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MATILDE FERNANDEZ

Instituto de Astrofisica de Andalucia (CSIC)
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E-mail:   matilde   the_at_symbol   iaa.es
...the picture


Overwiev:
                      Research interests and curriculum vitae ( postscript version).
                                  - T Tauris stars far from molecular clouds
                                  - The accretion process in classical T Tauri stars
                                  - Mass determination of pre-main sequence stars
                      Other interesting things, like caves and mountains
Fotos trineo: 1, 2, 3, 4.

RESEARCH INTERESTS & CURRICULUM VITAE

I am working on young, low mass (below 2 solar masses) stars. They have not yet reached the main sequence, that means that their main source of energy is not the hydrogen burning, contraction still plays a very important role. Nevertheless, they are already visible at optical (4000-8000 A) wavelengths; this is mainly the wavelength range in which I observe them. These stars are divided into two groups: the classical T Tauri stars, that undergo a moderate to strong mass accretion process, and the weak-line T Tauri stars, for which this process is quite attenuated or has already finished.
Almost all the research I do is related to the following topics. Feel free to ask or comment anything if you are interested in any of these topics.

Curriculum vitae.


T TAURI STARS FAR FROM MOLECULAR CLOUDS

X-ray observations from the ROSAT satellite led to the discovery of a significant fraction of low mass stars which are younger than 5 million years and located at large distances, up to 10 degrees, from any known star forming cloud. These stars are mostly weak-lined T Tauri stars, young objects in which the accretion of matter from a circumstellar disk is already finished or very weak. The origin of this previously unnoticed pre-main-sequence population is unclear. If the stars were formed in the dark clouds, a velocity much higher (larger than 3 km/s) than the typically measured average velocity (1-2 km/s) is required to carry the stars to their present locations.
Two models have been proposed to explain the origin of these stars: (1) ejection from their birth clouds with high velocities, as a result of close gravitational encounters, and (2) formation in small clouds, known as cloudlets, if as soon as stars form the molecular gas content disperses.

In order to check which model explains better the observations we have carried out optical photometry (in order to measure the rotational periods) and high resolution imaging (in order to search for companions). From the optical photometry, the rotational periods of 2/3 of the stars in the sample in the Lupus star forming region has been obtained. The fact that the periods are, on average, longer than what is observed on the cloud, supports the hypothesis of the formation on small cloudlets, that may have dissipated short after the star formation process (Fernández, Petr & Neuhauser, in preparation). A very small fraction, if any of the stars, may have been ejected from the big clouds by three body encounters.
The high resolution imaging on the Orion region has also been analyzed. The high binary frequency we found supports also the cloudlets hypothesis for the origin of those stars (Petr et al. in preparation).


THE ACCRETION PROCESS IN CLASSICAL T TAURI STARS

During my Ph.D.Thesis I studied a sample of 24 pre-main sequence stars (hereafter: first sample) in order to look for a relation between the optical continuum variability and the mass loss and accretion processes. With this goal, UBVRI photoelectric photometry and H alpha spectroscopy were carried out (Fernández 1995; Fernández et al. 1995). For those classical T Tauri stars for which the variability was well reproduced by hot spots, a relation was found between the amplitude of the variability and the H alpha equivalent width, in the sense that larger amplitudes corresponded to larger H alpha equivalent widths. Previous works found in the literature pointed to a relation between the H alpha emission, the luminosity of the forbidden lines and the excess of infrared luminosity, that means, a correlation between the accretion and mass loss processes. In this context the relation we found can be interpreted as a support for the explanation of the hot spots as generated by the accretion of mass onto the star (Fernández & Eiroa 1996).

Along these years photoelectric and CCD photometry of another sample of T Tauri stars was also carried out in the frame of the COYOTES (Coordinated Observations of Young ObjecTs from Earthbound Sites) program. The main goal of these observations was the study of the distribution of the rotational velocities of pre-main sequence and zero-age main sequence objects. The main results obtained so far in this program have been: (1) the observational evidence of the braking of classical T Tauri stars through the magnetic coupling between the stars and their inner accretion disks (Bouvier et al. 1993a, b), (2) to find a lower limit, of about 6 km/s, for the rotational velocity of the zero age main sequence Alpha Per G and K dwarfs (Allain et al. 1996), and (3) to obtain the rotational periods for stars classified as Post-T Tauri stars, thus filling the observational gap that previously existed between T Tauri stars and zero-age main sequence dwarfs (Bouvier et al. 1997).
I am also involved in the study of the accretion process in AA Tau. We have monitored its photometric, spectroscopic and polarimetric variations over a period of a month, and we propose a scenario, in the frame of the magnetospheric accretion theory, that explains the observations (Bouvier et al. 1999).

Due to the temperatures involved in the accretion process, the ultraviolet wavelength range provides very useful information. For this reason an ultraviolet monitoring program was carried out on one of the classical T Tauri stars, DI Cep, of the first sample. We observed it during 6 half shifts of the IUE satellite and the variability of features attributed to accretion was measured (Gómez de Castro & Fernández 1996).
In order to study with more detail the accretion process on classical T Tauri stars we carried out narrow band uvby photometry of some of these stars in the 3500-5500 Å wavelength range (Fernández, Schuster et al. in preparation). The anomalous behaviour already observed at short wavelengths for some stars in our first sample (the amplitude of the variability at 3600 Å being equal or smaller than that observed at 4400 Å) was also detected on more classical T Tauri stars. Nevertheless it became clear that, due to the abundance of emission lines in this region, flux calibrated spectroscopy should be the best approach to the study of the accretion process. Simultaneous photometry and spectroscopy or flux calibrated photometry was, thus, carried out on selected objects, most of them active classical T Tauri stars (Fernández et al. in preparation).

I am also interested in the study of other mechanisms responsible of the variability of other T Tauri stars. For this reason we observed spectroscopically, at very high resolution, some rapidly rotating weak-line T Tauri stars and these data allowed us to study the cool spots of some of them as well as the relation between these structures and their chromospheric activity (Fernández & Miranda 1998).
We have also selected a sample of T Tauri stars that show a behaviour between classical and weak line. We have performed simultaneous optical spectroscopic and photometric observations in order to study which are the mechanisms that play the most important role in their variability (Fernández, Alcalán, Covino, Chavarria-K et al. in preparation).

In the first sample that I mentioned, some Herbig Ae/Be stars were also included. These are the massive counterparts of the T Tauri stars, with masses from 3 to about 9 solar masses. Although they are all considered as one class of stars, quite different mechanisms seem to be responsible of their variability (Fernández & Eiroa, in preparation). We have also studied MWC 137, the central star of the S 266 ring nebula. Although it has been considered as a Herbig Ae/Be star candidate, our data support the hypothesis of MWC 137 being a B[e] supergiant located at 6 kpc or further from the Sun (Esteban & Fernández 1998).


MASS DETERMINATION OF PRE-MAIN SEQUENCE STARS

A large amount of weak line T Tauri stars have been found as optical counterparts of X-ray sources discovered by the ROSAT all sky survey in nearby star forming regions. High resolution optical spectroscopy carried out to confirm their young nature showed that several of them are spectroscopic binary systems. For most of these systems the photospheric lines of both stars are observed in the spectrum.
We are now searching for eclipsing binaries among these systems, because for a spectroscopic binary that is, simultaneously, an eclipsing binary, the masses of both stars can be determine with high precision. Up to now the mass of very few low mass young stars has been determine with such precision. The main goal of such determination is to constrain the theories that model the pre-main sequence evolution.
We have carried out CCD photometry on binary systems close to the Orion star forming region and we have found one of such seclipsing ystems (Covino et al. 2000). It is the first spectroscopic eclipsing binary for which both component are really pre-main sequence low mass stars.



OTHER INTERESTING THINGS

Caves in Mexico. They are really wonderful, and to look for them, and to find them. One of my favourites: Golondrinas. You can have a look to the entrance from the inside, to the ropes prepared in order to descent and to the "meadow", the platform at -330m.

The Alps. Mainly the glaciers, the high mountain.


Felipe


When Harry had been younger, he had dreamed and dreamed of some unknown relation coming to take him away. But it did never happen.
.
..
...
three... two... one
BUUUUMM!
The whole shack shivered and Harry sat bold upright, staring at the door. Someone was outside, knocking to come in ...

(Harry Potter and the philosopher's stone, J.K.Rowling)