arXiv subjects

J. M. Oliveira

Publications and source records attributed to J. M. Oliveira.

68 records · Page 4Linked to original sources

arXiv2004

Multi-Site observations of SU Aurigae

We present results from the 1996 MUSICOS (MUlti-SIte COntinuous Spectroscopy) campaign on the T Tauri star SU Aurigae. We find a 2.7-d periodicity in the He I (587.6 nm) line and somewhat longer, less well-pronounced periodicities in the Balmer lines and in Na D. Our observations support the suggestion that the wind and infall signatures are out of phase on SU Aur. We present Doppler images of SU Aur that have been obtained from least-squares deconvolved profiles. Images taken about one rotation apart show only limited overlap, in particular at low latitudes. This is in part due to limitations in signal-to-noise, and in part due to line profile deformations that arise from short-lived and/or non-surface features. The agreement at high latitudes is better and suggests that at least some longer-lived features are present. The analysis of Stokes V profiles yields a marginal magnetic field detection during one of the phases.

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arXiv2003

Magnetic topology and surface differential rotation on the K1 subgiant of the RS CVn system HR 1099

We present here spectropolarimetric observations of the RS CVn system HR 1099 (V711 Tau) secured from 1998 February to 2002 January with the spectropolarimeter MuSiCoS at the Telescope Bernard Lyot (Observatoire du Pic du Midi, France). We apply Zeeman-Doppler Imaging and reconstruct brightness and magnetic surface topologies of the K1 primary subgiant of the system, at five different epochs. We confirm the presence of large, axisymmetric regions where the magnetic field is mainly azimuthal, providing further support to the hypothesis that dynamo processes may be distributed throughout the whole convective zone in this star. We study the short-term evolution of surface structures from a comparison of our images with observations secured at close-by epochs by Donati et al. (2003) at the Anglo-Australian Telescope. We conclude that the small-scale brightness and magnetic patterns undergo major changes within a timescale of 4 to 6 weeks, while the largest structures remain stable over several years. We report the detection of a weak surface differential rotation (both from brightness and magnetic tracers) indicating that the equator rotates faster than the pole with a difference in rotation rate between the pole and the equator about 4 times smaller than that of the Sun. This result suggests that tidal forces also impact the global dynamic equilibrium of convective zones in cool active stars.

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arXiv2003

An L'-band survey for circumstellar disks around low-mass stars in the young sigma Orionis cluster

We present new K- and L'-band imaging of a representative sample of members of the young 3-5 Myr old sigma Orionis cluster. We identified objects with (K-L') excess by analysing colour-colour diagrams and comparing the observations with empirical main-sequence colours. The derived disk frequency depends on the method used: (54 +/- 15)% if measured directly from the JHKL' colour-colour diagram; or (46 +/-14)% if excesses are computed with respect to predicted photospheric colours (according to the objects spectral types, 2-sigma excess detections). We compare the (K-L') excess with other indicators and show that this is a robust and reliable disk indicator. We also compare the derived disk frequency with similarly aged clusters and discuss possible implications for disk lifetimes. The computed age of the sigma Ori cluster is very important: a cluster age of 3 Myr would support the overall disk lifetime of 6 Myr proposed in the literature, while an age > 4 Myr would point to a slower disk destruction rate.

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arXiv2003

Young stars and their circumstellar disks in the Sigma Orionis cluster

The sigma Orionis cluster is a young association evolving under the disruptive influence of its massive O-star namesake. We are analysing this cluster as part of a program to characterise the influence of O-stars on the early stages of stellar evolution. At an age of approximately 4 Myr, this cluster is at a crucial stage in terms of disk evolution and therefore it is a key case to better constrain disk dissipation timescales. We have obtained RI photometry and optical spectroscopy of the sigma Ori cluster; we have analysed the Li I and Na I features to establish cluster membership. We have thus gathered a unique sample of spectroscopically confirmed low-mass cluster members and brown dwarfs. Disk frequencies from K-band excesses from 2MASS suggest that less than 7% of the very low-mass sigma Ori members have disks, in stark contrast with even younger clusters (e.g. Trapezium). However, near-infrared disk frequencies have to be taken with caution. We are currently undertaking an L-band (imaging) and mid-infrared (imaging and spectroscopy) program to identify and probe the properties of circumstellar disks around young stars in this cluster. Preliminary results indicate that at least 30% of cluster members have circumstellar disks.

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arXiv2003

Cool stars in NGC 2547 and pre main sequence lithium depletion

We present the results of a spectroscopic survey of X-ray selected, low-mass candidate members of the young open cluster NGC 2547. Using a combination of photometry, spectroscopic indices and radial velocities we refine our candidate list and then use our spectroscopy to study the progression of lithium depletion in low-mass pre main sequence stars. We derive lithium abundances or upper limits for all our candidate members, which have effective temperatures in the range 5000>Teff>3200K, and compare these with predictions for lithium burning and depletion provided by a number of models and also with the lithium depletion seen in younger and older stars. We find that some models can reproduce the lithium abundance pattern of NGC 2547 if the cluster has an age of ~20-35Myr, which is also indicated by fits to low-mass isochrones in the Hertzsprung-Russell diagram. But the lack of significant further lithium depletion between NGC 2547 and older clusters argues for an age of at least 50Myr, more in keeping with the lack of lithium observed in even fainter NGC 2547 candidates. We show that reconciliation of these age estimates may require additions to the physics incorporated in current generations of pre main sequence models.

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arXiv2003

The lithium depletion boundary and the age of NGC 2547

We present the results of a photometric and spectroscopic survey of cool M dwarf candidates in the young open cluster NGC 2547. Using the 2dF fiber spectrograph, we have searched for the luminosity at which lithium remains unburned in an attempt to constrain the cluster age. The lack of a population of individual lithium-rich objects towards the faint end of our sample places a very strong lower limit to the cluster age of 35 Myr. However, the detection of lithium in the averaged spectra of our faintest targets suggests that the lithium depletion boundary lies at 9.5 < M(I) < 10.0 and that the cluster age is < 54 Myr. The age of NGC 2547 judged from fitting isochrones to low-mass pre-main-sequence stars in colour-magnitude diagrams is 20-35 Myr using the same evolutionary models. The sense and size of the discrepancy in age determined by these two techniques is similar to that found in another young cluster, IC 2391, and in the low-mass pre main-sequence binary system, GJ 871.1AB. We suggest that the inclusion of rotation or dynamo-generated magnetic fields in the evolutionary models could reconcile the two age determinations, but only at the expense of increasing the cluster ages beyond that currently indicated by the lithium depletion. Alternatively, some mechanism is required that increases the rate of lithium depletion in young, very low-mass fully convective stars.

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arXiv2002

Simultaneous Optical and X-ray Observations of Flares and Rotational Modulation on the RS CVn Binary HR 1099 (V711 Tau) from the MUSICOS 1998 Campaign

We present simultaneous and continuous observations of the H$α$, H$β$, He {\sc i} D$_{3}$, Na {\sc i} D$_{1}$,D$_{2}$ doublet and the Ca {\sc ii} H & K lines for the RS CVn system HR 1099. The spectroscopic observations were obtained during the MUSICOS 1998 campaign involving several observatories and instruments, both echelle and long-slit spectrographs. During this campaign, HR 1099 was observed almost continuously for more than 8 orbits of $2\fd8$. Two large optical flares were observed, both showing an increase in the emission of H$α$, Ca {\sc ii} H & K, H$β$ and He {\sc i} D$_{3}$ and a strong filling-in of the Na {\sc i} D$_{1}$,D$_{2}$ doublet. {Contemporary photometric observations were carried out with the robotic telescopes APT-80 of Catania and Phoenix-25 of Fairborn Observatories. Maps of the distribution of the spotted regions on the photosphere of the binary components were derived using the Maximum Entropy and Tikhonov photometric regularization criteria}. Rotational modulation was observed in H$α$ and He {\sc i} D$_{3}$ in anti-correlation with the photometric light curves. Both flares occurred at the same binary phase (0.85), suggesting that these events took place in the same active region. Simultaneous X-ray observations, performed by ASM on board RXTE, show several flare-like events, some of which correlate well with the observed optical flares. Rotational modulation in the X-ray light curve has been detected with minimum flux when the less active G5 V star was in front. A possible periodicity in the X-ray flare-like events was also found.

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arXiv2001

No disks around low-mass stars and brown dwarfs in the young sigma Orionis cluster?

We report on the analysis of 2MASS near-infrared data of a sample of low-mass stars and brown dwarfs in the sigma Orionis cluster. Youth and cluster membership have been spectroscopically confirmed using the Li I spectral line. We find little evidence in the JHKs colour-colour diagram for near-infrared excess emission for these cluster members. By comparison with model expectations, at most 2 out of 34 stars show (H-K) colour consistent with a near-infrared excess. This scarcity of near-infrared signatures of circumstellar disks in the lower-mass and substellar regimes of this cluster contrasts with findings in younger clusters, hinting at an age dependence of the disk frequency. Taking into account the apparent cluster age, our result supports the idea of a relatively fast (few Myr) disk dissipation and extends this conclusion to the substellar regime. We also find some evidence that, in this cluster, the disk frequency as measured by the Ks-band excess may be mass dependent.

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arXiv2001

MUSICOS 1998: Optical and X-rays Observations of Flares on the RS CVn Binary HR 1099

We present simultaneous and continuous observations of H_alpha, H_beta, NaI D_1, D_2 and HeI D_3 lines of the chromospherically active binary HR 1099. We have observed HR 1099 for more than 3 weeks almost continuously and monitored two flares. An increase in H_alpha, CaII H & K, H_beta, HeI D_3 and HeI lambda6678 and a strong filling-in of the NaI D_1, D_2 and MgI b triplet during one of the flares are observed. We have found that the flares took place at the same phase (0.85) of the binary orbit, and both of them seems to occur near the limb. Several X-rays flares were also detected by ASM on board RXTE. Rotational modulation in the X-rays light curve has been detected with maximum flux when the active K1IV star is in front.

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arXiv2001

MUSICOS 1998: Observations of Rotational Modulation and Flares on the RS CVn Binary HR 1099

We present simultaneous and continuous observations of H_alpha, H_beta, NaI D_1, D_2, HeI D_3 and CaII H & K lines of the chromospherically active binary HR 1099. We have observed HR 1099 for more than 3 weeks almost continuously and monitored two flares. An increase in H_alpha and CaII H & K, H_beta and HeI D_3 and a strong filling-in of the NaI D_1, D_2 during the flares are observed. We have found that the flares took place at the same phase (0.85) of the binary orbit, and both of them seems to occur near the limb.

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arXiv2000

Magnetospheric Accretion and Winds on the T Tauri Star SU Aurigae: Multi-Spectral Line Variability and Cross-Correlation Analysis

SU Aurigae is a T Tauri star that was well monitored during the MUSICOS 96 multi-site campaign. We present the results of the spectroscopic analysis of the circumstellar environment of this star, particularly of the Halpha, Hbeta, Na I D and He I D3 line profiles. The signatures of modulated outflows and mass accretion events are analysed, as well as transient spectral features. We compute the cross-correlation function (CCF) of several pairs of (velocity bins in) spectral lines to better investigate the profiles' temporal variability. We found increasing time lags between the variability of He I D3, Na I D and Hbeta. We propose this may be understood in terms of azimuthal distortion of the magnetic field lines due to the different rotation rates of the star and the disk. We find the slightly blueshifted absorption features in Halpha, Hbeta and Na I D to be anti-correlated with the accretion flow signatures. We propose that the transient absorption features in the blue wings of Halpha, Hbeta and Na I D (signatures of mass outflows), and flare brightenings are related to the disruption of distorted magnetospheric field lines.

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arXiv2000

The Circumstellar Environment of the TTS SU Aurigae

In this contribution we investigate how a disk determines the physical and geometrical properties of the circumstellar environment of the T Tauri SU Aurigae. Our model of the spectral energy distribution of this star includes the central young star, a flat black disk and a diffuse envelope. We also describe the inner interaction region between the disk and the star, the magnetosphere, by analysing the accretion and wind signatures in several spectral lines observed during the MUSICOS 96 campaign.

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arXiv1999

Spectral Line Variability in the Circumstellar Environment of the Classic al T Tauri Star SU Aurigae

SU Aurigae is a classical T Tauri star of spectral type G2. This star was one of the scientific targets of the MUSICOS 96 multi-site campaign that provided a wealth of high resolution cross-dispersed spectral data with a good continuous time coverage. We present the results of the analysis of the complex circumstellar environment of this star, with particular regard to magnetospheric models, in which the accretion from the disk is channelled onto the star along magnetic field lines. The signatures of modulated outflows and mass accretion events are present in the spectra, as well as transient spectral features. We computed auto-correlation and cross-correlation functions to better investigate the source of the profiles' variability. The comparison of the profiles of different spectral lines allows us to study the footprints of events effectively observed at different distances from the stellar surface.

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arXiv1999

Circumstellar emission and flares on FK Comae Berenices: Analysis and modelling of Balmer and He I D3 line variations

We present results on spectroscopic observations of the fast-rotating active giant FK Comae, obtained mainly with the ESA-MUSICOS spectrograph at the Isaac Newton Telescope (INT) in 1996 and 1997 and also with the Aurelie spectrograph at the Observatoire de Haute Provence (OHP) in 1997. The profiles analysed are those of the Balmer, Halpha and Hbeta, and He I D3 (5876 A) lines. We analyse the Balmer line variability and phase behaviour. We confirm these lines as highly variable, with excess emission that originates from extended structures and exhibits clear signs of rotational modulation. We have described the line profiles for two distinct states of activity, using different modelling approaches. Similar techniques were applied to the He I D3 spectra. A large flare event lasting several days was detected in both Balmer lines and in the He I D3 line. The energy released during this flare in H alpha is of the order of 10^37 erg, making it the largest Halpha flare reported on a cool star. Our results confirm the extreme complexity of the circumstellar environment of FK Comae. The activity level of this star is quite variable demanding different approaches to the line profile analysis.

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