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Observations of metal absorption systems in the spectra of distant quasars allow to constrain a possible variation of the fine-structure constant throughout the history of the Universe. Such a test poses utmost demands on the wavelength accuracy and previous studies were limited by systematics in the spectrograph wavelength calibration. A substantial advance in the field is therefore expected from the new ultra-stable high-resolution spectrograph Espresso, recently installed at the VLT. In preparation of the fundamental physics related part of the Espresso GTO program, we present a thorough assessment of the Espresso wavelength accuracy and identify possible systematics at each of the different steps involved in the wavelength calibration process. Most importantly, we compare the default wavelength solution, based on the combination of Thorium-Argon arc lamp spectra and a Fabry-P\'erot interferometer, to the fully independent calibration obtained from a laser frequency comb. We find wavelength-dependent discrepancies of up to 24m/s. This substantially exceeds the photon noise and highlights the presence of different sources of systematics, which we characterize in detail as part of this study. Nevertheless, our study demonstrates the outstanding accuracy of Espresso with respect to previously used spectrographs and we show that constraints of a relative change of the fine-structure constant at the $10^{-6}$ level can be obtained with Espresso without being limited by wavelength calibration systematics., Comment: 27 pages, accepted for publication in A&A

Ultra-hot giant exoplanets receive thousands of times Earth's insolation. Their high-temperature atmospheres (>2,000 K) are ideal laboratories for studying extreme planetary climates and chemistry. Daysides are predicted to be cloud-free, dominated by atomic species and substantially hotter than nightsides. Atoms are expected to recombine into molecules over the nightside, resulting in different day-night chemistry. While metallic elements and a large temperature contrast have been observed, no chemical gradient has been measured across the surface of such an exoplanet. Different atmospheric chemistry between the day-to-night ("evening") and night-to-day ("morning") terminators could, however, be revealed as an asymmetric absorption signature during transit. Here, we report the detection of an asymmetric atmospheric signature in the ultra-hot exoplanet WASP-76b. We spectrally and temporally resolve this signature thanks to the combination of high-dispersion spectroscopy with a large photon-collecting area. The absorption signal, attributed to neutral iron, is blueshifted by -11+/-0.7 km s-1 on the trailing limb, which can be explained by a combination of planetary rotation and wind blowing from the hot dayside. In contrast, no signal arises from the nightside close to the morning terminator, showing that atomic iron is not absorbing starlight there. Iron must thus condense during its journey across the nightside., Comment: Published in Nature (Accepted on 24 January 2020.) 33 pages, 11 figures, 3 tables

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We combine the available astrometric and photometric data for the 1993 microlensing event MACHO-LMC-5 to measure the mass of the lens, M=0.097 +/- 0.016 Msun. This is the most precise direct mass measurement of a single star other than the Sun. In principle, the measurement error could be reduced as low as 10% by improving the trig parallax measurement using, for example, the Space Interferometry Mission. Further improvements might be possible by rereducing the original photometric lightcurve using image subtraction or by obtaining new, higher-precision baseline photometry of the source. We show that the current data strongly limit scenarios in which the lens is a dark (i.e., brown-dwarf) companion to the observed M dwarf rather than being the M dwarf itself. These results set the stage for a confrontation between mass estimates of the M dwarf obtained from spectroscopic and photometric measurements and a mass measurement derived directly from the star's gravitational influence. This would be the first such confrontation for any isolated star other than the Sun., Comment: 20 pages inc 3 fig, submitted to ApJ

Recently obtained kinematic data has shown that the Large Magellanic Cloud (LMC) possesses an old stellar halo. In order to further characterize the properties of this halo, parametric King models are fit to the surface density of RR Lyrae stars. Using data from both the MACHO and OGLE II microlensing surveys, the model fits yield the center of their distribution at RA = 05:21.1+-0.8, Dec = -69:45+-6 (J2000) and a core radius of 1.42+-0.12 kpc. As a check the halo model is compared with RR Lyrae star counts in fields near the LMC's periphery previously surveyed with photographic plates. These data, however, require a cautious interpretation. Several topics regarding the LMC stellar halo are discussed. First, the properties of the halo imply a global mass-to-light ratio of M/L_V = 5.3+-2.1 and a total mass of 1.6+-0.6 10^10 M_sun for the LMC in good agreement with estimates based on the rotation curve. Second, although the LMC's disk and halo are kinematically distinct, the shape of the surface density profile of the halo is remarkably similar to that of the young disk. For example, the best-fit exponential scale length for the RR Lyrae stars is 1.47+-0.08 kpc, which compares to 1.46 kpc for the LMC's blue light. In the Galaxy, the halo and disk do not resemble each other like this. Finally, a local maximum in the LMC's microlensing optical depth due to halo-on-disk stellar self-lensing is predicted. For the parameters of the stellar halo obtained, this maximum is located near MACHO events LMC-4 and LMC-23, and is large enough to possibly account for these two events, but not for all of the observed microlensing., Comment: 11 pages, 1 figure, accepted to ApJ Letters

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The average of 14 recent measurements of the distance to the Large Magellanic Cloud (LMC) implies a true modulus of 18.50 +- 0.02 mag, and demonstrates a trend in the past 2 years of convergence toward a standard value. The distance indicators reviewed are the red clump, the tip of the red giant branch, Cepheid, RR Lyrae, and Mira variable stars, cluster main-sequence fitting, supernova 1987A, and eclipsing binaries. The eclipsing binaries yield a consistent distance on average; however, the internal scatter is twice as large as the average measurement error. I discuss parameters of LMC structure that pertain to distance indicators, and speculate that warps discovered using the color of the clump are not really warps., Comment: ver. 2, updated refs., 10 + 2 pages, now incl. Appendix w/ long-form abstract for "Highlights of Astronomy, Vol. 13," ed. O. Engvold after review paper for Proc. of JD 13 on "Extragalactic Binaries," eds. A. Gimenez and I. Ribas, invited talk from IAU XXV General Assembly

The oldest and most metal-poor Milky Way stars form a kinematically hot halo, which motivates the two major formation scenarios for our galaxy: extended hierarchical accretion and rapid collapse. RR Lyrae stars are excellent tracers of old and metal-poor populations. We measure the kinematics of 43 RR Lyrae stars in the inner regions of the nearby Large Magellanic Cloud (LMC) galaxy. The velocity dispersion, \sigma_{true}=53\pm10 km/s, indicates that a kinematically hot metal-poor old halo also exists in the LMC. This suggests that our galaxy and smaller late-type galaxies like the LMC have similar early formation histories., Comment: 8 pages, 2 figures; to be published in Science on Sept. 12, 2003

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This paper has been withdrawn by the author., Comment: Withdrawn

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The Hipparcos I-band calibration of horizontal-branch red clump giants as standard candles has lead to controversial results for the distance to the Large Magellanic Cloud (LMC). In an attempt to properly ascertain the corrections for interstellar extinction and clump age and metallicity, we analyze new multi-wavelength luminosity functions of the LMC red clump. Our photometry dataset in the K-band was obtained with the SOFI infrared imager at the European Southern Observatory's New Technology Telescope. In the V and I passbands, we employ data from WFPC2 onboard the Hubble Space Telescope. The LMC red clump is first identified in a K,(V-K) color-magnitude diagram. Our luminosity functions yield apparent magnitudes of K = 16.974, I = 18.206, and V = 19.233 (+- 0.009_r +- 0.02_s; random and systematic error, respectively). Compared directly to the Hipparcos red clump calibration (without a correction for age and metallicity), the LMC clump measurements imply a negative interstellar reddening correction. This unphysical result indicates a population difference between clumps. A modified calibration based on theoretical modeling yields an average reddening correction of E(B-V) = 0.089 +- 0.015_r, and a true LMC distance modulus of 18.493 +- 0.033_r +- 0.03_s. We reconcile our result with the short distance previously derived from OGLE II red clump data., Comment: To appear in The Astrophysical Journal, Letters

We derive general expressions for the LMC velocity field which we fit to kinematical data for 1041 carbon stars. We demonstrate that all previous studies of LMC kinematics have made unnecessary over-simplifications that have led to incorrect estimates of important structural parameters. We compile and improve LMC proper motion estimates to support our analysis. We find that the kinematically determined position angle of the line of nodes is 129.9 +/- 6.0 deg. The LMC inclination changes at a rate di/dt = -103 +/- 61 deg/Gyr, a result of precession and nutation induced by Milky Way tidal torques. The LMC rotation curve V(R) has amplitude 49.8 +/- 15.9 km/s, 40% lower than what has previously (and incorrectly) been inferred from e.g. HI. The dynamical center of the carbon stars is consistent with the center of the bar and the center of the outer isophotes, but not with the HI kinematical center. The enclosed mass inside 8.9 kpc is (8.7 +/- 4.3) x 10^9 M_sun, more than half of which is due to a dark halo. The LMC has a larger vertical thickness than has traditionally been believed. Its V/sigma is less than the value for the Milky Way thick disk. We discuss the implications for the LMC self-lensing optical depth. We determine the LMC velocity and orbit in the Galactocentric rest frame and find it to be consistent with the range of velocities that has been predicted by models for the Magellanic Stream. The Milky Way dark halo must have mass >4.3 x 10^{11} M_sun and extent >39 kpc for the LMC to be bound. We predict the LMC proper motion velocity field, and discuss techniques for kinematical distance estimation. [ABRIDGED], Comment: 57 pages, LaTeX, with 11 PostScript figures. Submitted to the Astronomical Journal

We discuss three aspects of post-AGB (PAGB) stars in old populations. (1) HST photometry of the nucleus of the planetary nebula (PN) K 648 in the globular cluster (GC) M15 implies a mass of 0.60 Msun, in contrast to the mean masses of white dwarfs in GCs of ~0.5 Msun. This suggests that K 648 is descended from a merged binary, and we infer that single Pop II stars do not produce visible PNe. (2) Yellow PAGB stars are the visually brightest stars in old populations (Mv ~ -3.3) and are easily recognizable because of their large Balmer jumps; thus they show great promise as a Pop II standard candle. Two yellow PAGB stars in the GC NGC 5986 have the same V magnitudes to within +/-0.05 mag, supporting an expected narrow luminosity function. (3) Using CCD photometry and a u filter lying below the Balmer jump, we have detected yellow PAGB stars in the halo of M31 and in its dwarf elliptical companion NGC 205. With the Milky Way zero point, we reproduce the Cepheid distance to M31, and find that NGC 205 is ~100 kpc further away than M31. The star counts imply a yellow PAGB lifetime of about 25,000 yr, and their luminosities imply masses near 0.53 Msun., Comment: 6 pages, 2 figures. To appear in proceedings of Torun, Poland, workshop on "Post-AGB Objects (Proto-Planetary Nebulae) as a Phase of Stellar Evolution," ed. S.K. Gorny

This paper has been withdrawn, Comment: This paper has been withdrawn by the author

We have obtained observations of the planetary nebula K648 in the Galactic globular cluster M15 with the Hubble Space Telescope's WFPC2 camera, covering an interval of 7 days. The frames provide both time-sampled broad-band photometry of the central star and high-resolution images of the nebula in the light of H-alpha, [OIII], and [NII]. In the deep narrow-band images, K648 is a fairly typical double-shelled elliptical, but with a bright arc at one end of the major axis that is especially prominent in [NII]; this feature is probably a collection of FLIERs. The nebula is surrounded by a faint, smooth elliptical halo, which appears undisturbed by any interaction with the interstellar medium. Adopting Teff = 40,000 +- 3,000 K based upon published spectral-line analyses, and employing our new broad-band optical flux data along with the known cluster distance, we find log L/L_sun = 3.78 +- 0.08 for the K648 central star. Theoretical post-asymptotic-giant-branch evolutionary tracks imply a mass of 0.60 +- 0.02 M_sun for this luminosity, which is significantly higher than the mean mass of white dwarfs in globular clusters and the halo field (0.50 +- 0.02). The K648 central star exhibits no significant photometric variability in our data; thus we find no direct evidence of a close binary companion. We suggest that the progenitor of K648 experienced mass augmentation in a close-binary merger, allowing it to evolve to a remnant of higher mass than those of the single stars in the cluster., Comment: Accepted for publication in The Astronomical Journal (23 pages, includes 3 figures, one color image)

The rotation of the disk of the Large Magellanic Cloud (LMC) is derived from the radial velocities of 422 carbon stars. New aspects of this analysis include the propagation of uncertainties in the LMC proper motion with a Monte Carlo, and a self-consistent modeling of the rotation curve and disk kinematics. The rotation curve is well fit by a truncated, finite-thickness exponential disk model with no dark halo. The velocity dispersion in radial bins decreases from 22 to 15 km/s, then increases to 20 km/s at larger radii. Constant-thickness models in virial equilibrium cannot be reconciled with the data even if the effects of LMC or Galactic dark halos are included. If the disk is virialized, the LMC disk is flared. We model the velocity dispersion at large radii (R > 6 kpc) as a maximal flared disk under the influence of the Galactic dark halo, which favors a mean density for the latter of 0.00025 M_sol/pc^3 at the LMC distance. LMC stellar kinematics play an important role in elucidating the nature of MACHOs, a dark population inferred from LMC microlensing. We have constructed a truncated and flared maximal disk model for the LMC which is kinematically based. Our model does not include a nonvirialized component such as tidal debris. Our upper limit on the self-lensing optical depth is in good agreement with that obtained from less sophisticated models, and is an order of magnitude too small to account for the MACHO microlensing signal [Abridged]., Comment: Accepted for publication in The Astrophysical Journal, 40 pages, 9 figures

The average near-infrared (K-band) luminosity of 238 Hipparcos red clump giants is derived and then used to measure the distance to the Galactic center. These Hipparcos red clump giants have been previously employed as I-band standard candles. The advantage of the K-band is a decreased sensitivity to reddening, and perhaps also a reduced systematic dependence on metallicity. In order to investigate the latter, and also to refer our calibration to a known metallicity zero-point, we restrict our sample of red clump calibrators to those with abundances derived from high-resolution spectroscopic data. The mean metallicity of the sample is [Fe/H] = -0.18 dex. The data are consistent with no correlation between M_K and [Fe/H], and only weakly constrain the slope of this relation. The luminosity function of the sample peaks at M_K = -1.61 +- 0.03 mag. Next, we assemble published optical and near-infrared photometry for about 20 red clump giants in a Baade's Window field with a mean metallicity nearly identical to that of the Hipparcos red clump. We derive the distance to the Galactic center: R = 8.24 +- 0.42 kpc. The uncertainty in this distance measurement is dominated by the small number of Baade's Window red clump giants examined here [abridged]., Comment: Accepted to The Astrophysical Journal, 28 pages, 7 figures, 1 table

Color-magnitude diagrams of globular clusters often exhibit a prominent horizontal branch (HB) and may also show features such as the red giant branch (RGB) bump and the asymptotic giant branch (AGB) bump. Stellar evolution theory predicts that the luminosities of these features will depend on the metallicity and age of the cluster. We calculate theoretical lines of 2 to 12 Gyr constant age RGB-bumps and AGB-bumps in the V(HB-Bump)--[Fe/H] diagram, which shows the brightness difference between the bump and the HB as a function of metallicity. In order to test the predictions, we identify giant branch bumps in new Hubble Space Telescope color-magnitude diagrams for 8 SMC clusters. First, we conclude that the SMC cluster bumps are RGB-bumps. The data for clusters younger than ~6 Gyr are in fair agreement the relative age dependent luminosities of the HB and RGB-bump. The V(HB-Bump)--[Fe/H] data for clusters older then ~6 Gyr demonstrate a less satisfactory agreement with our calculations. We conclude that ~6 Gyr is a lower bound to the age of clusters for which the Galactic globular cluster, age independent V(HB-Bump)--[Fe/H] calibration is valid. Application of the V(HB-bump)--[Fe/H] diagram to stellar population studies is discussed., Comment: Accepted for publication in the Astrophysical Journal, 30 pages, Latex aaspp4.sty, including 7 postscript figures

We have identified 4 Cepheids and 5 Miras using KPNO 4m BVRI images of an outer field in M101. The Cepheid and Mira periods range from 30 to 60 days and 350 to 800 days, respectively. We derive independent Cepheid and Mira distance moduli that agree within experimental uncertainties. We find a true distance modulus of 29.08 +- 0.13 mag., Comment: 28 pages, compressed uuencoded file contains 2 .ps files; finding chart not included, available via anonymous ftp at ftp://igpp.llnl.gov/pub/alves/m101_fig3.ps ; accepted for publication in AJ