arXiv subjects

Ramiro de la Reza

Publications and source records attributed to Ramiro de la Reza.

15 recordsLinked to original sources

arXiv2025

Lithium abundance and stellar rotation in the Galactic halo and thick disc: Contribution from low-mass giant field stars

The stellar evolution of lithium-rich (Li-rich) giant stars at very low metallicities remains largely unexplored to date. Using two recent large LAMOST catalogues of field, low-mass giant stars (both Li-rich and Li-poor) with metallicities from -4.0 to -1.0, we studied the conditions for Li enrichment and the distribution of stellar rotations in the Galactic halo and thick disc. Due to the scarcity of stars with [Fe/H] < -3.0, only three Li-rich RGB stars are known in this regime. The observational appearance of giants across the horizontal branch (HB) and asymptotic giant branch (AGB) stages (with Li abundances up to 6.15 dex) has been detected for metallicities > -2.5. Among these stars, we detected IR excesses indicative of giant stars losing mass, showing a recent episodic Li-enrichment process related to the Cameron-Fowler mechanism for the formation of new 7Li. Because stars with IR excesses are distributed across most metallicity values, we suggest this mechanism is at work throughout an important part of the Galaxy's evolutionary history. Based on these IR excesses, we identified three Li thresholds: about 1.5 dex for RGB stars, about 0.5 dex for HB stars, and about -0.5 dex for AGB stars, establishing a new criterion to characterise Li-rich giants. We carried out a study of stellar rotations in metal-poor giant stars, revealing that a plateau appears for velocities greater than 40 km/s up to near 90 km/s, with Li abundances from 1.02 to 1.82 dex. Among Li-rich giants with v sin i > 40 km/s, increasing rotation is observed as metallicity decreases from -1.0 to -2.5. The presence of RGB and HB Li-rich giants with rotations up to 90 km/s suggests that stellar models must account for extended 3He reservoir lifetimes as a source of 7Li. The velocity around 40 km/s appears to be a new critical value.

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arXiv2021

The bi-modal $^7$Li distribution of the Milky Way's thin-disk dwarf stars: The role of Galactic-scale events and stellar evolution

The lithium abundance, A(Li), in stellar atmospheres suffers from various enhancement and depletion processes during the star's lifetime. While several studies have demonstrated that these processes are linked to the physics of stellar formation and evolution, the role that Galactic-scale events play in the galactic A(Li) evolution is not yet well understood. We aim to demonstrate that the observed A(Li) bi-modal distribution, in particular in the FGK-dwarf population, is not a statistical artefact and that the two populations connect through a region with a low number of stars. We also want to investigate the role that Galactic-scale events play in shaping the A(Li) distribution of stars in the thin disk. We use statistical techniques along with a Galactic chemical evolution model for A(Li) that includes most of the well-known $^7$Li production and depletion channels. We confirm that the FGK main-sequence stars belonging to the Milky Way's thin disk present a bi-modal A(Li) distribution. We demonstrate that this bi-modality can be generated by a particular Milky Way star formation history profile combined with the stellar evolution's $^7$Li depletion mechanisms. We show that A(Li) evolution can be used as an additional proxy for the star formation history of our Galaxy.

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arXiv2015

Complex Organic and Inorganic Compounds in Shells of Lithium-rich K Giant Stars

Hydrocarbon organic material, as found in the interstellar medium, exists in complex mixtures of aromatic and aliphatic forms. It is considered to be originated from carbon enriched giant stars during their final stages of evolution, when very strong mass loss occurs in a few thousand years on their way to become planetary nebulae. We show here that the same organic compounds appear to be formed in previous stages of the evolution of giant stars. More specifically, during the first ascending giant branch K-type stars. According to our model this happens only when these stars are being abruptly enriched with lithium together with the formation of a circumstellar shell with a strong mass loss during just a few thousand years. This sudden mass loss is, on an average, a thousand times larger than that of normal Li-poor K giant stars. This shell would later be detached, specially when the star stops its Li enrichment and a rapid photospheric Li depletion occurs. In order to gain extra carbon-based material to form the organic hydrocarbons, and also to explain the presence of complex inorganic compounds in these stars, we propose an interaction of these strong winds with remaining asteroidal/cometary disks that already existed around these stars since they were dwarf A-type stars. The mechanism of interaction presented here is successful to explain the presence of inorganic compounds, however it is unable to produce new carbon free atoms to form the organic hydrocarbon compounds. Finally, we discuss some suggestions and speculations that can eventually help solving the long-standing puzzle of Li-rich giants.

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arXiv2012

Can Li-rich giants eject shells? Assembling the lithium puzzle in K giants

The existence of K giant stars with high Li abundance continues to challenge the standard theory of stellar evolution. All recent extensive surveys in the Galaxy show the same result: about 1 % of the mainly normal slow rotating K giants are Li rich. We explore here a model with two scenarios based on the important relation of Li-rich and Li-poor K giants with IR excesses. In this model, all K giant stars suffer a rapid enrichment and depletion of Li inducing the formation and ejection of circumstellar shells. The observational detection of these shells will not only validate this model, but also will give important hints on the mechanism of Li enrichment of these stars.

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arXiv2010

Spin Evolution of Accreting Young Stars. I. Effect of Magnetic Star-Disk Coupling

We present a model for the rotational evolution of a young, solar mass star interacting with an accretion disk. The model incorporates a description of the angular momentum transfer between the star and disk due to a magnetic connection, and includes changes in the star's mass and radius and a decreasing accretion rate. The model also includes, for the first time in a spin evolution model, the opening of the stellar magnetic field lines, as expected to arise from twisting via star-disk differential rotation. In order to isolate the effect that this has on the star-disk interaction torques, we neglect the influence of torques that may arise from open field regions connected to the star or disk. For a range of magnetic field strengths, accretion rates, and initial spin rates, we compute the stellar spin rates of pre-main-sequence stars as they evolve on the Hayashi track to an age of 3~Myr. How much the field opening affects the spin depends on the strength of the coupling of the magnetic field to the disk. For the relatively strong coupling (i.e., high magnetic Reynolds number) expected in real systems, all models predict spin periods of less than $\sim3$ days, in the age range of 1--3~Myr. Furthermore, these systems typically do not reach an equilibrium spin rate within 3~Myr, so that the spin at any given time depends upon the choice of initial spin rate. This corroborates earlier suggestions that, in order to explain the full range of observed rotation periods of approximately $1$--$10$ days, additional processes, such as the angular momentum loss from powerful stellar winds, are necessary.

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arXiv2009

Sulfur Abundances in the Orion Association B Stars

Sulfur abundances are derived for a sample of ten B main-sequence star members of the Orion association. The analysis is based on LTE plane-parallel model atmospheres and non-LTE line formation theory by means of a self-consistent spectrum synthesis analysis of lines from two ionization states of sulfur, SII and SIII. The observations are high-resolution spectra obtained with the ARCES spectrograph at the Apache Point Observatory. The abundance distribution obtained for the Orion targets is homogeneous within the expected errors in the analysis: A(S)=7.15+/-0.05. This average abundance result is in agreement with the recommended solar value (both from modelling of the photospheres in 1-D and 3-D, and meteorites) and indicates that little, if any, chemical evolution of sulfur has taken place in the last ~4.5 billion years. The sulfur abundances of the young stars in Orion are found to agree well with results for the Orion nebulae, and place strong constraints on the amount of sulfur depletion onto grains as being very modest or nonexistent. The sulfur abundances for Orion are consistent with other measurements at a similar galactocentric radius: combined with previous results for other OB-type stars produce a relatively shallow sulfur abundance gradient with a slope of -0.037+/-0.012 dex/kpc.

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arXiv2009

A 2MASS Analysis of the Stability of Southern Bok Globules

We used near-infrared 2MASS data to construct visual extinction maps of a sample of Southern Bok globules utilizing the NICE method. We derived radial extinction profiles of dense cores identified in the globules and analyzed their stability against gravitational collapse with isothermal Bonnor-Ebert spheres. The frequency distribution of the stability parameter xi_max of these cores shows that a large number of them are located in stable states, followed by an abrupt decrease of cores in unstable states. This decrease is steeper for globules with associated IRAS point sources than for starless globules. Moreover, globules in stable states have a Bonnor-Ebert temperature of T = 15 +- 6 K, while the group of critical plus unstable globules has a different temperature of T = 10 +- 3 K. Distances were estimated to all the globules studied in this work and the spectral class of the IRAS sources was calculated. No variations were found in the stability parameters of the cores and the spectral class of their associated IRAS sources. On the basis of 13CO J = 1-0 molecular line observations, we identified and modeled a blue-assymetric line profile toward a globule of the sample, obtaining an upper limit infall speed of 0.25 km/s.

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arXiv2004

The Metallicity of Post-T Tauri Stars: A preliminary approach to the understanding of the metal enrichment of stars harboring planets

The metallicity of young low mass Post-T Tauri stars in coeval associations is practically unknown. This work is the beginning of a systematic measurement of these metallicities based on high resolution spectra of low rotating members of these associations. Here, we present an application by examining the behavior of the Iron abundance with stellar mass and temperature of some members of an association 30 Myr old. This will test the possibility of explaining the high metallic content of stars with planets by means of injection of planetesimals during this early stage of evolution.

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arXiv2004

On the Rotation of Post-T Tauri Stars in Associations

Nearby associations are excellent objects for the study of the initial spin up phase during the PMS evolution. An empirical approach is adopted here to infer their rotations properties and relations to X-ray emission. Three nearby associations are considered. The TW Hya association with an age of 8 Myr, the Beta Pictoris moving group with 12 Myr and a combination of Tucana and Horologium associations (30 Myr). Two low and high rotation modes are considered for each association with stellar masses less than 1.5M and greater than 1.5M respectively. We infer representative equatorial rotation velocities Vo from the observed distribution of projected rotational velocities vsini. A spin up is found for the high rotation mode, whereas in the low rotation mode the Vo do not increase significantly. This insufficient increase of Vo is probably the cause of a decrease of the total mean specific angular momentum for the low mass stars between 8 and 30 Myr. However, for the high mass stars, where a sufficient spin up is present, the specific angular momentum is practically conserved in this same time interval. By supposing that the distribution of masses of these three associations follows a universal mass function, we estimate the number of members of these associations that remain to be detected. The analysis of rotation and stellar masses using luminosity X-rays indicators present similar properties, as the dependence on stellar mass and rotation, at least for the younger associations, to those obtained for T Tauri stars in the ONC. A strong desaturation effect appears at 30 Myr. This effect seems to be provoked by the minimum configuration of the stellar convection layers, attained for the first time for the higher mass stars at 30 Myr. The desaturation appears to be independent of rotation at this stage.

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arXiv2003

A Method to Search for Associations of Young Stars

In the SACY (Search for Associations Containing Young-stars) project we try to identify associations of stars younger than the Local Association among HIPPARCOS and/or TYCHO-2 stars later than G0 which are counterparts of the ROSAT X-ray bright sources. High-resolution spectra for the possible optical counterparts were obtained in order to assess both the youth and the spatial motion of each target. More than 1000 ROSAT sources were observed, covering a large area in the Southern Hemisphere. Associations are characterized mainly by the similarity in UVW velocity space of their proposed member, but other parameters, as evolutionary age, Li abundance and distribution in space must also be taken into account. We proposed a method to identify associations when proper motions and radial velocities are available, but no parallaxes. Using the method we found eleven associations in the SACY data.

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arXiv2003

SACY - a Search for Associations Containing Young stars

The scientific goal of the SACY (Search for Associations Containing Young-stars) was to identify possible associations of stars younger than the Pleiades Association among optical counterparts of the ROSAT X-ray bright sources. High-resolution spectra for possible optical counterparts later than G0 belonging to HIPPARCOS and/or TYCHO-2 catalogs were obtained in order to assess both the youth and the spatial motion of each target. More than 1000 ROSAT sources were observed, covering a large area in the Southern Hemisphere. The newly identified young stars present a patchy distribution in UVW and XYZ, revealing the existence of huge nearby young associations. Here we present the associations identified in this survey.

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arXiv2002

Rapidly-rotating lithium-rich K giants: The new case of the giant PDS365

PDS365 is a newly detected rapidly rotating (vsin i = 20 km/s) single low mass giant star which with HD233517 and HD219025 forms a remarkable ensemble of single K giants with the unique properties of rapid rotation, very strong Li lines, an asymmetrical Halpha profile, and a large far infrared excess. Their vsin i values are between 18 and 23 km/s, and LTE Li abundances, log e(Li), are between 2.9 and 3.9. Detailed analysis of PDS365 reveals it to be near one solar mass giant with a value of 12C/13C approximately equal to 12. A clear relation between high rotational velocities and very high Li abundances for K giant stars is found only when asymmetrical Halpha profiles and large far-infrared excesses are present. If we consider single K giants, we find that among rapid (vsin i >= 8 km/s) rotators, a very large proportion (~ 50%) are Li-rich giants. This proportion is in contrast with a very low proportion (~ 2%) of Li-rich stars among the much more common slowly rotating K giants. This striking difference is discussed in terms of proposed mechanisms for Li enrichment.

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arXiv2000

A New Association of Post-T Tauri Stars Near The Sun

Observing ROSAT sources in 20 x 25 deg centered at the high latitude active star ER Eri, we found evidences for a new young nearby association (~30Myr at~60pc), the Horologium Association (HorA), formed by at least 10 probable and 6 possible members, some being Post-T Tauri stars. We examine several requirements that characterize a young association and they, together, create a strong evidence for the reality of the HorA. In fact, the Li line intensities are between those of the oldest classical T Tauri stars and the ones of the Local Association stars. The space velocities of the HorA relative to the Sun, U= -9.5+/-1.0, V = -20.9 +/- 1.1, W = -2.1 +/- 1.9, are not far from those of the Local Association. We suggest that some hotter and non-X-ray active stars, with similar space velocities, could be massive members of the HorA, among them, the nearby Be star Achernar. The maximum of the mass distribution function of the HorA is around 0.8 solar masses. At its distance, the projected size of the HorA, ~50 pc, would be larger than our surveyed area and many other members could have been missed. We also observed 3 control regions, two at northern and southern galactic latitudes and a third one in the known TW Hya Association (TWA), and the properties and distribution of their young stars strengthen the reality of the HorA. Contrary to the TWA, the only known binaries in the HorA are 2 very wide systems. The HorA is much more isolated from clouds and older than the TWA and could give some clues about the lifetime of the disks around T Tauri stars. Actually, none of the proposed members is an IRAS source indicating an advanced stage of the evolution of their accreting disks. ER Eri itself was found to be a RS CVn-like system.

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arXiv2000

On 7Li Enrichment by Low Mass Metal Poor Red Giant Branch Stars

First-ascent red giants with strong and very strong Li lines have just been discovered in globular clusters. Using the stellar internal prompt 7Li enrichment - mass loss scenario, we explore the possibility of 7Li enrichment in the interstellar matter of the globular cluster M3 produced by these Li-rich giants. We found that enrichment as large as 70% or more compared to the initial 7Li content of M3 can be obtained during the entire life of this cluster. However, due to the several crossings of M3 into the Galactic plane, the new 7Li will be very probably removed by ram pressure into the disk. Globular clusters appear then as possible new sources of 7Li in the galactic disk. It is also suggested that the known Na/Al variations in stars of globular clusters could be somehow related to the 7Li variations and that the cool bottom mixing mechanism acting in the case of 7Li could also play a role in the case of Na and Al surface enrichments.

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arXiv1997

Discovery of a Luminous Quasar in the Nearby Universe

In the course of the Pico dos Dias survey (PDS), we identified the stellar like object PDS456 at coordinates alpha = 17h 28m 19.796s, delta = -14deg 15' 55.87'' (epoch 2000), with a relatively nearby (z = 0.184) and bright (B = 14.69) quasar. Its position at Galactic coordinates l_II = 10.4deg, b_II = +11.2deg, near the bulge of the Galaxy, may explain why it was not detected before. The optical spectrum of PDS456 is typical of a luminous quasar, showing a broad (FWHM ~ 4000 km/s) H_βline, very intense FeII lines and a weak [OIII]\lambda5007 line. PDS456 is associated to the infrared source IRAS 17254-1413 with a 60 \mum infrared luminosity L_{60} = 3.8 x 10^{45} erg/s. The relatively flat slopes in the infrared (α(25,60) = -0.33 and α(12,25) = -0.78) and a flat power index in the optical (F_ν \propto ν^{-0.72}) may indicate a low dust content. A good match between the position of PDS456 and the position of the X-ray source RXS J172819.3-141600 implies an X-ray luminosity L_x = 2.8 x 10^{44} erg/s. The good correlation between the strength of the emission lines in the optical and the X-ray luminosity, as well as the steep optical to X-ray index estimated (α_{ox} = -1.64) suggest that PDS456 is radio quiet. A radio survey previously performed in this region yields an upper limit for radio power at ~ 5 GHz of ~ 2.6 x 10^{30} erg/s/Hz. We estimate the Galactic reddening in this line-of-sight to be A_B \simeq 2.0, implying an absolute magnitude M_B = -26.7 (using H_0 = 75 km s^{-1} Mpc^{-1} and q_0 = 0). In the optical, PDS456 is therefore 1.3 times more luminous than 3C 273 and the most luminous quasar in the nearby (z \leq 0.3) Universe.

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