arXiv subjects

Helio J. Rocha-Pinto

Publications and source records attributed to Helio J. Rocha-Pinto.

24 records · Page 2Linked to original sources

arXiv2003

The Galactic Anticenter Stellar Stream

The recently discovered, ring-like structure just outside the Galactic disk in Monoceros is detected and traced among 2MASS M giant stars. We have developed a method to recover the signature of this structure from the distance probability density function of stars along a given line of sight. Its detection is possible even when the metallicity is unknown, provided that the structure is not too embedded in the disk. Application of this method reveals the presence of a large group of M giant stars at a Galactocentric distance of 18$\pm 2$ kpc, over $+36\degr<b<+12\degr$ and $100\degr < l <270\degr$. Evidence that the stream extends to high negative latitudes is also found. That the structure contains M giants shows that it contains populations of at least an order of magnitude higher abundance than the [Fe/H] = -1.6 mean metallicity previously reported for this system. The structural characteristics of the stellar stream as traced by M giants do not support the interpretation of this structure as a homogeneously dense ring that surrounds the Galaxy, but as a more localized structure, possibly a merging dwarf galaxy with tidal arms, like the Sagittarius dwarf galaxy.

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arXiv2000

An Intermittent Star Formation History in a 'Normal' Disk Galaxy: The Milky Way

The star formation rate history of the Milky Way is derived using the chromospheric age distribution for 552 stars in the solar neighborhood. The stars sample birthsites distributed over a very large range of distances because of orbital diffusion, and so give an estimate of the global star formation rate history. The derivation incorporates the metallicity dependence of chromospheric emission at a given age, and corrections to account for incompleteness, scale height-age correlations, and stellar evolutionary effects. The results show that the Milky Way has undergone fluctuations in the global star formation rate with amplitudes greater than a factor of 2-3 on timescales less than 0.2-1 Gyr. The actual history is likely to be more bursty than found here because of the smearing effect of age uncertainties. There is some evidence for a slow secular increase in the star formation rate, perhaps a record of the accumulation history of our galaxy. A smooth nearly-constant star formation rate history is strongly ruled out, confirming the result first discovered by Barry (1988) using a smaller sample and a different age calibration. This result suggests that galaxies can fluctuate coherently on large scales.

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arXiv1997

The G dwarf Metallicity Distribution and the Problem of Stellar Lifetimes Lower than the Disk Age

We examine the possibility of the inclusion of objects having lifetimes lower than the disk age in the database of recent determinations of the G dwarf metallicity distribution for the solar neighbourhood. As a preliminary step, stars with lifetimes potentially lower than the disk age are identified by means of a relation between (b-y), or mass, and [Fe/H], from evolutionary models of stars at the zero-age main sequence. We apply these results to the G dwarf samples of Rocha-Pinto and Maciel (1996) and Wyse and Gilmore (1995). We show that the majority of G dwarfs in these samples can be regarded as long-lived objects for chemical evolution purposes, provided the disk age is greater than 12 Gyr. We also find that the G dwarf problem is not an effect of the metallicity dependence of the stellar lifetimes.

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arXiv1997

History of the Star Formation Rate in the Local Disk from the G Dwarf Metallicity Distribution

We address the question of the occurrence of star formation bursts in the solar neighbourhood by using the metallicity distribution of the G dwarfs. We present a method to recover the star formation history using simultaneously the metallicity distribution and the age-metallicity relation. The method associates the number of stars in a given metallicity interval with the corresponding time interval predicted by the age-metallicity relation. We take into account corrections relative to observational errors, cosmic scatter and scale height effects. The method is tested by simulations of the chemical evolution of a galaxy with irregular star formation rate, and our results show that it is possible to recover the star formation history fairly well. The application of the method to the solar neighbourhood shows evidences for at least two strong events of star formation: a burst 8 Gyr ago, and a lull 2-3 Gyr ago. These features confirm some previous studies on the star formation history in the local disk.

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arXiv1996

The metallicity distribution of G dwarfs in the solar neighbourhood

We derive a new metallicity distribution of G dwarfs in the solar neighbourhood, using uvby photometry and up-to-date parallaxes. Our distribution comprises 287 G dwarfs within 25 pc from the Sun, and differs considerably from the classic solar neighbourhood distribution of Pagel & Patchett and Pagel by having a prominent single peak around [Fe/H] = -0.20 dex. The raw data are corrected for observational errors and cosmic scatter assuming a deviation of 0.1. In order to obtain the true abundance distribution, we use the correction factors given by Sommer-Larsen, which take into account the stellar scale heights. The distribution confirms the G dwarf problem, that is, the paucity of metal-poor stars relative to the predictions of the simple model of chemical evolution. Another feature of this distribution, which was already apparent in previous ones, is the small number of metal-rich stars again in comparison with the simple model. Our results indicate that it is very difficult to fit the simple model to this distribution, even with the definition of an `effective yield'. A comparison with several models from the literature is made. We find that models with infall are the most appropriate to explain the new metallicity distribution. We also show that the metallicity distribution is compatible with a major era of star formation occurring 5 to 8 Gyr ago, similar to results found by several authors.

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arXiv1994

A Model for the Chemical Evolution of the Galaxy with Refuses

A model is presented for the chemical evolution of the solar neighbourhood which takes into account three families of galactic objects, according to their condensation states: stars, refuses and gas. Stars are defined as every condensed objects with masses greater than or equal to the minimum mass which ignites hydrogen and which will give rise to an evolutionary track on the HR diagram to the left of Hayashi's limit; refuses include the remnants, which are compact objects resulting from stellar deaths, and the residues, which have masses not large enough to ignite hydrogen; gas is defined as the mass which can be condensed to form stars and/or residues. We have developed equations for the mass evolution of each family, and have studied the gas metallicity distribution within the framework of the instantaneous recycling approximation, adopting different initial conditions. In order to constrain the model parameters we have also used preliminary evaluations of comet cloud masses to investigate the role of the residues as sinks of heavy elements in the Galaxy.

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