Joana Oliveira - Academia.edu (original) (raw)
Papers by Joana Oliveira
Astronomy & Astrophysics, 2002
We present simultaneous and continuous observations of the Hα, Hβ, He i D3, Na i D1,D2 doublet an... more We present simultaneous and continuous observations of the Hα, Hβ, He i D3, Na i D1,D2 doublet and the Ca 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. d 8. Two large optical flares were observed, both showing an increase in the emission of Hα, Ca ii H & K, Hβ and He i D3 and a strong filling-in of the Na i D1,D2 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 Hα and He i D3 in anticorrelation 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.
Astronomy and Astrophysics, 2004
We present new mid-infrared observations of objects in the vicinity of the O-star σ Orionis, obta... more We present new mid-infrared observations of objects in the vicinity of the O-star σ Orionis, obtained with TIMMI-2 at ESO. By constraining their near-and mid-infrared spectral energy distributions, we established the nature of previously known IRAS sources and identified new mid-infrared sources as young stellar objects with circumstellar disks, likely massive members of the σ Ori cluster. For two of these objects we have obtained spectroscopy in the 8-13 µm range in order to investigate the chemistry of the dust grains. TX Ori exhibits a typical silicate emission feature at 10 µm, with a feature at about 11.2 µm that we identify as due to crystalline olivine. The IRAS 05358−0238 spectrum is very unusual, with a weak silicate feature and structure in the range 10-12 µm that may be explained as due to self-absorbed forsterite. We also provide the first evidence for the presence of circumstellar disks in the jet sources Haro 5-39/HH 447, V510 Ori/HH 444 and V603 Ori/HH 445.
Arthritis & Rheumatism, 2003
Even though we do not yet fully understand the formula for planet formation, we know where to fin... more Even though we do not yet fully understand the formula for planet formation, we know where to find the ingredients: in circumstellar disks around young low-mass stars. It is now believed that most solar mass stars form in OB associations. We propose to perform IRS spectroscopy of a sample of classical T Tauri stars (CTTS, pre-main-sequence stars with disks) in
Lecture Notes in Physics, 2001
ABSTRACT
The Astrophysical Journal, 2009
We investigate dust production and stellar mass loss in the Galactic globular cluster NGC 362. Du... more We investigate dust production and stellar mass loss in the Galactic globular cluster NGC 362. Due to its close proximity to the Small Magellanic Cloud (SMC), NGC 362 was imaged with the IRAC and MIPS cameras onboard the Spitzer Space Telescope as part of the Surveying the Agents of Galaxy Evolution (SAGE-SMC) Spitzer Legacy program. We detect several cluster members near the tip of the Red Giant Branch that exhibit infrared excesses indicative of circumstellar dust and find that dust is not present in measurable quantities in stars below the tip of the Red Giant Branch. We modeled the spectral energy distribution (SED) of the stars with the strongest IR excess and find a total cluster dust mass-loss rate of 3.0 +2.0 −1.2 × 10 −9 M ⊙ yr −1 , corresponding to a gas mass-loss rate of 8.6 +5.6 −3.4 × 10 −6 M ⊙ yr −1 , assuming [Fe/H] = −1.16. This mass loss is in addition to any dust-less mass loss that is certainly occurring within the cluster. The two most extreme stars, variables V2 and V16, contribute up to 45% of the total cluster dust-traced mass loss. The SEDs of the more moderate stars indicate the presence of silicate dust, as expected for low-mass, low-metallicity stars. Surprisingly, the SED shapes of the stars with the strongest mass-loss rates appear to require the presence of amorphous carbon dust, possibly in combination with silicate dust, despite their oxygen-rich nature. These results corroborate our previous findings in ω Centauri.
The Astronomical Journal, 2010
We present far-infrared spectra, λ=52-93 µm, obtained with the Spitzer Space Telescope in the Spe... more We present far-infrared spectra, λ=52-93 µm, obtained with the Spitzer Space Telescope in the Spectral Energy Distribution mode of its MIPS instrument, of a representative sample of the most luminous compact far-infrared sources in the Large Magellanic Cloud. These include carbon stars, OH/IR Asymptotic Giant Branch (AGB) stars, post-AGB objects and Planetary Nebulae, the R CrBtype star HV 2671, the OH/IR red supergiants WOH G064 and IRAS 05280−6910, the three B[e] stars IRAS 04530−6916, R 66 and R 126, the Wolf-Rayet star Brey 3a, the Luminous Blue Variable (LBV) R 71, the supernova remnant N 49, a large number of young stellar objects (YSOs), compact H ii regions and molecular cores, and a background galaxy at a redshift z ≃ 0.175. We use the spectra to constrain the presence and temperature of cold dust and the excitation conditions and shocks within the neutral and ionized gas, in the circumstellar environments and interfaces with the surrounding interstellar medium (ISM). First, we introduce a spectral classification scheme. Then, we measure line strengths, dust temperatures, and IR luminosities. Objects associated with star formation are readily distinguished from evolved stars by their cold dust and/or fine-structure lines. Evolved stars, including the LBV R 71, lack cold dust except in some cases where we argue that this is swept-up ISM. This leads to an estimate of the duration of the prolific dust-producing phase ("superwind") of several thousand years for both RSGs and massive AGB stars, with a similar fractional mass loss experienced despite the different masses. We tentatively detect line emission from neutral oxygen in the extreme RSG WOH G064, which suggests a large dust-free cavity with implications for the wind driving. In N 49, the shock between the supernova ejecta and ISM is revealed in spectacular fashion by its strong [O i] λ63-µm emission and possibly water vapour; we estimate that 0.2 M ⊙ of ISM dust was swept up. On the other hand, some of the compact H ii regions display pronounced [O iii] λ88µm emission. The efficiency of photo-electric heating in the interfaces of ionized gas and molecular clouds is estimated at 0.1-0.3%. We confirm earlier indications of a low nitrogen content in the LMC. Evidence for solid state emission features is found in both young and evolved objects, but the carriers of these features remain elusive; some of the YSOs are found to contain crystalline water ice. The spectra constitute a valuable resource for the planning and interpretation of observations with the Herschel Space Observatory and the Stratospheric Observatory For Infrared Astronomy (SOFIA).
Publications of the Astronomical Society of Australia, 2008
The Magellanic System represents one of the best places to study the formation and evolution of g... more The Magellanic System represents one of the best places to study the formation and evolution of galaxies. Photometric surveys of various depths, areas and wavelengths have had a significant impact on our understanding of the system; however, a complete picture is still lacking. VMC (the VISTA near-infrared Y JKs survey of the Magellanic System) will provide new data to derive the spatially resolved star formation history and to construct a three-dimensional map of the system. These data combined with those from other ongoing and planned surveys will give us an absolutely unique view of the system opening up the doors to truly new science!
Monthly Notices of the Royal Astronomical Society, 2004
We present here spectropolarimetric observations of the RS CVn system HR 1099 (V711 Tau) secured ... more 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 Télescope Bernard Lyot (Observatoire du Pic du Midi, France). We apply Zeeman-Doppler Imaging and reconstruct surface brightness and magnetic 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.
Monthly Notices of the Royal Astronomical Society, 2004
We present spectropolarimetric observations of the FK Com star HD 199178 obtained between 1998 De... more We present spectropolarimetric observations of the FK Com star HD 199178 obtained between 1998 December and August at the Télescope Bernard Lyot (Observatoire du Pic du Midi, France). We report the detection of a photospheric magnetic field and reconstruct its distribution by means of Zeeman-Doppler Imaging. We observe large regions where the magnetic field is mainly azimuthal, suggesting that the dynamo processes generating the magnetic activity of HD 199178 may be active very close to the stellar surface. We investigate the rapid evolution of surface brightness and magnetic structures from a continuous monitoring of the star over several weeks in 2002 and 2003. We report that significant changes occur in the distribution of cool spots and magnetic regions on typical timescales of the order of 2 weeks. Our spectropolarimetric observations also suggest that the surface of HD 199178 is sheared by differential rotation, with a difference in rotation rate between equatorial and polar regions of the order of 1.5 times that of the Sun.
Monthly Notices of the Royal Astronomical Society, 2004
We present results from the 1996 Multi-Site Continuous Spectroscopy (MUSICOS) campaign on the T T... more We present results from the 1996 Multi-Site Continuous Spectroscopy (MUSICOS) 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 due in part to limitations in signal-to-noise ratio, and in part 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.
Monthly Notices of the Royal Astronomical Society, 2003
Monthly Notices of the Royal Astronomical Society, 2003
We present the results of a spectroscopic survey of X-ray selected, low-mass candidate members of... more 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 > T eff > 3200 K, 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 − 35 Myr, 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 50 Myr, 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.
Monthly Notices of the Royal Astronomical Society, 2004
We present new K-and L ′ -band imaging of a representative sample of members of the young 3−5 Myr... more We present new K-and L ′ -band imaging of a representative sample of members of the young 3−5 Myr old σ Orionis cluster. We identified objects with (K − L ′ ) excess by analysing colour-colour diagrams and comparing the observations with empirical mainsequence 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-σ 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 σ 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.
Astronomy and Astrophysics, 2002
We report on the analysis of 2MASS nearinfrared data of a sample of low-mass stars and brown dwar... more We report on the analysis of 2MASS nearinfrared data of a sample of low-mass stars and brown dwarfs in the σ Orionis cluster. Youth and cluster membership have been spectroscopically confirmed using the Li i spectral line. We find little evidence in the JHK s colourcolour 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 s ) 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 K s -band excess may be mass dependent.
Astronomy & Astrophysics, 2011
The VISTA Magellanic Cloud (VMC) survey is assembling a deep, multi-epoch atlas of Y JK s photome... more The VISTA Magellanic Cloud (VMC) survey is assembling a deep, multi-epoch atlas of Y JK s photometry across the Magellanic Clouds. Prior to the VMC survey only the brightest Magellanic Cloud PNe (MCPNe) were accessible at near-infrared (NIR) wavelengths. It is now possible for the first time to assemble the NIR properties of MCPNe and to identify contaminating non-PNe mimics which are best revealed at NIR wavelengths (e.g. HII regions and symbiotic stars). To maintain the unique scientific niche that MCPNe occupy these contaminants must be removed. Here we conduct a VMC-led, multi-wavelength study of 102 objects previously classified as PNe that are located within the first six VMC tiles observed. We present images, photometry, lightcurves, diagnostic colour-colour diagrams and spectral energy distributions used to analyse the entire sample. At least five PNe have newly resolved nebula morphologies, a task previously only possible with the HST. A total 45/67 (67%) of Reid & Parker (RP) catalogued objects were reclassified as non-PNe, most of which were located in the vicinity of 30 Doradus. This sample included 16 field stars, 5 emission line stars, 19 HII regions, 4 symbiotic star candidates and 1 young stellar object. We discuss possible selection effects responsible for their inclusion in the RP catalogue and the implications for binary central star surveys targeting LMC PNe. A total of five new LMC symbiotic star candidates identified, compared to eight previously known, underlines the important role the VMC survey will have in advancing Magellanic symbiotic star studies.
Astronomy and Astrophysics, 2002
ω Ori (HD 37490, HR 1934) is a Be star known to have presented variations. In order to investigat... more ω Ori (HD 37490, HR 1934) is a Be star known to have presented variations. In order to investigate the nature and origin of its short-term and mid-term variability, a study is performed of several spectral lines (Hα, Hδ, He i 4471, 4713, 4921, 5876, 6678, C ii 4267, 6578, 6583, Mg ii 4481, Si iii 4553 and Si ii 6347), based on 249 high signal-to-noise high-resolution spectra taken with 8 telescopes over 22 consecutive nights during the MuSiCoS (Multi SIte COntinuous Spectroscopy) campaign in November-December 1998. The stellar parameters are revisited and the projected rotational velocity (v sin i = 179 km s −1 ) is redetermined using several methods. With the MuSiCoS 98 dataset, a time series analysis of line-profile variations (LPVs) is performed using the Restricted Local Cleanest (RLC) algorithm and a least squares method. The behaviour of the velocity of the centroid of the lines, the equivalent widths and the apparent vsini for several lines, as well as Violet and Red components of photospheric lines affected by emission (red He i lines, Si ii 6347, C ii 6578, 6583) are analyzed. The non-radial pulsation (NRP)
Astronomy and Astrophysics, 2006
Astronomy & Astrophysics, 2002
We present simultaneous and continuous observations of the Hα, Hβ, He i D3, Na i D1,D2 doublet an... more We present simultaneous and continuous observations of the Hα, Hβ, He i D3, Na i D1,D2 doublet and the Ca 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. d 8. Two large optical flares were observed, both showing an increase in the emission of Hα, Ca ii H & K, Hβ and He i D3 and a strong filling-in of the Na i D1,D2 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 Hα and He i D3 in anticorrelation 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.
Astronomy and Astrophysics, 2004
We present new mid-infrared observations of objects in the vicinity of the O-star σ Orionis, obta... more We present new mid-infrared observations of objects in the vicinity of the O-star σ Orionis, obtained with TIMMI-2 at ESO. By constraining their near-and mid-infrared spectral energy distributions, we established the nature of previously known IRAS sources and identified new mid-infrared sources as young stellar objects with circumstellar disks, likely massive members of the σ Ori cluster. For two of these objects we have obtained spectroscopy in the 8-13 µm range in order to investigate the chemistry of the dust grains. TX Ori exhibits a typical silicate emission feature at 10 µm, with a feature at about 11.2 µm that we identify as due to crystalline olivine. The IRAS 05358−0238 spectrum is very unusual, with a weak silicate feature and structure in the range 10-12 µm that may be explained as due to self-absorbed forsterite. We also provide the first evidence for the presence of circumstellar disks in the jet sources Haro 5-39/HH 447, V510 Ori/HH 444 and V603 Ori/HH 445.
Arthritis & Rheumatism, 2003
Even though we do not yet fully understand the formula for planet formation, we know where to fin... more Even though we do not yet fully understand the formula for planet formation, we know where to find the ingredients: in circumstellar disks around young low-mass stars. It is now believed that most solar mass stars form in OB associations. We propose to perform IRS spectroscopy of a sample of classical T Tauri stars (CTTS, pre-main-sequence stars with disks) in
Lecture Notes in Physics, 2001
ABSTRACT
The Astrophysical Journal, 2009
We investigate dust production and stellar mass loss in the Galactic globular cluster NGC 362. Du... more We investigate dust production and stellar mass loss in the Galactic globular cluster NGC 362. Due to its close proximity to the Small Magellanic Cloud (SMC), NGC 362 was imaged with the IRAC and MIPS cameras onboard the Spitzer Space Telescope as part of the Surveying the Agents of Galaxy Evolution (SAGE-SMC) Spitzer Legacy program. We detect several cluster members near the tip of the Red Giant Branch that exhibit infrared excesses indicative of circumstellar dust and find that dust is not present in measurable quantities in stars below the tip of the Red Giant Branch. We modeled the spectral energy distribution (SED) of the stars with the strongest IR excess and find a total cluster dust mass-loss rate of 3.0 +2.0 −1.2 × 10 −9 M ⊙ yr −1 , corresponding to a gas mass-loss rate of 8.6 +5.6 −3.4 × 10 −6 M ⊙ yr −1 , assuming [Fe/H] = −1.16. This mass loss is in addition to any dust-less mass loss that is certainly occurring within the cluster. The two most extreme stars, variables V2 and V16, contribute up to 45% of the total cluster dust-traced mass loss. The SEDs of the more moderate stars indicate the presence of silicate dust, as expected for low-mass, low-metallicity stars. Surprisingly, the SED shapes of the stars with the strongest mass-loss rates appear to require the presence of amorphous carbon dust, possibly in combination with silicate dust, despite their oxygen-rich nature. These results corroborate our previous findings in ω Centauri.
The Astronomical Journal, 2010
We present far-infrared spectra, λ=52-93 µm, obtained with the Spitzer Space Telescope in the Spe... more We present far-infrared spectra, λ=52-93 µm, obtained with the Spitzer Space Telescope in the Spectral Energy Distribution mode of its MIPS instrument, of a representative sample of the most luminous compact far-infrared sources in the Large Magellanic Cloud. These include carbon stars, OH/IR Asymptotic Giant Branch (AGB) stars, post-AGB objects and Planetary Nebulae, the R CrBtype star HV 2671, the OH/IR red supergiants WOH G064 and IRAS 05280−6910, the three B[e] stars IRAS 04530−6916, R 66 and R 126, the Wolf-Rayet star Brey 3a, the Luminous Blue Variable (LBV) R 71, the supernova remnant N 49, a large number of young stellar objects (YSOs), compact H ii regions and molecular cores, and a background galaxy at a redshift z ≃ 0.175. We use the spectra to constrain the presence and temperature of cold dust and the excitation conditions and shocks within the neutral and ionized gas, in the circumstellar environments and interfaces with the surrounding interstellar medium (ISM). First, we introduce a spectral classification scheme. Then, we measure line strengths, dust temperatures, and IR luminosities. Objects associated with star formation are readily distinguished from evolved stars by their cold dust and/or fine-structure lines. Evolved stars, including the LBV R 71, lack cold dust except in some cases where we argue that this is swept-up ISM. This leads to an estimate of the duration of the prolific dust-producing phase ("superwind") of several thousand years for both RSGs and massive AGB stars, with a similar fractional mass loss experienced despite the different masses. We tentatively detect line emission from neutral oxygen in the extreme RSG WOH G064, which suggests a large dust-free cavity with implications for the wind driving. In N 49, the shock between the supernova ejecta and ISM is revealed in spectacular fashion by its strong [O i] λ63-µm emission and possibly water vapour; we estimate that 0.2 M ⊙ of ISM dust was swept up. On the other hand, some of the compact H ii regions display pronounced [O iii] λ88µm emission. The efficiency of photo-electric heating in the interfaces of ionized gas and molecular clouds is estimated at 0.1-0.3%. We confirm earlier indications of a low nitrogen content in the LMC. Evidence for solid state emission features is found in both young and evolved objects, but the carriers of these features remain elusive; some of the YSOs are found to contain crystalline water ice. The spectra constitute a valuable resource for the planning and interpretation of observations with the Herschel Space Observatory and the Stratospheric Observatory For Infrared Astronomy (SOFIA).
Publications of the Astronomical Society of Australia, 2008
The Magellanic System represents one of the best places to study the formation and evolution of g... more The Magellanic System represents one of the best places to study the formation and evolution of galaxies. Photometric surveys of various depths, areas and wavelengths have had a significant impact on our understanding of the system; however, a complete picture is still lacking. VMC (the VISTA near-infrared Y JKs survey of the Magellanic System) will provide new data to derive the spatially resolved star formation history and to construct a three-dimensional map of the system. These data combined with those from other ongoing and planned surveys will give us an absolutely unique view of the system opening up the doors to truly new science!
Monthly Notices of the Royal Astronomical Society, 2004
We present here spectropolarimetric observations of the RS CVn system HR 1099 (V711 Tau) secured ... more 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 Télescope Bernard Lyot (Observatoire du Pic du Midi, France). We apply Zeeman-Doppler Imaging and reconstruct surface brightness and magnetic 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.
Monthly Notices of the Royal Astronomical Society, 2004
We present spectropolarimetric observations of the FK Com star HD 199178 obtained between 1998 De... more We present spectropolarimetric observations of the FK Com star HD 199178 obtained between 1998 December and August at the Télescope Bernard Lyot (Observatoire du Pic du Midi, France). We report the detection of a photospheric magnetic field and reconstruct its distribution by means of Zeeman-Doppler Imaging. We observe large regions where the magnetic field is mainly azimuthal, suggesting that the dynamo processes generating the magnetic activity of HD 199178 may be active very close to the stellar surface. We investigate the rapid evolution of surface brightness and magnetic structures from a continuous monitoring of the star over several weeks in 2002 and 2003. We report that significant changes occur in the distribution of cool spots and magnetic regions on typical timescales of the order of 2 weeks. Our spectropolarimetric observations also suggest that the surface of HD 199178 is sheared by differential rotation, with a difference in rotation rate between equatorial and polar regions of the order of 1.5 times that of the Sun.
Monthly Notices of the Royal Astronomical Society, 2004
We present results from the 1996 Multi-Site Continuous Spectroscopy (MUSICOS) campaign on the T T... more We present results from the 1996 Multi-Site Continuous Spectroscopy (MUSICOS) 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 due in part to limitations in signal-to-noise ratio, and in part 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.
Monthly Notices of the Royal Astronomical Society, 2003
Monthly Notices of the Royal Astronomical Society, 2003
We present the results of a spectroscopic survey of X-ray selected, low-mass candidate members of... more 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 > T eff > 3200 K, 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 − 35 Myr, 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 50 Myr, 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.
Monthly Notices of the Royal Astronomical Society, 2004
We present new K-and L ′ -band imaging of a representative sample of members of the young 3−5 Myr... more We present new K-and L ′ -band imaging of a representative sample of members of the young 3−5 Myr old σ Orionis cluster. We identified objects with (K − L ′ ) excess by analysing colour-colour diagrams and comparing the observations with empirical mainsequence 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-σ 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 σ 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.
Astronomy and Astrophysics, 2002
We report on the analysis of 2MASS nearinfrared data of a sample of low-mass stars and brown dwar... more We report on the analysis of 2MASS nearinfrared data of a sample of low-mass stars and brown dwarfs in the σ Orionis cluster. Youth and cluster membership have been spectroscopically confirmed using the Li i spectral line. We find little evidence in the JHK s colourcolour 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 s ) 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 K s -band excess may be mass dependent.
Astronomy & Astrophysics, 2011
The VISTA Magellanic Cloud (VMC) survey is assembling a deep, multi-epoch atlas of Y JK s photome... more The VISTA Magellanic Cloud (VMC) survey is assembling a deep, multi-epoch atlas of Y JK s photometry across the Magellanic Clouds. Prior to the VMC survey only the brightest Magellanic Cloud PNe (MCPNe) were accessible at near-infrared (NIR) wavelengths. It is now possible for the first time to assemble the NIR properties of MCPNe and to identify contaminating non-PNe mimics which are best revealed at NIR wavelengths (e.g. HII regions and symbiotic stars). To maintain the unique scientific niche that MCPNe occupy these contaminants must be removed. Here we conduct a VMC-led, multi-wavelength study of 102 objects previously classified as PNe that are located within the first six VMC tiles observed. We present images, photometry, lightcurves, diagnostic colour-colour diagrams and spectral energy distributions used to analyse the entire sample. At least five PNe have newly resolved nebula morphologies, a task previously only possible with the HST. A total 45/67 (67%) of Reid & Parker (RP) catalogued objects were reclassified as non-PNe, most of which were located in the vicinity of 30 Doradus. This sample included 16 field stars, 5 emission line stars, 19 HII regions, 4 symbiotic star candidates and 1 young stellar object. We discuss possible selection effects responsible for their inclusion in the RP catalogue and the implications for binary central star surveys targeting LMC PNe. A total of five new LMC symbiotic star candidates identified, compared to eight previously known, underlines the important role the VMC survey will have in advancing Magellanic symbiotic star studies.
Astronomy and Astrophysics, 2002
ω Ori (HD 37490, HR 1934) is a Be star known to have presented variations. In order to investigat... more ω Ori (HD 37490, HR 1934) is a Be star known to have presented variations. In order to investigate the nature and origin of its short-term and mid-term variability, a study is performed of several spectral lines (Hα, Hδ, He i 4471, 4713, 4921, 5876, 6678, C ii 4267, 6578, 6583, Mg ii 4481, Si iii 4553 and Si ii 6347), based on 249 high signal-to-noise high-resolution spectra taken with 8 telescopes over 22 consecutive nights during the MuSiCoS (Multi SIte COntinuous Spectroscopy) campaign in November-December 1998. The stellar parameters are revisited and the projected rotational velocity (v sin i = 179 km s −1 ) is redetermined using several methods. With the MuSiCoS 98 dataset, a time series analysis of line-profile variations (LPVs) is performed using the Restricted Local Cleanest (RLC) algorithm and a least squares method. The behaviour of the velocity of the centroid of the lines, the equivalent widths and the apparent vsini for several lines, as well as Violet and Red components of photospheric lines affected by emission (red He i lines, Si ii 6347, C ii 6578, 6583) are analyzed. The non-radial pulsation (NRP)
Astronomy and Astrophysics, 2006