846 research outputs found
Galactic Cepheids with Spitzer: I. Leavitt Law and Colors
Classical Cepheid variable stars have been important indicators of
extragalactic distance and Galactic evolution for over a century. The Spitzer
Space Telescope has opened the possibility of extending the study of Cepheids
into the mid- and far-infrared, where interstellar extinction is reduced. We
have obtained photometry from images of a sample of Galactic Cepheids with the
IRAC and MIPS instruments on Spitzer. Here we present the first mid-infrared
period-luminosity relations for Classical Cepheids in the Galaxy, and the first
ever Cepheid period-luminosity relations at 24 and 70 um. We compare these
relations with theoretical predictions, and with period-luminosity relations
obtained in recent studies of the Large Magellanic Cloud. We find a significant
period-color relation for the [3.6]-[8.0] IRAC color. Other mid-infrared colors
for both Cepheids and non-variable supergiants are strongly affected by
variable molecular spectral features, in particular deep CO absorption bands.
We do not find strong evidence for mid-infrared excess caused by warm (~500 K)
circumstellar dust. We discuss the possibility that recent detections with
near-infrared interferometers of circumstellar shells around delta Cep, l Car,
Polaris, Y Oph and RS Pup may be a signature of shocked gas emission in a
dust-poor wind associated to pulsation-driven mass loss.Comment: Accepted by The Astrophysical Journal on Nov 11, 200
A SEARCH FOR MASS LOSS ON THE CEPHEID INSTABILITY STRIP USING H i 21 cm LINE OBSERVATIONS
We present the results of a search for H i 21 cm line emission from the circumstellar environments of four Galactic Cepheids (RS Pup, X Cyg, ζ Gem, and T Mon) based on observations with the Karl G. Jansky Very Large Array. The observations were aimed at detecting gas associated with previous or ongoing mass loss. Near the long-period Cepheid T Mon, we report the detection of a partial shell-like structure whose properties appear consistent with originating from an earlier epoch of Cepheid mass loss. At the distance of T Mon, the nebula would have a mass (H I+He) of âŒ0.5M[subscript âš], or ~6% of the stellar mass. Assuming that one-third of the nebular mass comprises swept-up interstellar gas, we estimate an implied mass-loss rate of [superscript dot]MâŒ(0.6-2) x 10â»â” M[subscript âš] yrâ1. No clear signatures of circumstellar emission were found toward ζ Gem, RS Pup, or X Cyg, although in each case, line-of-sight confusion compromised portions of the spectral band. For the undetected stars, we derive model-dependent 3 upper limits on the mass-loss rates, averaged over their lifetimes on the instability strip, of âČ (0.3-6) x 10â»â¶ M[subscript âš] yrâ»Âč and estimate the total amount of mass lost to be less than a few percent of the stellar mass.National Science Foundation (U.S.) (AST-1310930)Chandra X-ray Center (U.S.) (NAS8-03060
Calibrating the projection factor for Galactic Cepheids
The projection factor (p), which converts the radial velocity to pulsational
velocity, is an important parameter in the Baade-Wesselink (BW) type analysis
and distance scale work. The p-factor is either adopted as a constant or
linearly depending on the logarithmic of pulsating periods. The aim of this
work is to calibrate the p-factor if a Cepheid has both the BW distance and an
independent distance measurement, and examine the p-factor for delta Cephei --
the prototype of classical Cepheids. We calibrated the p-factor for several
Galactic Cepheids that have both the latest BW distances and independent
distances either from Hipparcos parallaxes or main-sequence fitting distances
to Cepheid-hosted stellar clusters. Based on 25 Cepheids, the calibrated
p-factor relation is consistent with latest p-factor relation in literature.
The calibrated p-factor relation also indicates that this relation may not be
linear and may exhibit an intrinsic scatter. We also examined the discrepancy
of empirical p-factors for delta Cephei, and found that the reasons for this
discrepancy include the disagreement of angular diameters, the treatment of
radial velocity data, and the phase interval adopted during the fitting
procedure. Finally, we investigated the impact of the input p-factor in two BW
methodologies for delta Cephei, and found that different p-factors can be
adopted in these BW methodologies and yet result in the same angular diameters.Comment: 6 pages, 6 figures and 2 tables. A&A accepte
Cepheid limb darkening, angular diameter corrections, and projection factor from static spherical model stellar atmospheres
Context. One challenge for measuring the Hubble constant using Classical
Cepheids is the calibration of the Leavitt Law or period-luminosity
relationship. The Baade-Wesselink method for distance determination to Cepheids
relies on the ratio of the measured radial velocity and pulsation velocity, the
so-called projection factor and the ability to measure the stellar angular
diameters. Aims. We use spherically-symmetric model stellar atmospheres to
explore the dependence of the p-factor and angular diameter corrections as a
function of pulsation period. Methods. Intensity profiles are computed from a
grid of plane-parallel and spherically-symmetric model stellar atmospheres
using the SAtlas code. Projection factors and angular diameter corrections are
determined from these intensity profiles and compared to previous results.
Results. Our predicted geometric period-projection factor relation including
previously published state-of-the-art hydrodynamical predictions is not with
recent observational constraints. We suggest a number of potential resolutions
to this discrepancy. The model atmosphere geometry also affects predictions for
angular diameter corrections used to interpret interferometric observations,
suggesting corrections used in the past underestimated Cepheid angular
diameters by 3 - 5%. Conclusions. While spherically-symmetric hydrostatic model
atmospheres cannot resolve differences between projection factors from theory
and observations, they do help constrain underlying physics that must be
included, including chromospheres and mass loss. The models also predict more
physically-based limb-darkening corrections for interferometric observations.Comment: 8 pages, 6 figures, 2 tables, accepted for publication in A&
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VOF simulations of the contact angle dynamics during the drop spreading: Standard models and a new wetting force model
Introduction
In this study,a novel numerical implementation for the adhesion of liquid droplets impacting normally on solid dry surfaces is presented. The advantage of this new approach, compared to the majority of existing models, is that the dynamic contact angle forming during the surface wetting process is not inserted as a boundary condition, but is derived implicitly by the induced fluid flow characteristics (interface shape) and the adhesion physics of the gas-liquid-surface interface (triple line), starting only from the advancing and receding equilibrium contact angles. These angles are required in order to define the wetting properties of liquid phases when interacting with a solid surface.
Methodology
The physical model is implemented as a source term in the momentum equation of a Navier-Stokes CFD flow solver as an "adhesion-like" force which acts at the triple-phase contact line as a result of capillary interactions between the liquid drop and the solid substrate. The numerical simulations capture the liquid-air interface movement by considering the volume of fluid (VOF) method and utilizing an automatic local grid refinement technique in order to increase the accuracy of the predictions at the area of interest, and simultaneously minimize numerical diffusion of the interface.
Results
The proposed model is validated against previously reported experimental data of normal impingement of water droplets on dry surfaces at room temperature. A wide range of impact velocities, i.e. Weber numbers from as low as 0.2 up to 117, both for hydrophilic (Ξadv = 10° - 70°) and hydrophobic (Ξadv = 105° - 120°) surfaces, has been examined. Predictions include in addition to droplet spreading dynamics, the estimation of the dynamic contact angle; the latter is found in reasonable agreement against available experimental measurements.
Conclusion
It is thus concluded that theimplementation of this model is an effective approach for overcoming the need of a pre-defined dynamic contact angle law, frequently adopted as an approximate boundary condition for such simulations. Clearly, this model is mostly influential during the spreading phase for the cases of low We number impacts (We <80) since for high impact velocities, inertia dominates significantly over capillary forces in the initial phase of spreading
Identification of the Microlens in Event MACHO-LMC-20
We report on the identification of the lens responsible for microlensing
event MACHO-LMC-20. As part of a \textit{Spitzer}/IRAC program conducting
mid-infrared follow-up of the MACHO Large Magellanic Cloud microlensing fields,
we discovered a significant flux excess at the position of the source star for
this event. These data, in combination with high resolution near-infrared
\textit{Magellan}/PANIC data has allowed us to classify the lens as an early M
dwarf in the thick disk of the Milky Way, at a distance of kpc. This
is only the second microlens to have been identified, the first also being a M
dwarf star in the disk. Together, these two events are still consistent with
the expected frequency of nearby stars in the Milky Way thin and thick disks
acting as lenses.Comment: 6 pages, 4 figures, submitted to ApJ Letter
New Evidence for Mass Loss from delta Cephei from HI 21-cm Line Observations
Recently published Spitzer observations of the classical Cepheid archetype
delta Cephei revealed an extended dusty nebula surrounding this star and its
hot companion. The infrared emission resembles a bow shock aligned with the
direction of space motion of the star, indicating that delta Cep is undergoing
mass-loss through a stellar wind. Here we report HI 21-cm line observations
with the VLA to search for neutral atomic hydrogen associated with this wind.
Our VLA data reveal a spatially extended HI nebula (~13' or 1 pc across)
surrounding the position of delta Cep. The nebula has a head-tail morphology,
consistent with circumstellar ejecta shaped by the interaction between a
stellar wind and the ISM. We directly measure a mass of circumstellar hydrogen
M_HI\approx0.07M_odot, although the total HI mass may be larger. The HI data
imply a stellar wind with an outflow velocity V_o=35.6\pm1.2 km/s and a
mass-loss rate of M_dot=(1.0\pm0.8)x10**-6 M_dot/yr. We have computed
theoretical evolutionary tracks that include mass loss across the instability
strip and show that a mass-loss rate of this magnitude, sustained over the
preceding Cepheid lifetime of delta Cep, could be sufficient to resolve a
significant fraction of the discrepancy between the pulsation and evolutionary
masses for this star. (abridged)Comment: ApJ, in press (January 1, 2012). Version with full resolution figures
available at
http://www.haystack.mit.edu/hay/staff/lmatthew/matthews_deltaCep.pd
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Numerical investigation of droplet impingement onto hydrophobic and super-hydrophobic solid surfaces. The effect of Weber number and wettability
In this study, a new model for the wetting interaction between a liquid droplet and a solid surface is presented. Based on this model, a force which acts on the contact line is incorporated as a source term in the Navier-Stokes momentum equation. The advantage of the new model in comparison with the widely-used Brackbillâs model is that the contact angle is not inserted as a boundary condition, but is derived by the induced fluid flow and the adhesion physics of the liquid-surface combination. For the interface tracking, the Volume of Fluid (V.O.F) method is used, accompanied by an automatic local grid refinement technique in order to minimize the arithmetic diffusion of volume fraction and thus acquire more representative physical results. The new model is validated against experimental data for low and moderate We numbers both for hydrophilic and superhydrophobic surfaces. Results of the model are also compared against the standard Brackbillâs model for the implementation of the wetting force. The apparent contact angle during droplet spreading and recoiling is plotted in order to gain insight on the dynamic angle temporal evolution during the impingement process
The Cepheid mass discrepancy and pulsation-driven mass loss
Context. A longstanding challenge for understanding classical Cepheids is the
Cepheid mass discrepancy, where theoretical mass estimates using stellar
evolution and stellar pulsation calculations have been found to differ by
approximately 10 - 20%. Aims. We study the role of pulsation-driven mass loss
during the Cepheid stage of evolution as a possible solution to this mass
discrepancy. Methods. We computed stellar evolution models with a Cepheid
mass-loss prescription and various amounts of convective core overshooting. The
contribution of mass loss towards the mass discrepancy is determined using
these models, Results. Pulsation-driven mass loss is found to trap Cepheid
evolution on the instability strip, allowing them to lose about 5 - 10% of
their total mass when moderate convective core overshooting, an amount
consistent with observations of other stars, is included in the stellar models.
Conclusions. We find that the combination of moderate convective core
overshooting and pulsation-driven mass loss can solve the Cepheid mass
discrepancy.Comment: 4 pages, 2 figures and 2 tables. Accepted for publication A&A Letter
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