Ramiro de la Reza

A New Association of Post-T Tauri Stars near the Sun*

Observing ROSAT sources in an area 20° × 25° centered at the high-latitude (b = -59°) active star ER Eri, we found evidences for a nearby association, that we call the Horologium association (HorA), formed by at least 10 very young stars, some of them being bona fide post–T Tauri stars. We suggest other six stars as possible members of this proposed association. We examine several requirements that characterize a young stellar association. Although no one of them, isolated, gives an undisputed prove of the existence of the HorA, all together practically create a strong evidence for it. In fact, the Li line intensities are between those of the older classical T Tauri stars and the ones of the Local Association stars. The space velocity components 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, so that it could be one of its last episodes of star formation. In this region of the sky there are some hotter and non–X-ray active stars, with similar space velocities, that could be the massive members of the HorA, among them, the nearby Be star Achernar. The maximum of the mass distribution function of its probable members is around 0.7–0.9 M☉. We estimate its distance as ~60 pc and its size as ~50 pc. If spherical, this size would be larger than the surveyed area, and many other members could have been missed. ER Eri itself was found to be not a member, but a background RS CVn–like system. We also observed three 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 two very wide systems. The HorA is much more isolated from clouds and older (~30 Myr) 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 primitive accreting disks.

Rapidly Rotating Lithium-rich K Giants: The New Case of the Giant PDS 365

PDS 365 is a newly detected, rapidly rotating (v sin i = 20 km s-1), single, low-mass giant star that with HD 233517 and HD 219025 forms a remarkable ensemble of single K giants with the unique properties of rapid rotation, very strong Li lines, an asymmetrical Hα profile, and a large far-infrared excess. Their v sin i values are between 18 and 23 km s-1, and their LTE Li abundances, log e(Li), are between 2.9 and 3.9. Detailed analysis of PDS 365 reveals it to be a ~1 M⊙ 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 Hα profiles and large far-infrared excesses are present. If we consider single K giants, we find that among rapid (v sin i ≥ 8 km s-1) 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.

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 the disk due to a magnetic connection, and includes changes in the stars 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 ~3 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.

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 PDS 456 at coordinates a 5 17 h 28 m 19 s d 52 147159550 (epoch 2000), with a relatively nearby (z 5 0.184) and bright (B 5 14.69) quasar. Its position at Galactic coordinates l 5 107 b 51 117 .2, near the bulge of the Galaxy, may explain why it was not detected before. The optical spectrum of PDS 456 is typical of a luminous quasar, showing a broad (FWHM » 4000 km s 21 )H bline, very intense Fe ii lines, and a weak [O iii] l5007 line. PDS 456 is associated to the infrared source IRAS 1725421413 with a 60 mm infrared luminosity L60 5 3.8 # 10 45 ergs s 21 . The relatively flat slopes in the infrared [ a(25, 60) 52 0.33 and a(12, 25) 52 0.78] and a flat power index in the optical (Fn / n 20.72 ) may indicate a low dust content. A good match between the position of PDS 456 and the position of the X-ray source RXS J172819.32141600 implies an X-ray luminosity LX 5 2.8 # 10 44 ergs s 21 . 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 (aOX 52 1.64), suggests that PDS 456 is radio quiet. A radio survey previously performed in this region yields an upper limit for radio power at about 5 GHz of approximately 2.6 # 10 30 ergs 21 Hz 21 . We estimate the Galactic reddening in this line of sight to be AB . 2.0, implying an absolute magnitude MB 52 26.7 (using H0 5 75 km s 21 Mpc 21 and q0 5 0). In the optical, PDS 456 is therefore 1.3 times more luminous than 3C 273 and the most luminous quasar in the nearby (z • 0.3) universe. Subject headings: infrared: general — quasars: individual (PDS 456, IRAS 17254 21413) — X-rays: galaxies

SULFUR ABUNDANCES IN THE ORION ASSOCIATION B STARS

Sulfur abundances are derived for a sample of 10 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, S II and S III. 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 modeling of the photospheres in one-dimensional and three-dimensional, 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–1.

A TWO MICRON ALL SKY SURVEY ANALYSIS OF THE STABILITY OF SOUTHERN BOK GLOBULES

We used near-infrared Two Micron All Sky Survey 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 (ξmax) 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-asymmetric line profile toward a globule of the sample, obtaining an upper limit infall speed of 0.25 km s–1.

On the Rotation of Post-T Tauri Stars in Associations

Nearby associations or moving groups of post–T Tauri stars with ages between ~10 and 30 Myr are excellent objects for the study of the initial spin-up phase during the pre–main-sequence evolution. An empirical approach is adopted here for the first time with these stars to infer their rotations, properties, and relations to X-ray emission. Three nearby associations with distances less than 100 pc are considered: the TW Hydrae association (TWA) with an age of 8 Myr, the β Pictoris moving group (BPMG) with an age of 12 Myr, and a combination of Tucana and Horologium associations (Tuc/HorA; 30 Myr). Two low- and high-rotation modes are considered for each association, with stellar masses of 0.1 M⊙ ≤ M < 1.5 M⊙ and 1.5 M⊙ ≤ M ≤ 2.6 M⊙, respectively. Because no observed rotational periods are known for these stars, we use a mathematical tool to infer representative equatorial rotation velocities v0(eq) from the observed distribution of projected rotational velocities (v sin i). This is done for each mode and for each association. A spin-up is found for the high-rotation mode, whereas in the low-rotation mode the v0(eq) do not increase significantly. This insufficient increase of v0(eq) 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. A two-dimensional (mass and v sin i) K-S statistical test yields results compatible with a spin-up scenario. By supposing that the distribution of the 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 rotational and stellar masses using the luminosity X-ray indicators LX and LX/Lb present similar properties, as does the dependence on stellar mass and rotation, at least for the younger associations TWA and BPMG, to those obtained for T Tauri stars in the Orion Nebula Cluster (1 Myr). A strong desaturation effect appears at ~30 Myr, the age of Tuc/HorA, measured essentially by the early-G and late-F type stars. 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.

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.

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.

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 (7)Li enrichment-mass-loss scenario, we explore the possibility of (7)Li 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 (7)Li content of M3 can be obtained during the entire life of this cluster. However, because M3 will cross into the Galactic plane several times, the new (7)Li will be very probably removed by ram pressure into the disk. Globular clusters appear then as possible new sources of (7)Li 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 (7)Li variations and that the cool bottom process mixing mechanism acting in the case of (7)Li could also play a role in the case of Na and Al surface enrichments.