galaxy centre. The abundance ratios also show spatial variations. The X-ray derived Oxygen abundance is lower than that measured in the atmospheres of red supergiant stars, leading to the hypothesis that a significant fraction of Oxygen ions have already cooled off and no longer emit at energies > ~0.5 keV.

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arXiv2007

The First Empirical Mass Loss Law for Population II Giants

Using the Spitzer IRAC camera we have obtained mid-IR photometry of the red giant branch stars in the Galactic globular cluster 47 Tuc. About 100 stars show an excess of mid-infrared light above that expected from their photospheric emission. This is plausibly due to dust formation in mass flowing from these stars. This mass loss extends down to the level of the horizontal branch and increases with luminosity. The mass loss is episodic, occurring in only a fraction of stars at a given luminosity. Using a simple model and our observations we derive mass loss rates for these stars. Finally, we obtain the first empirical mass loss formula calibrated with observations of Population II stars. The dependence on luminosity of our mass loss rate is considerably shallower than the widely used Reimers Law. The results presented here are the first from our Spitzer survey of a carefully chosen sample of 17 Galactic Globular Clusters, spanning the entire metallicity range from about one hundredth up to almost solar.

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arXiv2007

The First Detailed Abundances for M giants in the inner Bulge from Infrared Spectroscopy

We report the first abundance analysis of 17 M giant stars in the inner Galactic bulge at $(l,b)=(0°,-1°)$, based on $R=25,000$ infrared spectroscopy $(1.5-1.8 ~μ\rm m)$ using NIRSPEC at the Keck telescope. Based on their luminosities and radial velocities, we identify these stars with a stellar population older than 1 Gyr. We find the iron abundance $\rm < [Fe/H] > = -0.22 \pm 0.03$, with a $1σ$ dispersion of $0.14\pm 0.024$. We also find the bulge stars have enhanced [$\rm alpha$/Fe] abundance ratio at the level of +0.3 dex relative to the Solar stars, and low $\rm <^{12}C/^{13}C>\approx 6.5\pm 0.3$. The derived iron abundance and composition for this inner bulge sample is indistinguishable from that of a sample of M giants our team has previously studied in Baade's Window $(l,b)=(0.9,-4)$. We find no evidence of any major iron abundance or abundance ratio gradient between this inner field and Baade's window.

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arXiv2007

Formation & evolution of the Galactic bulge: constraints from stellar abundances

We compute the chemical evolution of the Galactic bulge in the context of an inside-out model for the formation of the Milky Way. The model contains updated stellar yields from massive stars. The main purpose of the paper is to compare the predictions of this model with new observations of chemical abundance ratios and metallicity distributions in order to put constraints on the formation and evolution of the bulge. We computed the evolution of several alpha-elements and Fe and performed several tests by varying different parameters such as star formation efficiency, slope of the initial mass function and infall timescale. We also tested the effect of adopting a primary nitrogen contribution from massive stars. The [alpha/Fe] abundance ratios in the Bulge are predicted to be supersolar for a very large range in [Fe/H], each element having a different slope. These predictions are in very good agreement with most recent accurate abundance determinations. We also find a good fit of the most recent Bulge stellar metallicity distributions. We conclude that the Bulge formed on a very short timescale (even though timescales much shorter than about 0.1 Gyr are excluded) with a quite high star formation efficiency of about 20 Gyr$^{-1}$ and with an initial mass function more skewed toward high masses (i.e. x <= 0.95) than the solar neighbourhood and rest of the disk. The results obtained here are more robust than previous ones since they are based on very accurate abundance measurements.

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arXiv2007

The globular cluster NGC 1978 in the Large Magellanic Cloud

We have used deep high-resolution Hubble Space Telescope ACS observations to image the cluster NGC 1978 in the Large Magellanic Cloud. This high-quality photometric data set allowed us to confirm the high ellipticity (~0.30+-0.02) of this stellar system. The derived Color Magnitude Diagram allowed a detailed study of the main evolutionary sequences: in particular we have detected the so called Bump along the Red Giant Branch (at V_{555}=19.10+-0.10). This is the first detection of this feature in an intermediate-age cluster. Moreover the morphology of the evolutionary sequence and their population ratios have been compared with the expectations of different theoretical models (namely BaSTI, PEL and Padua) in order to quantify the effect of convective overshooting. The best agreement (both in terms of morphology and star counts) has been found the PEL (Pisa Evolutionary Library) isochrone with Z=0.008 (consistenly with the most recent determination of the cluster metallicity, [M/H]=-0.37 dex) and a mild overshooting efficiency (Lambda_{os}=0.1). By adopting this theoretical set an age of 1.9+-0.1 Gyr has been obtained.

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arXiv2006

Formation and evolution of the Galactic bulge: constraints from stellar abundances

We present results for the chemical evolution of the Galactic bulge in the context of an inside-out formation model of the Galaxy. A supernova-driven wind was also included in analogy with elliptical galaxies. New observations of chemical abundance ratios and metallicity distribution have been employed in order to check the model results. We confirm previous findings that the bulge formed on a very short timescale with a quite high star formation efficiency and an initial mass function more skewed toward high masses than the one suitable for the solar neighbourhood. A certain amount of primary nitrogen from massive stars might be required to reproduce the nitrogen data at low and intermediate metallicities.

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arXiv2006

New insights on chemical evolution of galaxies: XMM-Newton observations of M82

We report on the ongoing data analysis of a very deep (~100 ks) XMM-Newton observation of the starburst galaxy M82. We show some details of data analysis and a few results from spatially-resolved spectroscopy with the EPIC cameras. Since M82 is a bright object with a complex spectrum, the data reduction of such a deep observation has posed many challenges both about the involved astrophysical processes and the available data analysis techniques. Vertical (with respect to the galaxy plane) abundance gradients are discovered. The hints for an under-abundance of Oxygen stemming from our previous study are confirmed. The hot X-ray emitting gas is shown to have a multi-temperature distribution.

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arXiv2006

On the Iron content of NGC 1978 in the LMC: a metal rich, chemically homogeneous cluster

We present a detailed abundance analysis of giant stars in NGC 1978, a massive, intermediate-age stellar cluster in the Large Magellanic Cloud, characterized by a high ellipticity and suspected to have a metallicity spread. We analyzed 11 giants, all cluster members, by using high resolution spectra acquired with the UVES/FLAMES spectrograph at the ESO-Very Large Telescope. We find an iron content of [Fe/H]=-0.38 dex with very low (0.07 dex) dispersion, and a mean heliocentric radial velocity Vr=293.1 (with an error of 0.9 km/s) and a velocity dispersion (3.1 km/s), thus excluding the presence of a significant metallicity, as well as velocity, spread within the cluster.

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arXiv2005

Gaseous abundances in M82

We present the preliminary analysis of a deep (100ks) XMM-Newton observation of M82. The spatial distribution of the abundances of chemical elements (Fe, O, Ne, Mg, Si, S) is investigated through narrow-band imaging analisys and spatially-resolved spectroscopy. We find that the abundances of alpha-elements follow a bipolar distribution, these elements being more abundant in the gaseous outflow than in the galaxy centre. This behaviour is found to be more marked for lighter elements (O, Ne) than for heavier elements.

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arXiv2005

The First Detailed Abundances for M giants in Baade's Window from Infrared Spectroscopy

We report the first abundance analysis of 14 M giant stars in the Galactic bulge, based on R=25,000 infrared spectroscopy (1.5-1.8um) using NIRSPEC at the Keck II telescope. Because some of the bulge M giants reach high luminosities and have very late spectral type, it has been suggested that they are the progeny of only the most metal rich bulge stars, or possibly members of a younger bulge population. We find the iron abundance and composition of the M giants are similar to those of the K giants that have abundances determined from optical high resolution spectroscopy: <[Fe/H]>=-0.190 +/- 0.020 with a 1-sigma dispersion of 0.08 +/- 0.015. Comparing our bulge M giants to a control sample of local disk M giants in the Solar vicinity, we find the bulge stars are enhanced in alpha elements at the level of +0.3 dex relative to the Solar composition stars, consistent with other studies of bulge globular clusters and field stars. This small sample shows no dependence of spectral type on metallicity, nor is there any indication that the M giants are the evolved members of a subset of the bulge population endowed with special characteristics such as relative youth or high metallicity. We also find low 12C/13C < 10, confirming the prsence of extra-mixing processes during the red gaint phase of evolution

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arXiv2004

High resolution infrared spectra of bulge globular clusters: The extreme chemical abundances of Terzan 4 and Terzan 5

Using the NIRSPEC spectrograph at Keck II, we have obtained infrared echelle spectra covering the range $1.5-1.8 μ\rm m$ for the highly reddened bulge globular clusters Terzan 4 and Terzan 5. The clusters are of interest because a blue horizontal branch in Terzan 4 is consistent with very low metallicity, while Terzan 5 has been proposed as possibly the most metal rich known Galactic globular cluster. We report the first detailed abundance analysis for stars in these clusters, and we find [Fe/H]=--1.60 and --0.21 dex for Terzan 4 and Terzan 5, respectively, confirming the presence of a large metallicity spread in the bulge as well as in the halo of the Galaxy. We find $\rm [α/Fe]\approx+0.5$ dex in Terzan 4 and $\rm [α/Fe]\approx +0.3$ dex in Terzan 5, consistent with what has been found for field stars in the bulge. The enhanced $α-$element abundances are also consistent with rapid chemical enrichment, most likely by type II SNe. Finally, we measure very low $^{12}$C/$^{13}$C$\le10$ ratios in all the giant stars, confirming that extra-mixing mechanisms due to {\it cool bottom processing} occur during evolution along the RGB.

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