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D. D. Sasselov

Publications and source records attributed to D. D. Sasselov.

71 records · Page 4Linked to original sources
arXiv2000

Theoretical Transmission Spectra During Extrasolar Giant Planet Transits

The recent transit observation of HD 209458 b - an extrasolar planet orbiting a sun-like star - confirmed that it is a gas giant and determined that its orbital inclination is 85 degrees. This inclination makes possible investigations of the planet atmosphere. In this paper we discuss the planet transmission spectra during a transit. The basic tenet of the method is that the planet atmosphere absorption features will be superimposed on the stellar flux as the stellar flux passes through the planet atmosphere above the limb. The ratio of the planet's transparent atmosphere area to the star area is small, approximately 10^{-3} to 10^{-4}; for this method to work very strong planet spectral features are necessary. We use our models of close-in extrasolar giant planets to estimate promising absorption signatures: the alkali metal lines, in particular the Na I and K I resonance doublets, and the He I $2^3S$ - $2^3P$ triplet line at 1083.0 nm. If successful, observations will constrain the line-of-sight temperature, pressure, and density. The most important point is that observations will constrain the cloud depth, which in turn will distinguish between different atmosphere models. We also discuss the potential of this method for EGPs at different orbital distances and orbiting non-solar-type stars.

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arXiv2000

The DIRECT Project: Influence of Blending on the Cepheid Distance Scale. I. Cepheids in M31

We investigate the influence of blending on the Cepheid distance scale. Blending is the close association of a Cepheid with one or more intrinsically luminous stars. High-resolution HST images are compared to our ground-based data, obtained as part of the DIRECT project, for a sample of 22 Cepheids in the M31 galaxy. The average (median) V-band flux contribution from luminous companions which are not resolved on the ground-based images is about 19% (12%) of the flux of the Cepheid. This is a large effect - at the 10% level for distances. The current Cepheid distance estimates to M31 are all ground-based, and are thus affected (underestimated). We discuss indirect methods to find which Cepheids are blended, e.g. by the use of well-sampled light curves in at least two optical bands. More generally, our ground-based resolution in M31 corresponds to the HST resolution at about 10 Mpc. Blending leads to systematically low distances in the observed galaxies, and therefore to systematically high estimates of H_0; we discuss the issue and the implications.

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arXiv2000

Photometric Light Curves and Polarization of Close-in Extrasolar Giant Planets

The close-in extrasolar giant planets [CEGPs], $\ltorder$ 0.05 AU from their parent stars, may have a large component of optically reflected light. We present theoretical optical photometric light curves and polarization curves for the CEGP systems, from reflected planetary light. Different particle sizes of three condensates are considered. In the most reflective case, the variability is $\approx 100$ micromagnitudes, which will be easily detectable by the upcoming satellite missions MOST, COROT, and MONS, and possibly from the ground in the near future. The least reflective case is caused by small, highly absorbing grains such as solid Fe, with variation of much less than one micromagnitude. Polarization for all cases is lower than current detectability limits. We also discuss the temperature-pressure profiles and resulting emergent spectra of the CEGP atmospheres. We discuss the observational results of Tau Boo b by Cameron et al. (1999) and Charbonneau et al. (1999) in context of our model results. The predictions - the shape and magnitude of the light curves and polarization curves - are highly dependent on the size and type of condensates present in the planetary atmosphere.

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arXiv2000

Halting Planetary Migration

When Jupiter's Roche Lobe radius exceeded the scale height of the protoplanetary disk, Jupiter opened a gap in the disk. When the gap was wide enough, tidal torques from the disk interior and exterior to Jupiter were suppressed and migration continued on the accretion time scale. In the 'minimum solar nebula' about two Jupiter masses of gas remained between Jupiter and Saturn and about five Jupiter masses between Jupiter and Uranus. Unless all but a Jupiter mass of the outer disk was removed, Jupiter would have migrated into the Sun on the accretion time scale. So far, no mechanism has been postulated to remove the outer disk except for photo-evaporation which might work exterior to Saturn. For extra-solar Jupiters at an AU or less from the central star, photo-evaporation would not remove the outer disk. We propose a new mechanism, relying on the irradiation of the exposed edge of the gap by the Sun, to cause gas from the outer disk to cross the gap and become part of the inner disk, whence it was later accreted by the Sun.

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arXiv1999

On the Snow Line in Dusty Protoplanetary Disks

The snow line, in Hayashi's (1981) model, is where the temperature of a black body that absorbed direct sunlight and re-radiated as much as it absorbed, would be 170~K. It is usually assumed that the cores of the giant planets, e.g., Jupiter, form beyond the snow line. Since Hayashi, there have been a series of more detailed models of the absorption by dust of the stellar radiation, and of accretional heating, which alter the location of the snow line. We have attempted a "self-consistent" model of a T Tauri disk in the sense that we used dust properties and calculated surface temperatures that matched observed disks. We then calculated the midplane temperature for those disks, with no accretional heating or with small (<10^-8) accretion rates. Our models bring the snow line in to the neighbourhood of 1 AU; not far enough to explain the close planetary companions to other stars, but much closer than in recent starting lines for orbit migration scenarios.

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arXiv1999

DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries and Cepheids. V. Variables in the Field M31F

We undertook a long term project, DIRECT, to obtain the direct distances to two important galaxies in the cosmological distance ladder -- M31 and M33 -- using detached eclipsing binaries (DEBs) and Cepheids. While rare and difficult to detect, DEBs provide us with the potential to determine these distances with an accuracy better than 5%. The extensive photometry obtained in order to detect DEBs provides us with good light curves for the Cepheid variables. These are essential to the parallel project to derive direct Baade-Wesselink distances to Cepheids in M31 and M33. For both Cepheids and eclipsing binaries, the distance estimates will be free of any intermediate steps. As a first step in the DIRECT project, between September 1996 and October 1997 we obtained 95 full/partial nights on the F. L. Whipple Observatory 1.2 m telescope and 36 full nights on the Michigan-Dartmouth-MIT 1.3 m telescope to search for DEBs and new Cepheids in the M31 and M33 galaxies. In this paper, fifth in the series, we present the catalog of variable stars found in the field M31F $[(α,δ)= (10.\arcdeg10, 40.\arcdeg72), J2000.0]$. We have found 64 variable stars: 4 eclipsing binaries, 52 Cepheids and 8 other periodic, possible long period or non-periodic variables. The catalog of variables, as well as their photometry and finding charts, is available via anonymous ftp and the World Wide Web. The complete set of the CCD frames is available upon request.

astro-ph↗
arXiv1999

DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries and Cepheids. IV. Variables in the Field M31D

We undertook a long term project, DIRECT, to obtain the direct distances to two important galaxies in the cosmological distance ladder -- M31 and M33 -- using detached eclipsing binaries (DEBs) and Cepheids. While rare and difficult to detect, DEBs provide us with the potential to determine these distances with an accuracy better than 5%. The extensive photometry obtained in order to detect DEBs provides us with good light curves for the Cepheid variables. These are essential to the parallel project to derive direct Baade-Wesselink distances to Cepheids in M31 and M33. For both Cepheids and eclipsing binaries, the distance estimates will be free of any intermediate steps. As a first step in the DIRECT project, between September 1996 and October 1997 we obtained 95 full/partial nights on the F. L. Whipple Observatory 1.2 m telescope and 36 full nights on the Michigan-Dartmouth-MIT 1.3 m telescope to search for DEBs and new Cepheids in the M31 and M33 galaxies. In this paper, fourth in the series, we present the catalog of variable stars, most of them newly detected, found in the field M31D $[(α,δ)= (11.\arcdeg03, 41.\arcdeg27), J2000.0]$. We have found 71 variable stars: 5 eclipsing binaries, 38 Cepheids and 28 other periodic, possible long period or non-periodic variables. The catalog of variables, as well as their photometry and finding charts, is available via anonymous ftp and the World Wide Web. The complete set of the CCD frames is available upon request.

astro-ph↗
arXiv1999

DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries and Cepheids. III. Variables in the Field M31C

We undertook a long term project, DIRECT, to obtain the direct distances to two important galaxies in the cosmological distance ladder -- M31 and M33 -- using detached eclipsing binaries (DEBs) and Cepheids. While rare and difficult to detect, DEBs provide us with the potential to determine these distances with an accuracy better than 5%. The extensive photometry obtained in order to detect DEBs provides us with good light curves for the Cepheid variables. These are essential to the parallel project to derive direct Baade-Wesselink distances to Cepheids in M31 and M33. For both Cepheids and eclipsing binaries, the distance estimates will be free of any intermediate steps. As a first step in the DIRECT project, between September 1996 and October 1997 we obtained 95 full/partial nights on the F. L. Whipple Observatory 1.2 m telescope and 36 full nights on the Michigan-Dartmouth-MIT 1.3 m telescope to search for DEBs and new Cepheids in the M31 and M33 galaxies. In this paper, third in the series, we present the catalog of variable stars, most of them newly detected, found in the field M31C [(alpha,delta)=(11.10, 41.42) deg, J2000.0}]. We have found 115 variable stars: 12 eclipsing binaries, 35 Cepheids and 68 other periodic, possible long period or non-periodic variables. The catalog of variables, as well as their photometry and finding charts, is available via anonymous ftp and the World Wide Web. The complete set of the CCD frames is available upon request.

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arXiv1998

Identification and photometry of globular clusters in M31 and M33 galaxies

We have used the data from the DIRECT project to search for new globular cluster candidates in the M31 and M33 galaxies. We have found 67 new objects in M31 and 35 in M33 and confirmed 38 and 16 previously discovered ones. A $VI$ and $BVI$ photometry has been obtained for all the clusters in M31 and M33 respectively. Luminosity functions have been constructed for the clusters in each galaxy and compared with that of the Milky Way.

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arXiv1998

Extrasolar Giant Planets under Strong Stellar Irradiation

We investigate the effects on extrasolar giant planets [EGPs] of intense irradiation by their parent stars, describing the issues involved in treating the model atmosphere problem correctly. We treat the radiative transfer in detail, allowing the flux from the parent star to interact with all relevant depths of the planetary atmosphere, with no need for a pre-assumed albedo. We present a low-resolution optical and near-IR spectrum of a close-in EGP, focusing on the differences from an isolated planet. In our dust-free planetary atmospheres we find that Rayleigh scattering increases the EGP's flux by orders of magnitude shortward of the CaII H&K doublet (393 nm), and the spectral features of the parent star are exactly reflected. In the optical and near-IR the thermal absorption of the planet takes over, but the absorption features are changed by the irradiation. The inclusion of dust increases the reflected flux in the blue; the stellar spectral lines can be seen blueward of H-beta (486 nm).

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arXiv1997

DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries and Cepheids. II. Variables in the Field M31A

We undertook a long term project, DIRECT, to obtain the direct distances to two important galaxies in the cosmological distance ladder -- M31 and M33, using detached eclipsing binaries (DEBs) and Cepheids. While rare and difficult to detect, detached eclipsing binaries provide us with the potential to determine these distances with an accuracy better than 5%. The massive photometry obtained in order to detect DEBs provides us with the light curves for the Cepheid variables. These are essential to the parallel project to derive direct Baade-Wesselink distances to Cepheids in M31 and M33. For both Cepheids and eclipsing binaries the distance estimates will be free of any intermediate steps. As a first step of the DIRECT project, between September 1996 and January 1997 we have obtained 36 full nights on the Michigan-Dartmouth-MIT (MDM) 1.3-meter telescope and 45 full/partial nights on the F. L. Whipple Observatory (FLWO) 1.2-meter telescope to search for detached eclipsing binaries and new Cepheids in the M31 and the M33 galaxies. In this paper, second in the series, we present the catalog of variable stars, most of them newly detected, found in the field M31A ($α_{2000.0},δ_{2000}=11.34°,41.73°$). We have found 75 variable stars: 15 eclipsing binaries, 43 Cepheids and 17 other periodic, possible long period or non-periodic variables. The catalog of variables, as well as their photometry and finding charts, are available using the anonymous ftp service and the WWW. The CCD frames are available on request.

astro-ph↗
arXiv1997

DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries and Cepheids. I. Variables in the Field M31B

We undertook a long term project, DIRECT, to obtain the direct distances to two important galaxies in the cosmological distance ladder -- M31 and M33, using detached eclipsing binaries (DEBs) and Cepheids. While rare and difficult to detect, detached eclipsing binaries provide us with the potential to determine these distances with an accuracy better than 5%. The massive photometry obtained in order to detect DEBs provides us with good light curves for the Cepheid variables. These are essential to the parallel project to derive direct Baade-Wesselink distances to Cepheids in M31 and M33. For both Cepheids and eclipsing binaries the distance estimates will be free of any intermediate steps. As a first step of the DIRECT project, between September 1996 and January 1997 we have obtained 36 full nights on the Michigan-Dartmouth-MIT (MDM) 1.3-meter telescope and 45 full/partial nights on the F. L. Whipple Observatory (FLWO) 1.2-meter telescope to search for detached eclipsing binaries and new Cepheids in the M31 and the M33 galaxies. In this paper, first in the series, we present the catalog of variable stars, most of them newly detected, found in the field M31B ($α_{2000.0},δ_{2000}=11.20°,41.59°$). We have found 85 variable stars: 12 eclipsing binaries, 38 Cepheids and 35 other periodic, possible long period or non-periodic variables. The catalog of variables, as well as their photometry and finding charts, are available using the anonymous ftp service and the WWW.

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arXiv1997

EROS Variable Stars : Discovery of Beat Cepheids in the Small Magellanic Cloud and the effect of metallicity on pulsation

We report the discovery of eleven beat Cepheids in the Small Magellanic Cloud, using data obtained by the EROS microlensing survey. Four stars are beating in the fundamental and first overtone mode (F/1OT), seven are beating in the first and second overtone (1OT/2OT). The SMC F/1OT ratio is systematically higher than the LMC F/1OT, while the 1OT/2OT period ratio in the SMC Cepheids is the same as the LMC one.

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arXiv1996

The effect of metallicity on the Cepheid distance scale and its implications for the Hubble constant ($H_0$) determination

Recent HST determinations of the expansion's rate of the Universe (the Hubble constant, H_0) assumed that the Cepheid Period-Luminosity relation at V and I are independent of metallicity (Freedman, et al., 1996, Saha et al., 1996, Tanvir et al., 1995). The three groups obtain different vales for H_0. We note that most of this discrepancy stems from the asumption (by both groups) that the Period-Luminosity relation is independent of metallicity. We come to this conclusion as a result of our study of the Period-Luminosity relation of 481 Cepheids with 3 millions two colour measurements in the Large Magellanic Cloud and the Small Magellanic Cloud obtained as a by-product of the EROS microlensing survey. We find that the derived interstellar absorption corrections are particularly sensitive to the metallicity and when our result is applied to recent estimates based on HST Cepheids observations it makes the low-H_0 values higher and the high-H_0 value lower, bringing those discrepant estimates into agrement around $H_0 \approx 70 km/s Mpc^{-1}$.

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arXiv1996

Metallicity Effects on the Cepheid Extragalactic Distance Scale from EROS photometry in LMC and SMC

This is an investigation of the period-luminosity relation of classical Cepheids in samples of different metallicity. It is based on 481 Cepheids in the Large and Small Magellanic Clouds from the blue and red filter CCD observations (most similar to V_J & R_J) of the French EROS microlensing project. The data-set is complete and provides an excellent basis for a differential analysis between LMC and SMC. In comparison to previous studies of effects on the PL-relation, the EROS data-set offers extremely well-sampled light curves and well-filled instability strips. This allows reliable separation of Cepheids pulsating in the fundamental and the first overtone mode and derivation of differential reddening. Our main result concerns the determination of distances to galaxies which are inferred by using the LMC as a base and using two color photometry to establish the amount of reddening. We find a zero-point offset between SMC and LMC which amounts to a difference between inferred and true distance modulus of 0.14 \pm 0.06 mag in the VI_c system. The offset is exactly the same in both sets of PL-relations - of the fundamental and of the first overtone mode Cepheids. No effect is seen on the slopes of the PL-relations, although the fundamental and the first overtone mode Cepheids have different PL slopes. We attribute the color and the zero-point offset to the difference in metallicity between the SMC and LMC Cepheids. A metallicity effect of that small magnitude still has important consequencies for the inferred Cepheid distances and the determination of H_0. When applied to recent estimates based on HST Cepheid observations, our metallicity dependence makes the low-H_0 values (Sandage et al. 1994) higher and the high-H_0 values (Freedman et al. 1994b) lower, thus bringing those

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arXiv1996

EROS differential studies of Cepheids in the Magellanic Clouds : Stellar pulsation, stellar evolution and distance scale

We present a differential study of 500 Magellanic Cepheids with 3 million measurements obtained as a by-product of the EROS microlensing survey. The data-set is unbiased and provides an excellent basis for a differential analysis between LMC and SMC. We investigate the pulsational properties of the Cepheids as a function of metallicity and the constraints they give to stellar pulsation, stellar evolution theory and opacities. Moreover, our empirical analysis shows a small metallicity effect on the period-luminosity relation which has a significant implication to recent distance estimates based on HST observations of Cepheids in galaxies as far as the Virgo cluster. When the color shift of Cepheids due to metallicity is interpreted as reddening in the determination of the true distance modulus of a target galaxy a systematic error is made. Accounting for that error makes the low-H_0 values (Sandage et al.) higher and the high-H_0 (Freedman et al.) value lower, bringing those discrepant estimates into agrement around $H_0 \approx 70$ km/s Mpc^{-1}.

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