32 research outputs found
Insights from Asteroseismology of Massive Stars: The Need for Additional Angular Momentum Transport Mechanisms
In massive stars, rotation and oscillatory waves can have a tight interplay.
In order to assess the importance of additional angular momentum transport
mechanisms other than rotation, we compare the asteroseismic properties of a
uniformly rotating model and a differentially rotating one. Accordingly, we
employ the observed period spacing of 36 dipole g-modes in the Kepler
M target KIC 7760680 to discriminate between these two models. We favor
the uniformly rotating model, which fully satisfies all observational
constraints. Therefore, efficient angular momentum transport by additional
mechanisms such as internal gravity waves, heat-driven modes and magnetic field
is needed during early main sequence evolution of massive stars.Comment: Proceedings of "Seismology of the Sun and the Distant Stars 2016".
Editors: M\'ario J. P. F. G. Monteiro, Margarida S. Cunha, Jo\~ao Miguel T.
Ferreir
On the shape of core overshooting in stellar model computations, and asteroseismic tests
Slowly pulsating B stars (SPB) and Dor stars pulsate in high-order
gravity (g-) modes. The frequencies of g-modes are sensitive to the detailed
structure and evolution history of stars having convective cores. Receding
convective cores in OB-type stars leave behind a chemically inhomogenous
radiative zone. Once a g-mode has radial nodes near the
boundaries of these layers, the mode gets trapped and its period deviates from
asymptotic period spacing. Careful study of such trapped modes allows
constraining the extent of such layers by fitting individual pulsation
frequencies. We employ 19 consecuitve dipole g-modes of a very rich Kepler SPB
pulsator, KIC 10526294, to demonstrate the power of mode trapping in B-stars in
studying the thermal and chemical stratification in the overshooting layer.Comment: 4 pages, 4 figures; to appear in the proceedings of "Physics of
Evolved Stars 2015 - A conference dedicated to the memory of Olivier
Chesneau
Sub-Inertial Gravity Modes in the B8V Star KIC 7760680 Reveal Moderate Core Overshooting and Low Vertical Diffusive Mixing
KIC 7760680 is so far the richest slowly pulsating B star, by exhibiting 36
consecutive dipole () gravity (g-) modes. The monotonically decreasing
period spacing of the series, in addition to the local dips in the pattern
confirm that KIC 7760680 is a moderate rotator, with clear mode trapping in
chemically inhomogeneous layers. We employ the traditional approximation of
rotation to incorporate rotational effects on g-mode frequencies. Our detailed
forward asteroseismic modelling of this g-mode series reveals that KIC 7760680
is a moderately rotating B star with mass M. By
simultaneously matching the slope of the period spacing, and the number of
modes in the observed frequency range, we deduce that the equatorial rotation
frequency of KIC 7760680 is 0.4805 day, which is 26\% of its Roche break
up frequency. The relative deviation of the model frequencies and those
observed is less than one percent. We succeed to tightly constrain the
exponentially-decaying convective core overshooting parameter to . This means that convective core overshooting can
coexist with moderate rotation. Moreover, models with exponentially-decaying
overshoot from the core outperform those with the classical step-function
overshoot. The best value for extra diffusive mixing in the radiatively stable
envelope is confined to (with in cm sec), which is notably smaller than theoretical
predictions.Comment: 12 Figures, 2 Tables, all data publicly available for download;
accepted for publication in Astrophysical Journa
Tight asteroseismic constraints on core overshooting and diffusive mixing in the slowly rotating pulsating B8.3V star KIC 10526294
KIC 10526294 is a very slowly rotating and slowly pulsating late B-type star.
Its 19 consecutive dipole gravity modes constitute a series with almost
constant period spacing. This unique collection of identified modes probes the
near-core environment of this star and holds the potential to reveal the size
and structure of the overshooting zone on top of the convective core, as well
as the mixing properties of the star. We pursue forward seismic modelling based
on adiabatic eigenfrequencies of equilibrium models for eight extensive
evolutionary grids tuned to KIC 10526294, by varying the initial mass,
metallicity, chemical mixture, and the extent of the overshooting layer on top
of the convective core. We examine models for both OP and OPAL opacities and
test the occurrence of extra diffusive mixing. We find a tight mass,
metallicity relation within the ranges ~ 3.13 to 3.25 Msun and ~ 0.014
to 0.028. We deduce that an exponentially decaying diffusive core overshooting
prescription describes the seismic data better than a step function formulation
and derive a value of between 0.017 and 0.018. Moreover, the inclusion
of extra diffusive mixing with a value of between 1.75 and
2.00 dex (with in cm^2/sec) improves the goodness-of-fit based on
the observed and modelled frequencies with a factor 11 compared to the case
where no extra mixing is considered, irrespective of the combination
within the allowed seismic range. The inclusion of diffusive mixing in addition
to core overshooting is essential to explain the structure in the observed
period spacing pattern of this star. Moreover, we deduce that an exponentially
decaying prescription for the core overshooting is to be preferred over a step
function. Our best models for KIC 10526294 approach the seismic data to a level
that they can serve future inversion of its stellar structure.Comment: 13 pages, 4 tables, 12 figures, accepted for publication in Astronomy
& Astrophyic
The internal rotation profile of the B-type star KIC10526294 from frequency inversion of its dipole gravity modes and statistical model comparison
The internal angular momentum distribution of a star is key to determine its
evolution. Fortunately, the stellar internal rotation can be probed through
studies of rotationally-split non-radial oscillation modes. In particular,
detection of non-radial gravity modes (g modes) in massive young stars has
become feasible recently thanks to the Kepler space mission. Our aim is to
derive the internal rotation profile of the Kepler B8V star KIC 10526294
through asteroseismology. We interpret the observed rotational splittings of
its dipole g modes using four different approaches based on the best seismic
models of the star and their rotational kernels. We show that these kernels can
resolve differential rotation the radiative envelope if a smooth rotational
profile is assumed and the observational errors are small. Based on Kepler
data, we find that the rotation rate near the core-envelope boundary is well
constrained to nHz. The seismic data are consistent with rigid
rotation but a profile with counter-rotation within the envelope has a
statistical advantage over constant rotation. Our study should be repeated for
other massive stars with a variety of stellar parameters in order to deduce the
physical conditions that determine the internal rotation profile of young
massive stars, with the aim to improve the input physics of their models.Comment: 52 pages, 32 figures, accepted for publication in The Astrophysical
Journa
Separated Fringe Packet Observations with the CHARA Array II: Andromeda, HD 178911, and {\xi} Cephei
When observed with optical long-baseline interferometers (OLBI), components
of a binary star which are sufficiently separated produce their own
interferometric fringe packets; these are referred to as Separated Fringe
Packet (SFP) binaries. These SFP binaries can overlap in angular separation
with the regime of systems resolvable by speckle interferometry at single,
large-aperture telescopes and can provide additional measurements for
preliminary orbits lacking good phase coverage, help constrain elements of
already established orbits, and locate new binaries in the undersampled regime
between the bounds of spectroscopic surveys and speckle interferometry. In this
process, a visibility calibration star is not needed, and the separated fringe
packets can provide an accurate vector separation. In this paper, we apply the
SFP approach to {\omega} Andromeda, HD 178911, and {\xi} Cephei with the CLIMB
three-beam combiner at the CHARA Array. For these systems we determine
component masses and parallax of 0.9630.049 and
0.8600.051 and 39.541.85 milliarcseconds (mas) for
{\omega} Andromeda, for HD 178911 of 0.8020.055 and
0.6220.053 with 28.261.70 mas, and masses of
1.0450.031 and 0.4080.066 and
38.102.81 mas for {\xi} Cephei.Comment: 28 pages, 4 tables, 6 figures, accepted to AJ May 201
Ensemble Asteroseismology of the Young Open Cluster NGC 2244
Our goal is to perform in-depth ensemble asteroseismology of the young open
cluster NGC2244 with the 2-wheel Kepler mission. While the nominal Kepler
mission already implied a revolution in stellar physics for solar-type stars
and red giants, it was not possible to perform asteroseismic studies of massive
OB stars because such targets were carefully avoided in the FoV in order not to
disturb the exoplanet hunting. Now is an excellent time to fill this hole in
mission capacity and to focus on the metal factories of the Universe, for which
stellar evolution theory is least adequate.
Our white paper aims to remedy major shortcomings in the theory of stellar
structure and evolution of the most massive stars by focusing on a large
ensemble of stars in a carefully selected young open cluster. Cluster
asteroseismology of very young stars such as those of NGC2244 has the major
advantage that all cluster stars have similar age, distance and initial
chemical composition, implying drastic restrictions for the stellar modeling
compared to asteroseismology of single isolated stars with very different ages
and metallicities.
Our study requires long-term photometric measurements of stars with visual
magnitude ranging from 6.5 to 15 in a large FoV with a precision better than 30
ppm for the brightest cluster members (magnitude below 9) up to 500 ppm for the
fainter ones, which is well achievable with 2-Wheel Kepler, in combination with
high-precision high-resolution spectroscopy and spectro-polarimetry of the
brightest pulsating cluster members. These ground-based spectroscopic data will
be assembled with the HERMES and CORALIE spectrographs (twin 1.2m Mercator and
Euler telescopes, La Palma, Canary Islands and La Silla, Chile), as well as
with the spectro-polarimetric NARVAL instrument (2m BLT at the Pic du Midi,
French Pyrenees), to which we have guaranteed access.Comment: 10 pages, 3 figures, white paper submitted in response to the NASA
call for community input for science investigations the Kepler 2-Wheel
spacecraf
Asteroseismology of massive stars with the TESS mission: the runaway Beta Cep pulsator PHL 346 = HN Aqr
We report an analysis of the first known Beta Cep pulsator observed by the
TESS mission, the runaway star PHL 346 = HN Aqr. The star, previously known as
a singly-periodic pulsator, has at least 34 oscillation modes excited, 12 of
those in the g-mode domain and 22 p modes. Analysis of archival data implies
that the amplitude and frequency of the dominant mode and the stellar radial
velocity were variable over time. A binary nature would be inconsistent with
the inferred ejection velocity from the Galactic disc of 420 km/s, which is too
large to be survivable by a runaway binary system. A kinematic analysis of the
star results in an age constraint (23 +- 1 Myr) that can be imposed on
asteroseismic modelling and that can be used to remove degeneracies in the
modelling process. Our attempts to match the excitation of the observed
frequency spectrum resulted in pulsation models that were too young. Hence,
asteroseismic studies of runaway pulsators can become vital not only in tracing
the evolutionary history of such objects, but to understand the interior
structure of massive stars in general. TESS is now opening up these stars for
detailed asteroseismic investigation.Comment: accepted for ApJ