110 research outputs found
Flow Boiling and Condensation Experiment Flight Hardware Development
The Flow Boiling and Condensation Experiment (FBCE) to be manifested on the International Space Station (ISS) consists of a fluid system and the associated electronics to provide for conditioning the test fluid (normal-PerFluorohexane or nPFH-C6F14) to the proper thermodynamic state prior to entering a test module, which can be interchangeable based on the science objectives. Two separate test modules have been manufactured for the FBCE, the Flow Boiling Module (FBM), which investigates flow boiling for a subcooled liquid, saturated liquid, or two phase mixture, and the Condensation Module Heat Transfer (CM-HT), which investigates condensation of a flowing saturated or superheated vapor. The test fluid heating is accomplished using the Bulk Heater Module (BHM), which heats the fluid to various states based on the demands of the currently installed test module. ISS Internal Thermal Control System (ITCS) water is utilized to cool the test fluid prior to entering the circulation pump, and is also utilized for cooling for condensation in CM-HT, for cooling of a camera in FBM. An adjustable pressure bellows-type accumulator is used to set the pressure at the inlet of the test section, but does not provide active pressure control during testing. The flow of the test fluid is achieved using a gear pump controlled by a coriolis flow meter, which also provides the flow rate measurement. Flow rates for the ITCS water loops are measured and controlled using coriolis flow meters with directly controlled proportional valves. During execution of FBCE operations, the FBM is scheduled to collect data for three months before being exchanged with CM-HT for another three month data collection run. In this work, we present the development of the flight hardware, the associated challenges experienced during the development such as packaging flight system hardware, and the lessons learned in overcoming the encountered challenges
Theoretical study of Doradus pulsations in pre-main sequence stars
The question of the existence of pre-main sequence (PMS) ~Doradus
(~Dor) has been raised by the observations of young clusters such as
NGC~884 hosting ~Dor members. We have explored the properties of
~Dor type pulsations in a grid of PMS models covering the mass range
and we derive the theoretical instability
strip (IS) for the PMS ~Dor pulsators. We explore the possibility of
distinguishing between PMS and MS ~Dor by the behaviour of the period
spacing of their high order -modes (-modes).Comment: 5 pages, 6 figures, Proc. HELAS IV Conference, Lanzarote, February
2010. Eds T. Roca Cort\'es, P. Pall\'e and S. Jim\'enez Reyes. Accepted in
Astron. Nac
Vibrational Instability of Metal-Poor Low-Mass Main-Sequence Stars
We find that low-degree low-order g-modes become unstable in metal-poor
low-mass stars due to the -mechanism of the pp-chain. Since the
outer convection zone of these stars is limited only to the very outer layers,
the uncertainty in the treatment of convection does not affect the result
significantly. The decrease in metallicity leads to decrease in opacity and
hence increase in luminosity of a star. This makes the star compact and results
in decrease in the density contrast, which is favorable to the
-mechanism instability. We find also instability for high order
g-modes of metal-poor low-mass stars by the convective blocking mechanism.
Since the effective temperature and the luminosity of metal-poor stars are
significantly higher than those of Pop I stars, the stars showing
Dor-type pulsation are substantially less massive than in the case of Pop I
stars. We demonstrate that those modes are unstable for about
stars in the metal-poor case.Comment: 4 pages, 4 figures, To be published in Astrophysics and Space Science
Proceedings series (ASSP). Proceedings of the "20th Stellar Pulsation
Conference Series: Impact of new instrumentation and new insights in stellar
pulsations", 5-9 September 2011, Granada, Spai
Spectroscopic Pulsational Frequency Identification and Mode Determination of Gamma Doradus Star HD135825
We present the mode identification of frequencies found in spectroscopic
observations of the Gamma Doradus star HD135825. Four frequencies were
successfully identified: 1.3150 +/- 0.0003 1/d; 0.2902 +/- 0.0004 1/d; 1.4045
+/- 0.0005 1/d; and 1.8829 +/- 0.0005 1/d. These correspond to (l, m) modes of
(1,1), (2,-2), (4,0) and (1,1) respectively. Additional frequencies were found
but they were below the signal-to-noise limit of the Fourier spectrum and not
suitable for mode identification. The rotational axis inclination and vsini of
the star were determined to be 87 degrees (nearly edge-on) and 39.7 km/s
(moderate for Gamma Doradus stars) respectively. A simultaneous fit of these
four modes to the line profile variations in the data gives a reduced chi
square of 12.7. We confirm, based on the frequencies found, that HD135825 is a
bona fide Gamma Doradus star.Comment: Accepted to MNRAS 2012 March
Spectrum Analysis of Bright Kepler Gamma Doradus Candidate Stars
Ground-based spectroscopic follow-up observations of the pulsating stars
observed by the Kepler satellite mission are needed for their asteroseismic
modelling. We aim to derive the fundamental parameters for a sample of 26 Gamma
Doradus candidate stars observed by the Kepler satellite mission to accomplish
one of the required preconditions for their asteroseismic modelling and to
compare our results with the types of pulsators expected from the existing
light curve analysis. We use the spectrum synthesis method to derive the
fundamental parameters like Teff, logg, [M/H], and vsini from newly obtained
spectra and compute the spectral energy distribution from literature photometry
to get an independent measure of Teff. We find that most of the derived Teff
values agree with the values given in the Kepler Input Catalogue. According to
their positions in the HR-diagram three stars are expected Gamma Dor stars, ten
stars are expected Delta Sct stars, and seven stars are possibly Delta Sct
stars at the hot border of the instability strip. Four stars in our sample are
found to be spectroscopic binary candidates and four stars have very low
metallicity where two show about solar C abundance. Six of the 10 stars located
in the Delta Sct instability region of the HR-diagram show both Delta Sct and
Gamma Dor-type oscillations in their light curves implying that Gamma Dor-like
oscillations are much more common among the Delta Sct stars than predicted by
theory. Moreover, seven stars showing periods in the Delta Sct and the Delta
Sct-Gamma Dor range in their light curves are located in the HR-diagram left of
the blue edge of the theoretical Delta Sct instability strip. The consistency
of these findings with recent investigations based on high-quality Kepler data
implies the need for a revision of the theoretical Gamma Dor and Delta Sct
instability strips.Comment: 10 pages, 3 figures, 7 tables; accepted for publication in MNRA
Constraining the properties of delta Scuti stars using spectroscopic eclipsing binary systems
Many stars exhibit stellar pulsations, favoring them for asteroseismic
analyses. Interpreting the oscillations requires some knowledge of the
oscillation mode geometry (spherical degree, radial and azimuthal orders). The
delta Scuti stars (1.5 - 2.5 M_sol) often show just one or few pulsation
frequencies. Although this may promise a successful seismological analysis, we
may not know enough about either the mode or the star to use the oscillation
frequency to improve the determination of the stellar model, or probe the
star's structure. For the observed frequencies to be used successfully as
seismic probes of these objects, we need to concentrate on stars for which we
can reduce the number of free parameters in the problem, such as binary systems
or open clusters. We investigate how much our understanding of a delta Scuti
star is improved when it is in a detached eclipsing binary system instead of
being a single field star. We use singular value decomposition to explore the
precision we expect in stellar parameters (mass, age and chemical composition)
for both cases. We examine how the parameter uncertainties propagate to the
luminosity - effective temperature diagram and determine when the effort of
obtaining a new measurement is justified. We show that for the single star, a
correct identification of the oscillation mode is necessary to produce strong
constraints on the stellar model properties, while for the binary system the
observations without the pulsation mode provide the same or better constraints
on the stellar parameters. In the latter case, ...Comment: emulateapj 16 pages, accepted Ap
On the relationship between the Delta Scuti and Gamma Doradus pulsators
We searched for Delta Scuti-type pulsations amongst known and candidate Gamma
Doradus stars. We acquired 270 h of observations and monitored a total of 26
stars. One target, HD 209295, turned out to be a member of both classes of
pulsating star. We classify six of our targets as new bona fide Gamma Doradus
stars, nine more as good Gamma Doradus candidates, and three as ellipsoidal
variables. The remainder comprise one Delta Scuti star and five unexplained
variables.
We revise the blue border of the Gamma Doradus phenomenon towards cooler
temperatures. This new blue edge extends from a temperature of about 7550 K on
the ZAMS to 7400 K one magnitude above it. Five bona fide Gamma Doradus stars
we observed are located inside the Delta Scuti instability strip, but none of
them exhibited observable Delta Scuti pulsations. We therefore suggest that
Gamma Doradus stars are less likely to be Delta Scuti pulsators compared to
other normal stars in the same region of the lower instability strip. In
addition, we show that there is a clear separation between the pulsation
constants Q of Delta Scuti and Gamma Doradus stars. The Gamma Doradus stars
known to date all have Q>0.23 d.Comment: 12 pages, 9 figures. MNRAS, in pres
A spectroscopic study of southern (candidate) gamma Doradus stars. I. Time series analysis
We present results of a spectroscopic study of 37 southern (candidate) gamma
Doradus stars based on echelle spectra. The observed spectra were
cross-correlated with the standard template spectrum of an F0-type star for an
easier detection of binary and intrinsic variations. We identified 15 objects
as spectroscopic binaries, including 7 new ones, and another 3 objects are
binary suspects. At least 12 objects show composite spectra. We could determine
the orbital parameters for 9 binaries, of which 4 turn out to be ellipsoidal
variables. For 6 binaries, we estimated the expected time-base of the orbital
variations. Clear profile variations are observed for 17 objects, pointing
towards stellar pulsation. For 8 of them, we have evidence that the main
spectroscopic and photometric periods coincide. Our results, in combination
with prior knowledge from the literature, lead to the classification of 10
objects as new bona-fide gamma Doradus stars, 1 object as new bona-fide delta
Scuti star, and 8 objects as constant stars. Finally, we determined the
projected rotational velocity with two independent methods. The resulting vsini
values range from 3 to 135 km/s. For the bona-fide gamma Doradus stars, the
majority has vsini below 60 km/s.Comment: 13 pages (+ 10 pages online material), 10 (+16) figures. Accepted for
publication by A&
Confirmation of simultaneous p and g mode excitation in HD 8801 and Gamma Peg from time-resolved multicolour photometry of six candidate "hybrid" pulsators
We carried out a multi-colour time-series photometric study of six stars
claimed as "hybrid" p and g mode pulsators in the literature. Gamma Peg was
confirmed to show short-period oscillations of the Beta Cep type and
simultaneous long-period pulsations typical of Slowly Pulsating B (SPB) stars.
From the measured amplitude ratios in the Stromgren uvy passbands, the
stronger of the two short period pulsation modes was identified as radial; the
second is l=1. Three of the four SPB-type modes are most likely l=1 or 2.
Comparison with theoretical model calculations suggests that Gamma Peg is
either an 8.5 solar mass radial fundamental mode pulsator or a 9.6 solar mass
first radial overtone pulsator. HD 8801 was corroborated as a "hybrid" Delta
Sct Gamma Dor star; four pulsation modes of the Gamma Dor type were detected,
and two modes of the Delta Sct type were confirmed. Two pulsational signals
between the frequency domains of these two known classes of variables were
confirmed and another was newly detected. These are either previously unknown
types of pulsation, or do not originate from HD 8801. The O-type star HD 13745
showed small-amplitude slow variability on a time scale of 3.2 days. This
object may be related to the suspected new type of supergiant SPB stars, but a
rotational origin of its light variations cannot be ruled out at this point. 53
Psc is an SPB star for which two pulsation frequencies were determined and
identified with low spherical degree. The behaviour of 53 Ari and Iota Her is
consistent with non-variability during our observations, and we could not
confirm light variations of the comparison star 34 Psc previously suspected.
The use of signal-to-noise criteria in the analysis of data sets with strong
aliasing is critically discussed.Comment: 14 pages, 10 figures, accepted by MNRA
CoRoT\,102699796, the first metal-poor Herbig Ae pulsator: a hybrid Sct- Dor variable?
We present the analysis of the time series observations of CoRoT\,102699796
obtained by the CoRoT satellite that show the presence of five independent
oscillation frequencies in the range 3.6-5 c/d. Using spectra acquired with
FLAMES@VLT, we derive the following stellar parameters: spectral type F1V,
T=7000200 K, log(g)=, [M/H]=,
sin= km/s, L/L=21. Thus, for the first
time we report the existence of a metal poor, intermediate-mass PMS pulsating
star. Ground-based and satellite data are used to derive the spectral energy
distribution of CoRoT\,102699796 extending from the optical to mid-infrared
wavelengths. The SED shows a significant IR excess at wavelengths greater than
. We conclude that CoRoT\,102699796 is a young Herbig Ae (F1Ve) star
with a transitional disk, likely associated to the HII region [FT96]213.1-2.2.
The pulsation frequencies have been interpreted in the light of the
non-radial pulsation theory, using the LOSC code in conjunction with static and
rotational evolutionary tracks. A minimization algorithm was used to find the
best-fit model with M=1.84 M, T=6900 K which imply an
isochronal age of t2.5 Myr. This result is based on the interpretation of
the detected frequencies as -modes of low-moderate -value. To our
knowledge, this is the first time that such modes are identified in a
intermediate-mass PMS pulsating star. Since CoRoT\,102699796 lies in the region
of the HR diagram where the Sct and Dor instability strips
intersect, we argue that the observed pulsation characteristics are
intermediate between these classes of variables, i.e. CoRoT\,102699796 is
likely the first PMS hybrid Dor- Sct pulsator ever studied.Comment: 13 pages, 7 figures. Accepted for publication in Monthly Notices of
the RA
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