arXiv subjects

Ricardo Dorda

Publications and source records attributed to Ricardo Dorda.

11 recordsLinked to original sources

arXiv2022

Strong lithium lines in red supergiants at different metallicities

Current models of stellar evolution predict that stars more massive than $\sim6\:$M$_{\odot}$ should have completely depleted all lithium (Li) in their atmospheres by the time when they reach the He core burning phase. Against this, a non-negligible number of red giants with masses $>6\:$M$_{\odot}$ presenting strong Li lines have recently been reported. Motivated by this finding, we have carried out a spectroscopic survey of red supergiants (RSGs) in the Perseus Arm and a selection of young open clusters in the Magellanic Clouds to assess the presence of the Li I $λ$6708 doublet line. Based on a sample of >70 objects, close to one third of RSGs in the Perseus Arm display noticeable Li lines, with perhaps a trend towards a lower fraction among more luminous stars. The samples in the Magellanic Clouds are not as large, but hint at a metallicity dependence. Twenty one RSGs in 5 LMC clusters show a very high fraction of Li detection, around 40%. Conversely, 17 RSGs in 5 SMC clusters give only one secure detection. The interpretation of these observational results is not straightforward, but a mechanism for Li production seems most likely. Further characterisation work is ongoing, while theoretical studies into this matter are urgently needed.

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arXiv2022

Westerlund 1 under the light of Gaia EDR3: Distance, isolation, extent, and a hidden population

(Abridged) We have used Gaia EDR3 data, together with spectra of a large sample of luminous stars in the field surrounding Westerlund 1, to explore the extent of the cluster. We carry out a non-parametric analysis of proper motions and membership determination. We investigate the reddening and proper motions of several dozen OB stars and red supergiants < 1 deg away from Wd 1. We identify a population of kinematic members of Wd 1 that largely includes the known spectroscopic members. From their EDR3 parallaxes, we derive a distance to the cluster of $4.23^{+0.23}_{-0.21}$kpc. Extinction in this direction increases by a large amount around 2.8 kpc, due to dark clouds associated to the Scutum-Crux arm. As a consequence, we hardly see any stars at distances comparable to that of the cluster. The proper motions of Wd 1, however, are very similar to those of stars in the field surrounding it, but distinct. We find a second, astrometrically well-defined population in the foreground ($d\approx2\:$kpc), likely connected to the possible open cluster BH197. Wd 1 is very elongated, an effect not driven by the very heavy extinction to the East and South. We find a low-density halo extending up to 10' from the cluster centre, mainly in the NW quadrant. A few OB stars at larger distances from the cluster, most notably the LBV MN48, share its proper motions, suggesting that Wd 1 has little or no peculiar motion with respect to the field population of the Norma arm. However, we are unable to find any red supergiant that could belong to an extended population related to the cluster, although we observe several dozen such objects in the foreground. We find a substantial population of luminous OB members obscured by several more magnitudes of extinction than most known members. These objects, mostly located in the central region of the cluster, increase the population of OB supergiants by about 25%.

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arXiv2021

A massive open cluster hiding in full sight

Obscuration and confusion conspire to limit our knowledge of the inner Milky Way. Even at moderate distances, the identification of stellar systems becomes compounded by the extremely high density of background sources. Here we provide a very revealing example of these complications by unveiling a large, massive, young cluster in the Sagittarius arm that has escaped detection until now despite containing more than 30 stars brighter than $G=13$. By combining Gaia DR2 astrometry, Gaia and 2MASS photometry and optical spectroscopy, we find that the new cluster, which we name Valparaiso~1, located at $\sim2.3\:$kpc, is about 75~Ma old and includes a large complement of evolved stars, among which we highlight the 4~d classical Cepheid CM~Sct and an M-type giant that probably represents the first detection of an AGB star in a Galactic young open cluster. Although strong differential reddening renders accurate parameter determination unfeasible with the current dataset, direct comparison to clusters of similar age suggests that Valparaiso~1 was born as one of the most massive clusters in the Solar Neighbourhood, with an initial mass close to $10^{4}\:$M$_{\odot}$.

astro-ph.GA↗

arXiv2020

Cluster membership for the long period Cepheid calibrator SV Vul

Classical Cepheids represent the first step of the distance scale ladder. Claims of tension between the locally calculated Hubble constant and the values deduced from $Planck$'s results have sparked new interest in these distance calibrators. Cluster membership provides an independent distance measurement, as well as astrophysical context for studies of their stellar properties. Here we report the discovery of a young open cluster in the vicinity of SV Vul, one of the most luminous Cephedis known in the Milky Way. $Gaia$ DR2 data show that SV Vul is a clear astrometric and photometric member of the new cluster, which we name Alicante 13. Although dispersed, Alicante 13 is moderately well populated, and contains three other luminous stars, one early-A bright giant and two low-luminosity red supergiants. The cluster is about 30 Ma old at a nominal distance of 2.5 kpc. With this age, SV Vul should have a mass around $10\:$M$_{\odot}$, in good accordance with its luminosity, close to the highest luminosity for Cepheids allowed by recent stellar models.

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arXiv2018

An atlas of cool supergiants from the Magellanic Clouds and typical interlopers. A guide for the classification of luminous red stars

We present an atlas composed of more than 1500 spectra of late-type stars (spectral types from G to M) observed simultaneously in the optical and calcium triplet spectral ranges. These spectra were obtained as part of a survey to search for cool supergiants in the Magellanic Clouds and were taken over four epochs. We provide the spectral and luminosity classification for each spectrum (71% are supergiants, 13% are giants or luminous giants, 4% are carbon or S stars, and the remaining 12% are foreground stars of lesser luminosities). We also provide a detailed guide for the spectral classification of luminous late-type stars, the result of the extensive classification work done for the atlas. Although this guide is based on classical criteria, we have put them together and re-elaborated them for modern CCD-spectra as these criteria were scattered among many different works and mainly conceived for use with photographic plate spectra. The result is a systematic, well-tested process for identifying and classifying luminous late-type stars, illustrated with CCD spectra of standard stars and the classifications of our own catalogue.

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arXiv2018

Berkeley 51, a young open cluster with four yellow supergiants

The heavily obscured open cluster Berkeley~51 shows characteristics typical of young massive clusters, even though the few previous studies have suggested older ages. We combine optical ($UBV$) and 2MASS photometry of the cluster field with multi-object and long-slit optical spectroscopy for a large sample of stars. We apply classical photometric analysis techniques to determine the reddening to the cluster, and then derive cluster parameters via isochrone fitting. We find a large population of B-type stars, with a main sequence turn-off at B3$\,$V, as well as a large number of supergiants with spectral types ranging from F to M. We use intermediate resolution spectra of the evolved cool stars to derive their stellar parameters and find an essentially solar iron abundance. Under the plausible assumption that our photometry reaches stars still close to the ZAMS, the cluster is located at $d\approx5.5\:$kpc and has an age of $\sim60\:$Ma, though a slightly younger and more distant cluster cannot be ruled out. Despite the apparent good fit of isochrones, evolved stars seem to reside in positions of the CMD far away from the locations where stellar tracks predict Helium burning to occur. Of particular interest is the presence of four yellow supergiants, two on the ascending branch and two others close to or inside the instability strip.

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arXiv2017

The red supergiant population in the Perseus arm

We present a new catalogue of cool supergiants in a section of the Perseus arm, most of which had not been previously identified. To generate it, we have used a set of well-defined photometric criteria to select a large number of candidates (637) that were later observed at intermediate resolution in the the Infrared Calcium Triplet spectral range, using a long-slit spectrograph. To separate red supergiants from luminous red giants, we used a statistical method, developed in previous works and improved in the present paper. We present a method to assign probabilities of being a red supergiant to a given spectrum and use the properties of a population to generate clean samples, without contamination from lower-luminosity stars. We compare our identification with a classification done using classical criteria and discuss their respective efficiencies and contaminations as identification methods. We confirm that our method is as efficient at finding supergiants as the best classical methods, but with a far lower contamination by red giants than any other method. The result is a catalogue with 197 cool supergiants, 191 of which did not appear in previous lists of red supergiants. This is the largest coherent catalogue of cool supergiants in the Galaxy.

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arXiv2016

Characterisation of red supergiants in the Gaia spectral range

The infrared Calcium Triplet and its nearby spectral region have been used for spectral and luminosity classification of late-type stars, but the samples of cool supergiants (CSGs) used have been very limited (in size, metallicity range, and spectral types covered). The spectral range of the Gaia Radial Velocity Spectrograph (RVS) covers most of this region but does not reach the main TiO bands in this region, whose depths define the M sequence. We study the behaviour of spectral features around the Calcium Triplet and develop effective criteria to identify and classify CSGs, comparing their efficiency with other methods previously proposed. We measure the main spectral features in a large sample (almost 600) of CSGs from three different galaxies, and we analyse their behaviour through a principal component analysis. Using the principal components, we develop an automatised method to differentiate CSGs from other bright late-type stars, and to classify them. The proposed method identifies a high fraction of the supergiants (SGs) in our test sample, which cover a wide metallicity range (SGs from the SMC, the LMC, and the Milky Way) and with spectral types from G0 up to late-M. In addition, it is capable to separate most of the non-SGs in the sample, identifying as SGs only a very small fraction of them. A comparison of this method with other previously proposed shows that it is more efficient and selects less interlopers. A way to automatically assign a spectral type to the SGs is also developed. We apply this study to spectra at the resolution and spectral range of the Gaia RVS, with a similar success rate. The method developed identifies and classifies CSGs in large samples, with high efficiency and low contamination, even in conditions of wide metallicity and spectral-type ranges.

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arXiv2016

Spectral type, temperature and evolutionary stage in cool supergiants

In recent years, temperature scales in cool supergiants (CSGs) have been disputed, and the possibility that spectral types (SpTs) do not depend primarily on temperature has been raised. We explore the relations between different observed parameters and the capability of deriving accurate intrinsic stellar parameters from them through the analysis of the largest spectroscopic sample of CSGs to date from SMC and LMC. We explore possible correlations between different observational parameters, also making use of near- and mid-infrared colours and literature on photometric variability. Direct comparison between the behaviour of atomic lines (Fe I, Ti I, and Ca II) in the observed spectra and synthetic atmospheric models provides compelling evidence that effective temperature is the prime underlying variable driving the SpT sequence in CSGs. However, there is a clear correlation between SpT and luminosity, with later ones tending to correspond to more luminous stars with heavier mass loss. The population of CSGs in the SMC is characterised by a higher degree of spectral variability, early spectral types (centred on type K1) and low mass-loss rates (at least as measured by dust-sensitive mid-infrared colours). The population in the LMC displays less spectroscopic variability and later spectral types. The distribution of spectral types is not single-peaked. Instead, the brightest CSGs have a significantly different distribution from less luminous objects, presenting mostly M subtypes (centred on M2), and increasing mass-loss rates for later types. In conclusion, the observed properties of CSGs in the SMC and the LMC cannot be described correctly by standard evolutionary models. The very strong correlation between spectral type and bolometric luminosity, supported by all data from the Milky Way, cannot be reproduced at all by current evolutionary tracks.

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arXiv2015

A new survey of cool supergiants in the Magellanic Clouds

In this study, we conduct a pilot program aimed at the red supergiant population of the Magellanic Clouds. We intend to extend the current known sample to the unexplored low end of the brightness distribution of these stars, building a more representative dataset with which to extrapolate their behaviour to other Galactic and extra-galactic environments. We select candidates using only near infrared photometry, and with medium resolution multi-object spectroscopy, we perform spectral classification and derive their line-of-sight velocities, confirming the nature of the candidates and their membership to the clouds. Around two hundred new RSGs have been detected, hinting at a yet to be observed large population. Using near and mid infrared photometry we study the brightness distribution of these stars, the onset of mass-loss and the effect of dust in their atmospheres. Based on this sample, new a priori classification criteria are investigated, combining mid and near infrared photometry to improve the observational efficiency of similar programs as this.

astro-ph.SR↗

arXiv2013

The population of M-type supergiants in the starburst cluster Stephenson 2

The open cluster Stephenson 2 contains the largest collection of red supergiants known in the Galaxy, and at present is the second most massive young cluster known in the Milky Way. We have obtained multi-epoch, intermediate-resolution spectra around the CaII triplet for more than 30 red supergiants in Stephenson~2 and its surroundings. We find a clear separation between a majority of RSGs having spectral types M0-M2 and the brightest members in the NIR, which have very late spectral types and show strong evidence for heavy mass loss. The distribution of spectral types is similar to that of RSGs in other clusters, such as NGC 7419, or associations, like Per OB1. The cluster data strongly support the idea that heavy mass loss and maser emission is preferentially associated with late-M spectral types, suggesting that they represent an evolutionary phase.

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