390 research outputs found
UvA-DARE (Digital Academic Repository) Astronomy & Astrophysics Variability and pulsations in the Be star 66 Ophiuchi
No full textVariability and pulsations in the Be star 66 Ophiuchi Floquet, M.; Neiner, C.; Janot-Pacheco, E.; Hubert, A.M.; Jankov, S.; Zorec, J.; Briot, D.; Chauville, J.; Leister, N.V.; Percy, J.R.; Ballerau, D.; Bakos, A.G. Published in: Astronomy & Astrophysics Link to publication Citation for published version (APA): Floquet, M., Neiner, C., Janot-Pacheco, E., Hubert, A. M., Jankov, S., Zorec, J., ... Bakos, A. G. (2002). Variability and pulsations in the Be star 66 Ophiuchi. Astronomy & Astrophysics, 392, 137-149. General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. They are attributed to non-radial pulsations and can be associated with mode degree = 2 and = 3, respectively. Inspection of Stokes V profiles suggests the presence of a weak Zeeman signature but further observations are needed to confirm the detection of a magnetic field in 66 Oph
Accretion discs, low-mass protostars and planets: probing the impact of magnetic fields on stellar formation
No full textWhereas the understanding of most phases of stellar evolution made
considerable progress throughout the whole of the twentieth century, stellar
formation remained rather enigmatic and poorly constrained by observations
until about three decades ago, when major discoveries (e.g., that protostars
are often associated with highly collimated jets) revolutionized the field.
At this time, it became increasingly clearer that magnetic fields were
playing a major role at all stages of stellar formation.
We describe herein a quick overview of the main breakthroughs that observations
and theoretical modelling yielded for our understanding of how stars (and their
planetary systems) are formed and on how much these new worlds are shaped by
the presence of magnetic fields, either those pervading the interstellar medium
and threading molecular clouds or those produced through dynamo processes in
the convective envelopes of protostars or in the accretion discs from which
they feed
Asteroseismologie van magnetische massieve sterren
No full textAbout ten percent of stars with spectral type O, B, or A have a detectable, stable and strong large-scale magnetic field at their surface, which most often resembles a magnetic dipole. These large-scale magnetic fields extend into the radiative layers of the OBA stars and theory and simulations predict that they alter the internal structure and physical properties of these stars. In particular, it is expected that these large-scale magnetic fields enforce uniform rotation in the radiative layers and may suppress convective core overshooting. This has consequences for the evolution of these magnetic hot stars and it has implications for galactic evolution. Indeed, the most massive stars are important chemical furnaces, producing much of the heavy elements in the Universe. Therefore, we observed and investigated the internal structure of magnetic hot stars. To do so, asteroseismology, i.e., the study of stellar pulsations, is the best method as the oscillation properties are directly related to the internal physical conditions. Various types of stellar oscillations are known and they are classified based on their dominant restoring force. Of these, gravity modes are governed by the buoyancy force and have their strongest probing power in the near core region, which is the domain of our interest.
Our first objective was to identify pulsating magnetic hot stars and characterize their magnetic and seismic properties. To do so, we constructed a sample of 16 magnetic candidate stars by following indirect observational diagnostics for the presence of a large-scale magnetic field. Ground-based high-resolution optical spectropolarimetry, taken with the ESPaDOnS instrument, enabled the confirmation of the presence of a large-scale magnetic field for 12 of the magnetic candidate stars. The stars without a detected magnetic field still showed indications of the presence of a weak large-scale magnetic field through chemical peculiarities. For two known magnetic stars, namely HD 43317 and o Lup, the geometry and strength of the large-scale magnetic field were characterized in detail by studying the variability in the measured longitudinal magnetic field at various rotation phases, and by analysing time series of spectropolarimetric observations obtained with the ESPaDOnS, Narval, and HARPSpol instruments. For each star in our sample, we obtained high-cadence and high-precision space-based photometry from either the BRITE, CoRoT, or K2 mission with at least a 60 d timebase to investigate (periodic) variability. Only for a few magnetic stars did we detect coherent periodic variability, uncorrelated to the rotation, that indicated the presence of stellar pulsations. The stars HD 158596, HD 177765, and o Lup were indicative for one or a few stellar pulsation modes, while HD 43317 revealed tens of stellar pulsations mode frequencies that pointed towards gravity modes. In HD 43317, many of the pulsation mode frequencies were lower or similar to the rotation frequency. Hence, the Coriolis force acts as an additional restoring force for these modes, implying that they are gravito-inertial modes.
Following these results, we investigated the B3.5V star HD 43317 in detail to meet our objective of observationally determining the internal structure of a magnetic hot star. We did this by means of forward seismic modelling, where the observed stellar pulsation mode frequencies in the CoRoT data covering ~150 d were fit to those of gravito-intertial modes computed with the pulsation code GYRE, coupled to MESA stellar structure models. We identified the pulsation mode frequencies as overlapping (l,m) = (1,-1) and (2,-1) mode series. The small convective core overshooting region derived from the seismic modelling was in line with the theoretical predictions. Yet, some of the parameters for the best fitted models were also compatible with literature values for non-magnetic pulsators within the derived uncertainties. This was due to degeneracies between stellar structure models with similar values for the asymptotic period spacing of the gravito-inertial modes, leading to skew and large confidence intervals for various model parameters. We conclude that the CoRoT time series of ~150 d is too short to lead to stringent constraints and tests of the stellar interior to discriminate between magnetic and non-magnetic pulsating hot stars.
From our detailed modelling efforts of the best studied pulsating magnetic hot star HD 43317, we were unable to observationally corroborate the theoretical predictions of an altered internal structure for magnetic hot stars. Simplifications and approximations were made during the forward seismic modelling due to the limited frequency resolution of the CoRoT data in terms of its time base. Further efforts to include magnetism in the pulsation codes, or magnetism, rotation, and angular momentum transport in the evolutionary models, are worthwhile to test whether magnetic signatures are present in the numerous (non-magnetic) gravito-inertial pulsators recently found in the nominal Kepler database (which has a ten times better frequency resolution compared to CoRoT).status: Publishe
U.: Seismic modelling of the late Be stars HD 181231 and HD 175869 observed with CoRoT: a laboratory for mixing processes
No full textReceived...; accepted... Context. HD181231 and HD175869 are two late rapidly rotating Be stars, which have been observed using high-precision photometry with the CoRoT satellite during about five consecutive months and 27 consecutive days, respectively. An analysis of their light curves, by Neiner and collaborators and Gutierrez-Soto and collaborators respectively, showed that several independent pulsation g-modes are present in these stars. Fundamental parameters have also been previously determined by these authors using spectroscopy. Aims. We aim to model these results to infer seismic properties of HD181231 and HD175869, and constrain internal transport processes of rapidly rotating massive stars. Methods. We used an adiabatic (NRO) and a non-adiabatic (Tohoku) oscillation code that accounts for the combined action of Coriolis and centrifugal accelerations on stellar pulsations as needed for rapid rotator modelling. We coupled these codes with a 2D (ROTORC) stellar structure model to take the rotational deformation of the star into account. The action of transport processes was parametrised with the mixing parameter α, which represents the ”non-standard” extension of the convective core, and determined by matching observed pulsation frequencies assuming a single star evolution scenario. In a second step, we used (Geneva) evolution models to evaluate the contribution of the secula
Spectroscopy of be stars
No full textThis chapter describes non supergiant B-type stars that show emission lines, called Be stars. The emission is caused by the presence of a circumstellar decretion disk. Many physical phenomena are thought to be involved in these stars, such as rapid rotation, pulsations and magnetic fields, and give rise to variations. Spectroscopy is used as a diagnostic tool to study Be stars, by professional astronomers as well as by amateurs
Quantitative classification of WR nuclei of planetary nebulae
No full textWe analyse 42 emission-line nuclei of Planetary Nebulae (PNe), in the
framework of a large spectrophotometric survey of [WC] nuclei of PNe conducted
since 1994, using low/medium resolution spectra obtained at ESO and at OHP.
We construct a grid of selected line-intensities (normalized to
\ion{C}{iv}-5806 Å = 100) ordered by decreasing ionisation potential
going from 871 to 24 eV. In this grid, the stars appear to belong clearly to prominent O (hot
[WO1-4] types) or C (cooler [WC4-11] types) line-sequences, in agreement with
the classification of massive WR stars applied to Central Stars of Planetary
Nebulae (CSPNe) by Crowther et al. [CITE] (CMB98).
We propose 20 selected line ratios and the FWHM of \ion{C}{iv} and \ion{C}{iii} lines as
classification diagnostics, which agree well with the 7 line ratios and the FWHM proposed
by CMB98. This classification based on
ionisation is related to the evolution of the temperature and of the stellar
wind, reflecting the mass-loss history.
In particular, inside the hot [WO4]-class, we discover four stars showing very
broad lines over the whole spectral range. These stars possibly mark the
transition from the initial momentum-driven phase to the later energy-driven
phase of the CSPNe along their evolution
from the post-Asymptotic Giant Branch (post-AGB) phase through [WC] late, [WC4] and [WO]-types.
The HR diagram and the diagram linking the terminal velocity and the
temperature indicate highly dispersed values of the stellar mass for our
sample, around a mean mass higher than for normal CSPNe.
The distribution of the 42 stars along the ionisation sequence shows 24% of
[WO1-3], 21% of [WO4], 17% of [WC4] hot stars, and 26% of [WC9-11] cool
stars. The [WC5-8] classes remain poorly represented (12%).
This distribution is confirmed on the basis of a large compilation of the
127 known emission-lines CSPNe, which represent about 5% of the known PNe
Ionization-induced star formation - IV. Triggering in bound clusters
No full textWe present a detailed study of star formation occurring in bound star-forming clouds under the influence of internal ionizing feedback from massive stars across a spectrum of cloud properties. We infer which objects are triggered by comparing our feedback simulations with control simulations in which no feedback was present. We find that feedback always results in a lower star formation efficiency and usually but not always results in a larger number of stars or clusters. Cluster mass functions are not strongly affected by feedback, but stellar mass functions are biased towards lower masses. Ionization also affects the geometrical distribution of stars in ways that are robust against projection effects, but may make the stellar associations more or less subclustered depending on the background cloud environment. We observe a prominent pillar in one simulation which is the remains of an accretion flow feeding the central ionizing cluster of its host cloud and suggest that this may be a general formation mechanism for pillars such as those observed in M16. We find that the association of stars with structures in the gas such as shells or pillars is a good but by no means foolproof indication that those stars have been triggered and we conclude overall that it is very difficult to deduce which objects have been induced to form and which formed spontaneously simply from observing the system at a single time.Peer reviewe
Pre-main-sequence stars in the star-forming complex Sh 2-284
No full textLocated at the Galactic anticentre, Sh 2-284 is an H ii region that harbours several young open clusters. Dolidze 25, a rare metal-poor (Z similar to 0.004) young cluster, is one of these. Given its association with Sh 2-284, it is reasonable to assume a low metallicity for the whole H ii region. Sh 2-284 is expected to host a significant population of pre-main-sequence (PMS) stars of both low and intermediate mass (Herbig Ae stars). We aim to characterize these stars by means of a spectroscopic and photometric survey conducted with VIMOS@VLT and complemented with additional optical and infrared observations. In this survey we selected and characterized 23 PMS objects. We derived the effective temperature, the spectral energy distribution and luminosity of these objects, and, using theoretical PMS evolutionary tracks with the appropriate metallicity, we estimated the mass and the age of the studied objects. We also estimated a distance of 4 kpc for Sh 2-284, using the spectroscopic parallax of three OB stars. From the age determination we concluded that triggered star formation is occurring in this region. Our results show that a significant fraction of the young stellar objects (YSOs) may have preserved their disc/envelopes, in contrast with what is found in other recent studies of low-metallicity star-forming regions in the Galaxy. Finally, among the 23 bona fide PMS stars, we identified eight stars that are good candidates to be pulsators of the delta Scuti type
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