arXiv subjects

M. Fernandez

Publications and source records attributed to M. Fernandez.

43 records · Page 3Linked to original sources

arXiv2004

Rotation periods of Post-T Tauri stars in Lindroos systems

We present a rotational study of Post-T Tauri stars (PTTSs) in Lindroos systems, defined as binaries with early type primaries on the main-sequence (MS) and late-type secondaries on the pre-main-sequence (PMS) phase. The importance of this study in comparison with previous ones is that the Lindroos sample is not X-ray selected so we avoid a possible bias towards fast rotators. In this preliminary study we have monitored eleven stars in the UBVRI bands during two campaigns of ten consecutive nights each. Eight of the observed PTTSs show periodic modulations in their lightcurves and the derived periods range from 1.9d to 8.0d. The comparison of these results with theoretical rotational tracks based on disk-star locking theory shows that star-disk decoupling times of 1-20 Myr could reproduce the rotational properties of the targets, assuming an initial rotation period of ~8d and a mass of 1 Mo. We have studied the rotation-activity relations of Lindroos PTTSs and compared them with those found in other groups of PMS and zero-age main-sequence (ZAMS) ~1 Mo stars. The Lindroos sample displays activity-rotation relations very similar to those found in TTSs. It contains a mixture of very active stars, with Lx/Lbol ratios close to the saturation level of -3, and less active (unsaturated) stars. This could be the result of different star-disk decoupling times. Future monitoring of a larger and unbiased sample of PTTS will be important to confirm the significance of these results.

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arXiv2003

The weak-line T Tauri star V410Tau I. A multi-wavelength study of variability

We present the results of an intensive coordinated monitoring campaign in the optical and X-ray wavelength ranges of the low-mass, pre-main sequence star V410Tau carried out with the aim to study the relation between various indicators for magnetic activity that probe emission from different atmospheric layers: optical photometric star spot (rotation) cycle, chromospheric Halpha emission, and coronal X-rays. Two X-ray pointings were carried out with the Chandra satellite simultaneously with the optical observations, and centered near the maximum and minimum levels of the optical lightcurve. A relation of their different count levels to the rotation period of the dominating spot is not confirmed by a third Chandra observation carried out some months later, during another minimum of the 1.87d cycle. Similarly we find no indications for a correlation of the Halpha emission with the spots' rotational phase. The extraordinary stability of the largest spot is confirmed by long-term photometric and radial velocity measurements. Joining our optical photometry with previous data we provide a new estimate for the dominant periodicity of V410Tau. This updated value removes systematic offsets of the time of minimum observed in data taken over the last decade. Furthermore, the combination of the new data with published measurements taken during the last decade allows us to examine long-term changes in the mean light level of the photometry of V410Tau. A variation on the timescale of 5.4yr is suggested. Assuming that this behavior is truely cyclic V410Tau is the first pre-main sequence star on which an activity cycle is detected.

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arXiv2003

Rotational periods of very young brown dwarfs and very low-mass stars in ChaI

We have studied the photometric variability of very young brown dwarfs and very low-mass stars (masses well below 0.2 M_sun) in the ChaI star forming region. We have determined photometric periods in the Gunn i and R band for the three M6.5-M7 type brown dwarf candidates ChaHa2, ChaHa3 and ChaHa6 of 2.2 to 3.4 days. These are the longest photometric periods found for any brown dwarf so far. If interpreted as rotationally induced they correspond to moderately fast rotational velocities, which is fully consistent with their v sini values and their relatively large radii. We have also determined periods for the two M5-M5.5 type very low-mass stars B34 and CHXR78C. In addition to the Gunn i and R band data, we have analysed JHK_s monitoring data of the targets, which have been taken a few weeks earlier and confirm the periods found in the optical data. Upper limits for the errors in the period determination are between 2 and 9 hours. The observed periodic variations of the brown dwarf candidates as well as of the T Tauri stars are interpreted as modulation of the flux at the rotation period by magnetically driven surface features, on the basis of a consistency with v sini values as well as (R-i) color variations typical for spots. Furthermore, the temperatures even for the brown dwarfs in the sample are relatively high (>2800K) because the objects are very young. Therefore, the atmospheric gas should be sufficiently ionized for the formation of spots on one hand and the temperatures are too high for significant dust condensation and hence variabilities due to clouds on the other hand.

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arXiv2002

Coordinated Multiwavelength Observations of V410Tau

In November 2001 we undertook a coordinated observing campaign to study the connection between X-ray and optical variability in the weak-line T Tauri star V410Tau. The observing plan included three 15 ksec observations with Chandra using the Advanced CCD Imaging Spectrometer for Spectroscopy scheduled for different phases of the known 1.87 d starspot cycle. Photometric and spectroscopic monitoring of V410Tau involving telescopes on three different continents was scheduled simultaneously with the Chandra exposures.

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arXiv2000

The distance to the nearest star-forming clouds: MBM12 and MBM20

We present high-resolution spectra (R ~ 49,000) of stars that have parallax measurements from the Hipparcos satellite and are projected along the line of sight to the two nearest known star forming clouds to the Sun: MBM12 and MBM20. The spectra were obtained with the FOCES Echelle Spectrograph at the 2.2 meter telescope in Calar Alto, Spain and the wavelength range was chosen to include the interstellar NaI D lines at 5889.950 Angstroms and 5895.924 Angstroms. Since the stars are at a range of distances, we use their spectra along with their parallaxes from Hipparcos to determine the distance to the molecular gas. The stars in front of the cloud do not show interstellar NaI D absorption features while the stars behind the cloud do show interstellar absorption features. We find that both clouds are somewhat more distant than previously estimated. The revised distance to MBM12 is 58+-5 pc < d < 90+-12 pc and the distance to MBM20 is 112+-15 pc < d < 161+-21 pc.

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arXiv1999

On the possibility of ground-based direct imaging detection of extra-solar planets: The case of TWA-7

We show that {\em ground-based} direct imaging detection of extra-solar planets is possible with current technology. As an example, we present evidence for a possible planetary companion to the young T Tauri star 1RXSJ104230.3$-$334014 (=TWA-7), discovered by ROSAT as a member of the nearby TW Hya association. In an HST NICMOS F160W image, an object is detected that is more than 9 mag fainter than TWA-7, located $2.445 \pm 0.035^{\prime \prime}$ south-east at a position angle of $142.24 \pm 1.34^{\circ}$. One year later using the ESO-NTT with the SHARP speckle camera, we obtained H- and K-band detections of this faint object at a separation of $2.536 \pm 0.077^{\prime \prime}$ and a position angle of $139.3 \pm 2.1^{\circ}$. Given the known proper motion of TWA-7, the pair may form a proper motion pair. If the faint object orbits TWA-7, then its apparent magnitudes of H=$16.42 \pm 0.11$ and K=$16.34 \pm 0.15$ mag yield absolute magnitudes consistent with a $\sim 10^{6.5}$ yr old $\sim 3$ M$_{\rm jup}$ mass object according to the non-gray theory by Burrows et al. (1997). At $\sim 55$ pc, the angular separation of $\sim 2.5^{\prime \prime}$ corresponds to $\sim 138$~AU. However, position angles and separations are slightly more consistent with a background object than with a companion.

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arXiv1999

ROSAT PSPC observations of T Tauri stars in MBM12

We present the ROSAT PSPC pointed and ROSAT All-Sky Survey (RASS) observations and the results of our low and high spectral resolution optical follow-up observations of the T Tauri stars (TTS) and X-ray selected T Tauri star candidates in the region of the high galactic latitude dark cloud MBM12 (L1453-L1454, L1457, L1458). Previous observations have revealed 3 "classical" T Tauri stars and 1 "weak-line" T Tauri star along the line of sight to the cloud. Because of the proximity of the cloud to the sun, all of the previously known TTS along this line of sight were detected in the 25 ks ROSAT PSPC pointed observation of the cloud. We conducted follow-up optical spectroscopy at the 2.2-meter telescope at Calar Alto to look for signatures of youth in additional X-ray selected T Tauri star candidates. These observations allowed us to confirm the existence of 4 additional TTS associated with the cloud and at least 2 young main sequence stars that are not associated with the cloud and place an upper limit on the age of the TTS in MBM12 ~ 10 Myr. The distance to MBM12 has been revised from the previous estimate of 65+-5 pc to 65+-35 pc based on results of the Hipparcos satellite. At this distance MBM12 is the nearest known molecular cloud to the sun with recent star formation. We estimate a star-formation efficiency for the cloud of 2-24%. We have also identified a reddened G9 star behind the cloud with Av ~ 8.4-8.9 mag. Therefore, there are at least two lines of sight through the cloud that show larger extinctions (Av > 5 mag) than previously thought for this cloud. This higher extinction explains why MBM12 is capable of star-formation while most other high-latitude clouds are not.

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