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D. Barrado y Navascues

Publications and source records attributed to D. Barrado y Navascues.

72 records · Page 4Linked to original sources
arXiv2001

The Substellar Mass Function in sigma Orionis

We combine results from imaging searches for substellar objects in the sigma Orionis cluster and follow-up photometric and spectroscopic observations to derive a census of the brown dwarf population in a region of 847 arcmin^2. We identify 64 very low-mass cluster member candidates in this region. We have available three color (IZJ) photometry for all of them, spectra for 9 objects, and K photometry for 27% of our sample. These data provide a well defined sequence in the I vs I-J, I-K color magnitude diagrams, and indicate that the cluster is affected by little reddening despite its young age (~5 Myr). Using state-of-the-art evolutionary models, we derive a mass function from the low-mass stars (0.2 Msol) across the complete brown dwarf domain (0.075 Msol to 0.013 Msol), and into the realm of free-floating planetary-mass objects (<0.013 Msol). We find that the mass spectrum (dN/dm ~ m^{-alpha}) increases toward lower masses with an exponent alpha = 0.8+/-0.4. Our results suggest that planetary-mass isolated objects could be as common as brown dwarfs; both kinds of objects together would be as numerous as stars in the cluster. If the distribution of stellar and substellar masses in sigma Orionis is representative of the Galactic disk, older and much lower luminosity free-floating planetary-mass objects with masses down to about 0.005 Msol should be abundant in the solar vicinity, with a density similar to M-type stars.

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arXiv2000

WIYN Open Cluster Study 5. Lithium Depletion and Metallicity in G and K Dwarfs of the Open Cluster M35

We present an analysis of high quality spectra of members of the young cluster M35. By using a multi-fiber spectrograph, we are able to collect high signal-to-noise, high resolution spectra of a sample of photometric candidate members. Accurate radial velocities are used to establish the membership status and rotational velocities are measured using cross-correlation. We also derive the metal content of the cluster, [Fe/H]= -0.21$\pm$0.10, based on spectral synthesis. Finally, we derive the lithium abundances of the bona fide cluster members and compare the results with members of other clusters. For example, M35 shows a smaller range in both rotation rates and lithium abundances as compared to the Pleiades. We discuss possible roles of various parameters. Our high quality M35 database of lithium abundances and rotational velocities are perfectly suited to be used as a laboratory to test theoretical models dealing with the lithium depletion phenomenon.

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arXiv1999

A Lithium Age for the Young Cluster IC2391

We have identified a large number of possible very low mass members of the cluster IC2391 based primarily on their location in an I versus (R-I)_C CM diagram. We have obtained new photometry and low resolution ($Δλ= 2.7$ Å) spectroscopy of 19 of these objects (14.9 $\le$ I_C $\le$ 17.5) in order to confirm cluster membership. We identify 15 of our targets as likely cluster members based on their spectral types, radial velocity, EW(NaI8200Å), and H(alpha) emission strengths. One of these stars has a definite lithium detection and two other (fainter) stars have possible lithium detections. We find the lithium depletion boundary in IC2391 is at I_C=16.2, which implies an age for IC2391 of 53$\pm$5 Myr. While this is considerably older than the age most commonly attributed for this cluster (~35 Myr), the correction factor to the IC2391 age is comparable to those recently derived for the Pleiades and alpha Per clusters and can be explained by new models for high mass stars that incorporate a modest amount of convective core overshooting.

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arXiv1999

The Lithium Depletion Boundary and the Age of the Young Open Cluster IC~2391

We have obtained new photometry and intermediate resolution ($Δλ= 2.7$ Å ) spectra of 19 of these objects (14.9 $\le$ $I_c$ $\le$ 17.5) in order to confirm cluster membership. We identify 15 of our targets as likely cluster members based on their $VRI$ photometry, spectral types, radial velocity, and H$α$ emission strengths. Higher S/N spectra were obtained for 8 of these probable cluster members in order to measure the strength of the lithium 6708 Å doublet and thus obtain an estimate of the cluster's age. One of these 8 stars has a definite lithium detection and two other (fainter) stars have possible lithium detections. A color-magnitude diagram for our program objects shows that the lithium depletion boundary in IC~2391 is at $I_c$=16.2. Using recent theoretical model predictions, we derive an age for IC~2391 of 53$\pm$5 Myr. While this is considerably older than the age most commonly attributed for this cluster ($\sim$35 Myr) this result for IC~2391 is comparable those recently derived for the Pleiades and Alpha Persei clusters and can be explained by new models for high mass stars that incorporate a modest amount of convective core overshooting.

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arXiv1999

The Castor Moving Group: The age of Fomalhaut and Vega

We have recomputed the kinematic properties of several of dozens nearby stars, to try to verify if Castor has a cohort of stars sharing the same space motion and age. We used kinematics, the location of the stars in CM diagrams, their lithium abundances, and their activity, to establish that the moving group seems to be real and to reject several stars which were believed to be part of this group. Of the initial 26 stars, we show that probably only 16 stars are physically associated. The moving group contains several A spectral type stars. Among them, Vega and Fomalhaut, two of the prototypes of the beta Pic type stars. Since this association includes several late spectral type stars, we used their properties to estimate their age and, therefore, the age of the group and that of Vega and Fomalhaut. Our estimate for that age is 200$\pm$100 Myr.

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arXiv1999

The age-mass relation for chromospherically active binaries: III. Lithium depletion in giant components

We present a study of the lithium abundances of a sample of evolved components of Chromospherically Active Binary Systems. We show that a significant part of them have lithium excesses, independently of their mass and evolutionary stage. Therefore, it can be concluded that Li abundance does not depend on age for giant components of CABS. These overabundances appear to be closely related to the stellar rotation, and we interpret them as a consequence of the transfer of angular momentum from the orbit to the rotation as the stars evolve in and off the Main Sequence, in a similar way as it happens in the dwarf components of the same systems and in the Tidally Locked Binaries belonging to the Hyades and M67.

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arXiv1999

The Age of Beta Pic

We have reanalyzed data for the proposed moving group associated with beta Pic in order to determine if the group (or part of it) is real, and, if so, to derive an improved age estimate for beta Pic. By using new, more accurate proper motions from PPM and Hipparcos and a few new radial velocities, we conclude that on kinematic grounds, two M dwarfs have space motions that coincide with that of beta Pic to within 1 km/s with small error bars. Based on a CM diagram derived from accurate photometry and Hipparcos parallaxes, these two possible proper-motion companions to beta Pic are very young; we derive an age of $\sim$20 Myr by comparison to theoretical tracks from D'Antona & Mazzitelli. In fact, the proposed beta Pic companions comprise two of the three youngest M dwarfs in the sample of 160 dM stars for which we have data. The chromospheric and coronal activity of these two stars also confirm that they are quite young. We argue that the probability that two of the three youngest nearby M dwarfs would accurately share the space motion of beta Pic by chance is quite small, and therefore we believe that beta Pic and the two M dwarfs (GL 799 and GL 803) were formed together. The estimated age for beta Pic is then 20$\pm$10 Myr.

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arXiv1999

Deep Photometric Survey of IC2391

We present a deep survey of the young southern cluster IC2391. We have covered 2 sq. degrees in the R,I filter. We have selected several dozens VLM stars and BD candidates based on the position of these objects in the Color-Magnitude Diagram.

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arXiv1999

Brown Dwarfs: Toward a New Age Scale for Young Open Cluster

We review our the status of the photometric searches of brown dwarfs and vely low mass stars in several young open clusters and present some results on the spectroscopic follow-up, including new cluster ages based on the position of the lithium depletion boundary.

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arXiv1999

Mass Function at the end of the Main Sequence: the M35 Open Cluster

We present very deep RIc photometry of the M35 open cluster. We have covered 30' \times 30', equivalent to about a fourth of the total area of the cluster. The data range from I_c=17.5 to 23.5 magnitudes, and the color interval is $0.5 \le (R-I)_c \le 2.5$. Roughly, these values correspond from $0.6 M_\odot$ to $0.1 M_\odot$ in the cases of members of the cluster. By using the location of the stars on a Color-Magnitude Diagram, we have selected possible members of this cluster and obtained the Luminosity and Mass Functions of the M35 open cluster and compared them with those from the Pleiades.

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arXiv1998

Stellar Activity in Coeval Open Clusters: Praesepe and The Hyades

Randich and Schmitt [1995, A&A 298, 115] found that the coronal activity of solar-type and low mass stars in Praesepe is significantly lower than that of stars in the Hyades cluster. We have carried out several tests in order to find a possible explanation for this result. We have measured radial velocities of two groups of Praesepe stars (a dF-dK sample and a dM sample) and have measured H$α$ as a chromospheric activity index for the dM sample. We conclude that the Praesepe catalog used in the X-ray analysis does not contain a significant number of non-members. The comparison of the H$α$ equivalent widths for the M dwarfs in Praesepe with those in the Hyades indicates that, at least for stars in this mass range, the Praesepe stars are as active or more active than their Hyades counterparts. We have also analyzed a few ROSAT PSPC pointings of Praesepe in order to obtain a new and independent estimate of the X-ray luminosities and upper limits for a small sample of Praesepe members concluding that the small differences between the old and new upper limits are not large enough to explain the dichotomy in the X-ray properties of Praesepe and the Hyades. Therefore, our examination of the available data does not provide a clear reason to explain why the X-ray luminosity functions of the two clusters are different. Part of the explanation could be found in the binaries. Speculatively, these clusters could have different orbital period distributions, with more short period binaries among the Hyades, which would show larger coronal activity.

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arXiv1997

Why is the X-ray luminosity function different for the Hyades and Praesepe?

We have studied dF-dK and dM stars belonging to Praesepe in order to determine membership via radial velocities and to compare several stellar properties, such as X-ray luminosities, H$α$ equivalent width, etc, between this cluster and Hyades, which have similar age. We show that all properties are analogous in both clusters except the distribution of Lx. This fact could be related to difficulties in the analysis of the Lx upper limits or to more fundamental reasons, such as binarity rates and differences in the properties of the binaries of each cluster.

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arXiv1997

Lithium and binarity in M67

A study of the lithium abundances in binaries of the old open cluster M67 is presented. Abundances were estimated using curves of growth and equivalent widths of the LiI6707.8 Å doublet. We have corrected of the effects of the companion on the measured equivalent width and on the color indices by deconvolving the photometry, computing colors and magnitudes for each component. As happens in the Hyades cluster, there are some lithium excesses in main-sequence and evolved TLBS binaries with respect other binaries and single stars, although the M67 data have large errors.

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arXiv1997

Chromospheric Emission from Red Giants in the Open Cluster NGC 6940

The observation of the Ca II H and K lines in red giants in NGC 6940 allows the strength of chromospheres and their behavior to be evaluated in a population whose evolution is well understood. Spectra in the Ca II lines have been obtained for giant stars in this cluster. Emission reversals are present in some objects. The absolute flux in the chromospheric emission is determined as a function of effective temperature for stars on the red giant branch. The stellar surface flux in the Ca II lines decreases smoothly with increasing $(B-V)$ in contradiction to model predictions.

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arXiv1997

Lithium Abundance in Binaries of the Hyades Open Cluster

We have derived accurate and homogeneous Lithium abundances in 49 main sequence binary systems belonging to the Hyades Open Cluster by using a deconvolution method to determine individual magnitudes and colors for the primary and secondary components of the binary. The input parameters of the model are the observed Li equivalent width, the actual distance to the binary, the integrated apparent magnitude and the integrated colors of the binaries -BV(RI)_K. We show that the general behavior is the same in binaries and in single stars (Li is depleted faster in K stars than in G stars and there is a deep dip for mid-F stars). However, there is a larger scatter in the abundances of binary systems than in single stars. Moreover, in general, binary systems have an overabundance, which is more conspicuous in close binaries. In fact, there is a cut-off period, which can be estimated as P(orb)~9 d. This value is in excellent agreement with the theoretical predition of Zahn (1994).

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arXiv1997

The Age of Gl879 and Fomalhaut

We estimate here the age of one of the prototypes of the Beta Pic-like stars, Fomalhaut, based on the properties of its common proper motion companion Gl879. By combining constraints derived from the lithium abundance, rotational velocity, HR diagram position, and coronal activity we conclude that the age for Gl879, and hence the age for Fomalhaut, is 200$\pm$100 Myr. This age estimate agrees quite well with the completely independent age estimate derived directly from isochrone--fitting to Fomalhaut's position in an HR diagram, and thus confirms that circumstellar dust disks can persist in A stars for several hundred Myr.

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arXiv1997

The Age-Mass Relation for Chromospherically Active Binaries. II. Lithium Depletion in Dwarf Components

We present an extensive study of lithium abundances in dwarf components of chromospherically active binary stars (CABS). Since most of these binaries have known radii, masses and ages, this kind of data is especially useful for comparisons with theoretical models which try to explain the Li depletion phenomenon. We show that a significant part of these stars have clear Li overabundances. These excesses are evident when comparing our sample of CABS with binary and single stars belonging to open clusters of different ages, namely Pleiades, Hyades, NGC752, M67 and NGC188. The Li excesses are more conspicuous for masses in the range 0.75-0.95 Mo, indicating that the rate of Li depletion has been less pronounced in CABS than in single stars. This phenomenon is interpreted in the context of transport of angular momentum from the orbit to the stellar rotation due to tidal effects. Other explanations, however, can not be ruled out. This is the case of transport of material induced by internal gravity waves, which could be inhibited due to the presence of strong magnetic fields associated with the effective dynamo in CABS. The confirmed existence of a relation between Li and fluxes in CaII H&K lines can also be accommodated within both scenarios.

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