arXiv subjects

Miriam Garcia

Publications and source records attributed to Miriam Garcia.

31 records · Page 2Linked to original sources

arXiv2019

Stellar Physics with High-Resolution UV Spectropolarimetry

Current burning issues in stellar physics, for both hot and cool stars, concern their magnetism. In hot stars, stable magnetic fields of fossil origin impact their stellar structure and circumstellar environment, with a likely major role in stellar evolution. However, this role is complex and thus poorly understood as of today. It needs to be quantified with high-resolution UV spectropolarimetric measurements. In cool stars, UV spectropolarimetry would provide access to the structure and magnetic field of the very dynamic upper stellar atmosphere, providing key data for new progress to be made on the role of magnetic fields in heating the upper atmospheres, launching stellar winds, and more generally in the interaction of cool stars with their environment (circumstellar disk, planets) along their whole evolution. UV spectropolarimetry is proposed on missions of various sizes and scopes, from POLLUX on the 15-m telescope LUVOIR to the Arago M-size mission dedicated to UV spectropolarimetry.

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arXiv2019

Ongoing star formation at the outskirts of Sextans A: Spectroscopic detection of early-O type stars

With both nebular- and stellar-derived abundances of $\lesssim$ 1/10 Zsun and low foreground extinction, Sextans A is a prime candidate to replace the Small Magellanic Cloud as reservoir of metal-poor massive stars and reference to study the metal-poor Universe. We report the discovery of two early-O type stars in Sextans A, the earliest O-stars with metallicity < 1/7 Zsun known to date, and two additional O9 stars. Colour-excess estimates towards individual targets, enabled by spectral typing, manifest that internal reddening is neither uniform nor negligible. The four targets define a new region of star formation far from the optically-brightest centre of the galaxy and from its conspicuous HII shells, but not devoid of neutral hydrogen. In fact, we detect a spatial correlation between OB-stars and HI in Sextans A and other dIrr's that leads us to propose that the neutral phase may be fundamental to star formation in low-density environments. According to the existing evidence at least two of the targets formed in isolation, thus suggestive of an stochastic sampling of the initial mass function that would enable low-mass galaxies like Sextans A to form very massive stars. The discovery of these four stars provide spatially-resolved, spectroscopic confirmation of recent findings suggesting that dwarf galaxies can sustain star formation despite the low density of the gas phase.

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arXiv2017

Massive stars in the Sagittarius Dwarf Irregular Galaxy

Low metallicity massive stars hold the key to interpret numerous processes in the past Universe including re-ionization, starburst galaxies, high-redshift supernovae and GRBs. The Sagittarius Dwarf Irregular Galaxy (SagDIG, 12+log(O/H)=7.37) represents an important landmark in the quest for analogues accessible with 10-m class telescopes. This paper presents low-resolution spectroscopy executed with the Gran Telescopio Canarias that confirms that SagDIG hosts massive stars. The observations unveiled three OBA-type stars and one red supergiant candidate. Pending confirmation from high-resolution follow-up studies, these could be the most metal-poor massive stars of the Local Group.

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arXiv2017

The LUVOIR Ultraviolet Multi-Object Spectrograph (LUMOS): Instrument Definition and Design

The Large Ultraviolet / Optical / Infrared Surveyor (LUVOIR) is one of four large mission concepts currently undergoing community study for consideration by the 2020 Astronomy and Astrophysics Decadal Survey. The LUVOIR Ultraviolet Multi-Object Spectrograph, LUMOS, is being designed to support all of the UV science requirements of LUVOIR, from exoplanet host star characterization to tomography of circumgalactic halos to water plumes on outer solar system satellites. LUMOS offers point source and multi-object spectroscopy across the UV bandpass, with multiple resolution modes to support different science goals. The instrument will provide low (R = 8,000-18,000) and medium (R = 30,000-65,000) resolution modes across the far-ultraviolet (FUV: 100-200 nm) and near-ultraviolet (NUV: 200-400 nm) windows, and a very low resolution mode (R = 500) for spectroscopic investigations of extremely faint objects in the FUV. Imaging spectroscopy will be accomplished over a 3 x 1.6 arcminute field-of-view by employing holographically-ruled diffraction gratings to control optical aberrations, microshutter arrays (MSA), advanced optical coatings for high-throughput in the FUV, and next generation large-format photon-counting detectors. The spectroscopic capabilities of LUMOS are augmented by an FUV imaging channel (100-200nm, 13 milliarcsecond angular resolution, 2 x 2 arcminute field-of-view) that will employ a complement of narrow and medium-band filters. We present an overview of LUMOS' observing modes and estimated performance curves for effective area, spectral resolution, and imaging performance. Example "LUMOS 100-hour Highlights" observing programs are presented to demonstrate the potential power of LUVOIR's ultraviolet spectroscopic capabilities.

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arXiv2017

Low-metallicity (sub-SMC) massive stars

The double distance and metallicity frontier marked by the SMC has been finally broken with the aid of powerful multi-object spectrographs installed at 8-10m class telescopes. VLT, GTC and Keck have enabled studies of massive stars in dwarf irregular galaxies of the Local Group with poorer metal content than the SMC. The community is working to test the predictions of evolutionary models in the low-metallicity regime, set the new standard for the metal-poor high-redshift Universe, and test the extrapolation of the physics of massive stars to environments of decreasing metallicity. In this paper, we review current knowledge on this topic.

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arXiv2016

On the binary origin of FS CMa stars: young massive clusters as test beds

FS CMa stars are low-luminosity objects showing the B[e] phenomenon whose evolutionary origin is yet to be unraveled. Various binary-related hypotheses have been recently proposed, two of them involving the spiral-in evolution of the binary orbit. The latter occurs more often in dense stellar environments like young massive clusters (YMCs). Hence, a systematic study of FS CMa stars in YMCs would be crucial to find out how these objects are created. In YMCs, two FS CMa stars have been confirmed and three candidates have been found through a search method based on narrow-band photometry at Paschen-$α$ and the neighboring continuum. We apply this method to archival data from the Paschen-$α$ survey of the Galactic Center region, yielding a new candidate in the Quintuplet cluster. Limitations of this method and other alternatives are briefly discussed.

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arXiv2014

Winds of low-metallicity OB-type stars: HST-COS spectroscopy in IC1613

We present the first quantitative UV spectroscopic analysis of resolved OB stars in IC1613. Because of its alleged very low metallicity (<~1/10 Zo, from HII regions), studies in this Local Group dwarf galaxy could become a significant step forward from the SMC towards the extremely metal-poor massive stars of the early Universe. We present HST-COS data covering the ~1150-1800Å wavelength range with resolution R~2500. We find that the targets do exhibit wind features, and these are similar in strength to SMC stars. Wind terminal velocities were derived from the observed PCygni profiles with the SEI method. The vinf-Z relationship has been revisited. The terminal velocity of IC1613 O-stars is clearly lower than Milky Way counterparts, but there is no clear difference between IC1613 and SMC or LMC analogue stars. We find no clear segregation with host galaxy in the terminal velocities of B-supergiants, nor in the vinf/vesc ratio of the whole OB star sample in any of the studied galaxies. Finally, we present first evidence that the Fe-abundance of IC1613 OB stars is similar to the SMC, in agreement with previous results on red supergiants. With the confirmed ~1/10 solar oxygen abundances of B-supergiants, our results indicate that IC1613's [alpha/Fe] ratio is sub-solar.

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arXiv2013

The Effective Temperatures of O-type Stars from UV spectroscopy

We present an analysis of high resolution spectra in the far-UV -- UV range (~905-2000Å) with non-LTE, spherical, hydrodynamical, line-blanketed models, of three O-type Galactic stars, and derive their photospheric and wind parameters. These data extend previously analyzed samples and fill a gap in spectral type coverage. The combined sample confirms a revised (downward) effective temperature scale with respect to canonical calibrations, as found in our previous works from UV and optical spectra, and in recent works by other authors.

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arXiv2012

The young stellar population of IC 1613. III. New O-type stars unveiled by GTC-OSIRIS

Recent findings hint that the winds of massive stars with poorer metallicity than the SMC may be stronger than predicted by theory. Besides calling the paradigm of radiation driven winds into question, this result would impact the predicted evolutionary paths of massive stars, their calculated ionizing radiation and mechanical feedback and the role these objects play at different stages of the Universe. The field needs a systematic study of the winds of a large set of very metal poor massive stars, but the sampling of spectral types is particularly poor in the very early types. This paper's goal is to increase the list of known O-type stars in the dwarf irregular galaxy IC1613, whose metallicity is smaller than the SMC's by roughly a factor 2. Using the reddening-free Q-parameter, evolutionary masses and GALEX photometry, we built a list of very likely O-type stars. We obtained low-resolution R~1000 GTC-OSIRIS spectra for a fraction of them and performed spectral classification, the only way to unequivocally confirm candidate OB-stars. We have discovered 8 new O-type stars in IC1613, increasing the list of 7 known O-type stars in this galaxy by a factor of 2. The best quality spectra were analyzed with the model atmosphere code FASTWIND to derive stellar parameters. We present the first spectral type -- effective temperature scale for O-stars beyond the SMC. The derived effective temperature calibration for IC1613 is about 1000K hotter than the scale at the SMC. The analysis of an increased list of O-type stars will be crucial for the studies of the winds and feedback of massive stars at all ages of the Universe.

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arXiv2012

Red supergiants around the obscured open cluster Stephenson 2

Several clusters of red supergiants have been discovered in a small region of the Milky Way close to the base of the Scutum-Crux Arm and the tip of the Long Bar. Population synthesis models indicate that they must be very massive to harbour so many supergiants. Among them, Stephenson 2, with a core grouping of 26 RSGs, is a strong candidate to be the most massive cluster in the Galaxy. It is located close to a region where a strong over-density of RSGs had been found. We explore the actual cluster size and its possible connection to this over-density. We have performed a cross-match between DENIS, USNO-B1 and 2MASS to identify candidate obscured luminous red stars around Ste 2, and in a control nearby region, finding >600 candidates. More than 400 are sufficiently bright in I to allow observation with a 4-m class telescope. We have observed a subsample of ~250 stars, using AF2 on the WHT telescope in La Palma, obtaining intermediate-resolution spectroscopy in the 7500--9000A range. We derive spectral types and luminosity classes for all these objects and measure their radial velocities. Our targets are G and K supergiants, late (>=M4) M giants, and M-type bright giants (luminosity class II) and supergiants. We find ~35 RSGs with radial velocities similar to Ste 2 members, spread over the two areas surveyed. In addition, we find ~40 RSGs with radial velocities incompatible in principle with a physical association. Our results show that Ste 2 is not an isolated cluster, but part of a huge structure likely containing hundreds of RSGs, with radial velocities compatible with the terminal velocity at this Galactic longitude (and a distance ~6kpc). In addition, we find evidence of several populations of massive stars at different distances along this line of sight [ABRIDGED].

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arXiv2010

Exploring the connection of weak winds and magnetic fields

The theory of radiatively driven winds successfully explains the key points of the stellar winds of hot massive stars. However, there is an apparent break-down of this paradigm at L/Lsun<5.2: the stellar wind momentum is smaller than predicted for low luminosity early-type stars from metal poor environments, and there are also some Galactic examples. In this work we explore whether magnetic fields are playing a role.

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arXiv2010

In savvy pursuit of Local Group blue massive stars

The important role of metallicity on massive star evolution and the combination of multi-object spectrographs and 10m class telescopes, have lead to numerous systematic studies of massive stars in Local Group galaxies. While color based quests of blue massive stars are relatively successful, they must be confirmed with spectroscopy and usually lead to lists dominated by B-type modest-mass stars. We have developed a friends of friends code to find OB associations in Local Group galaxies, presented in Garcia et al. (2009). One of the key points of the method is the photometric criterion to choose candidate OB stars from the reddening-free Q parameter, that could be easily extended to include from GALEX to near-IR photometry. While not a new idea, one of our code's strong advantages is the automatic determination of evolutionary masses for the members, enabling a quick and more insightful choice of candidates for spectroscopy, and the identification of potential advanced evolutionary stages. We present our work on the very metal-poor irregular IC 1613 (Garcia et al. 2010). The association properties are not only a powerful aid towards finding the most interesting candidate massive stars, but also reveal the galaxy's structure and recent star formation history.

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arXiv2004

The Effective Temperatures of Hot Stars II. The Early-O Types

We derive the stellar parameters of a sample of Galactic early-O type stars by analysing their UV and Far-UV spectra from FUSE (905-1187A), IUE, HST-STIS and ORFEUS (1200-2000A). The data have been modeled with spherical, hydrodynamic, line-blanketed, non-LTE synthetic spectra computed with the WM-basic code. We obtain effective temperatures ranging from Teff = 41,000 K to 39,000 K for the O3-O4 dwarf stars, and Teff = 37,500 K for the only supergiant of the sample (O4 If+). Our values are lower than those from previous empirical calibrations for early-O types by up to 20%. The derived luminosities of the dwarf stars are also lower by 6 to 12%; however, the luminosity of the supergiant is in agreement with previous calibrations within the error bars. Our results extend the trend found for later-O types in a previous work by Bianchi & Garcia.

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