Sabine Frink

The Distance to SN 1999em in NGC 1637 from the Expanding Photosphere Method

ABSTRACT We present 30 optical spectra and 49 photometric epochs sampling the first 517 days after discovery of supernova (SN) 1999em and derive its distance through the expanding photosphere method (EPM). SN 1999em is shown to be a Type II‐plateau (II‐P) event, with a photometric plateau lasting until about 100 days after explosion. We identify the dominant ions responsible for most of the absorption features seen in the optical portion of the spectrum during the plateau phase. Using the weakest unblended absorption features to estimate photospheric velocity, we find the distance to SN 1999em to be \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfon...

Discovery of a Substellar Companion to the K2 III Giant ɩ Draconis

We report precise radial velocity measurements of the K giant ι Dra (HD 137759, HR 5744, HIP 75458), carried out at Lick Observatory, which reveal the presence of a substellar companion orbiting the primary star. A Keplerian fit to the data yields an orbital period of about 536 days and an eccentricity of 0.70. Assuming a mass of 1.05 M☉ for ι Dra, the mass function implies a minimum companion mass m2 sin i of 8.9 MJ, making it a planet candidate. The corresponding semimajor axis is 1.3 AU. The nondetection of the orbital motion by Hipparcos allows us to place an upper limit of 45 MJ on the companion mass, establishing the substellar nature of the object. We estimate that transits in this system could occur already for inclinations as low as 815, as a result of the large diameter of the giant star. The companion to ι Dra is the first brown dwarf or planet found to orbit a giant rather than a main-sequence star.

Planetary Companions to HD 12661, HD 92788, and HD 38529 and Variations in Keplerian Residuals of Extrasolar Planets*

Precision Doppler observations at the Lick and Keck observatories have revealed Keplerian velocity variations in the stars HD 12661, HD 92788, and HD 38529. HD 12661 (G6 V) has an orbital period of 252.7 ± 2.7 days, velocity semiamplitude K = 88.4 ± 2.0 m s-1, and orbital eccentricity e = 0.23 ± 0.024. Adopting a stellar mass of 1.07 M☉, we infer a companion mass of M sin i = 2.79 MJ and a semimajor axis of a = 0.79 AU. HD 92788 (G5 V) has an orbital period of 326.7 ± 3.2 days, velocity semiamplitude K = 99.9 ± 2.4, and orbital eccentricity e = 0.30 ± 0.06. The adopted stellar mass of 1.06 M☉ yields a companion mass of M sin i = 3.34 MJ and a semimajor axis of a = 0.95 AU. HD 38529 (G4 IV) has an orbital period of 14.3 ± 0.8 days, velocity semiamplitude K = 53.8 ± 2.0 m s-1, and eccentricity e = 0.27 ± 0.03. The stellar mass of 1.4 M☉ sets M sin i = 0.77 MJ, with a semimajor axis of a = 0.13 AU for this companion. In addition to the 14.3 day periodicity, the velocity residuals for HD 38529 show curvature over the three years of observations. Based on a measurement of Ca II H and K emission, all three stars are chromospherically inactive. Based on both spectral synthesis modeling and narrowband photometry, HD 12661, HD 92788, and HD 38529 all appear to be metal-rich stars, reinforcing the correlation of high metallicity in the host stars of gas giant extrasolar planets. We examine the velocity residuals to the Keplerian fits for a subsample of 12 planet-bearing stars that have been observed longer than two years at the Lick Observatory. Five of the 12 (Ups Andromedae, τ Boo, 55 Cnc, HD 217107, and HD 38529) exhibit coherent variations in the residual velocities that are consistent with additional companions. Except for Upsilon Andromedae, the source of the velocity variation remains speculative pending completion of one full orbit. GJ 876 exhibits residual velocities with high rms scatter (24 m s-1), lacking identifiable coherence. The residual velocities for six of the 12 stars (51 Peg, 70 Vir, 16 Cyg B, ρ CrB, 47 UMa, and HD 195019) exhibit rms velocity scatter of ~7 m s-1, consistent with errors. The residual velocity trends suggest that known planet-bearing stars appear to harbor a distant (>3 AU) detectable companion more often than other stars in our planet survey.

Search for young stars among ROSAT All-Sky Survey X-ray sources in and around the R CrA dark cloud ⋆

We present the ROSAT All-Sky Survey data in a 126 deg 2 area in and around the CrA star forming region. With low-resolution spectroscopy of unidentied ROSAT sources we could nd 19 new pre-main sequence stars, two of which are classical T Tauri stars, the others being weak-lined. The spectral types of these new T Tauri stars range from F7 to M6. The two new classical T Tauri stars are located towards two small cloud-lets outside of the main CrA cloud. They appear to be 10 Myrs old, by comparing their location in the H R diagram with isochrones for an assumed distance of 130 pc, the distance of the main CrA dark cloud. The new o-cloud weak-line T Tauri stars may have formed in similar cloudlets, which have dispersed re- cently. High-resolution spectra of our new T Tauri stars show that they have signicantly more lithium absorption than zero-age main-sequence stars of the same spectral type, so that they are indeed young. From those spectra we also obtained rotational and radial velocities. For some stars we found the proper motion in published catalogs. The direction and velocity of the 3D space motion { south relative to the galatic plane { of the CrA T Tauri stars is consistent with the dark cloud being formed originally by a high-velocity cloud impact onto the galactic plane, which triggered Send oprint requests to :R. Neuh¨ ? Partly based on observations collected at the 1.52 m and 3.5 m telescopes of the European Southern Observatory, Chile, in programs 55.E-0549, 57.E-0646, and 63.L-0023, and on ob- servations collected at the 0.9 m, 1.5 m, and 4.0 m CTIO telescope. the star formation in CrA. We also present VRIJHK photometry for most of the new T Tauri stars to derive their luminosities, ages, and masses.

Timescales of Disk Evolution and Planet Formation: HST, Adaptive Optics, and ISO Observations of Weak-Line and Post-T Tauri Stars*

We present high spatial resolution HST and ground-based adaptive optics observations and high-sensitivity ISO (ISOCAM & ISOPHOT) observations of a sample of X-ray selected weak-line (WTTS) and post? (PTTS) T Tauri stars located in the nearby Chamaeleon T and Scorpius-Centaurus OB associations. HST/NICMOS and adaptive optics observations aimed at identifying substellar companions (young brown dwarfs) at separations ?30 AU from the primary stars. No such objects were found within 300 AU of any of the target stars, and a number of faint objects at larger separations can very likely be attributed to a population of field (background) stars. ISOCAM observations of 5 to 15 Myr old WTTSs and PTTSs in ScoCen reveal infrared excesses which are clearly above photospheric levels and which have a spectral index intermediate between that of younger (1 to 5 Myr) T Tauri stars in Chamaeleon and that of pure stellar photospheres. The difference in the spectral index of the older PTTSs in ScoCen compared with the younger classical and weak-line TTSs in Cha can be attributed to a deficiency of smaller size (0.1 to 1 ?m) dust grains relative to larger size (?5 ?m) dust grains in the disks of the PTTSs. The lack of small dust grains is either due to the environment (effect of nearby O stars and supernova explosions) or due to disk evolution. If the latter is the case, it would hint that circumstellar disks start to become dust depleted at an age between 5 to 15 Myr. Dust depletion is very likely related to the build-up of larger particles (ultimately rocks and planetesimals) and thus an indicator for the onset of the period of planet formation.

A Strategy for Identifying the Grid Stars for the Space Interferometry Mission

We present a strategy to identify several thousand stars that are astrometrically stable at the microarcsecond level for use in the Space Interferometry Mission (SIM) astrometric grid. The requirements on the grid stars make this a rather challenging task. Taking a variety of considerations into account, we argue for K giants as the best type of stars for the grid, mainly because they can be located at much larger distances than any other type of star owing to their intrinsic brightness. We show that it is possible to identify suitable candidate grid K giants from existing astrometric catalogs. However, double stars have to be eliminated from these candidate grid samples, since they generally produce much larger astrometric jitter than tolerable for the grid. The most efficient way to achieve this is probably by means of a radial velocity survey. To demonstrate the feasibility of this approach, we repeatedly measured the radial velocities for a preselected sample of 86 nearby Hipparcos K giants with precisions of 5-8 m s–1. The distribution of the intrinsic radial velocity variations for the bona fide single K giants shows a maximum around 20 m s–1, which is small enough not to severely affect the identification of stellar companions around other K giants. We use the results of our observations as input parameters for Monte Carlo simulations on the possible design of a radial velocity survey of all grid stars. Our favored scenario would result in a grid which consists to 68% of true single stars and to 32% of double or multiple stars with periods mostly larger than 200 years, but only 3.6% of all grid stars would display astrometric jitter larger than 1 μas. This contamination level is probably tolerable.

The PRIMA Astrometric Planet Search project

The PRIMA facility will implement dual-star astrometry at the VLTI. We have formed a consortium that will build the PRIMA differential delay lines, develop an astrometric operation and calibration plan, and deliver astrometric data reduction software. This will enable astrometric planet surveys with a target precision of 10μas. Our scientific goals include determining orbital inclinations and masses for planets already known from radial-velocity surveys, searches for planets around stars that are not amenable to high-precision radial-velocity observations, and a search for large rocky planets around nearby low-mass stars.

Preparing the PRIMA astrometric planet search: selecting suitable target and reference stars

ESOs PRIMA (Phase-Referenced Imaging and Micro-arcsecond Astrometry) facility at the VLT Interferometer on Cerro Paranal in Chile is expected to be fully operational in only a few years from now. With PRIMA/VLTI, it will then be possible to perform relative astrometry with an accuracy of the order of 10 microarcseconds over angles of about 10 arcseconds. The main science driver for this astrometric capability is a systematic search for extrasolar planets around nearby stars. Target stars as well as reference stars for this astrometric planet search have to be very carefully chosen in order to make the measurements robust and effective. Most importantly, reference stars have to be astrometrically stable to only a few microarcseconds in order to provide a suitable reference for the astrometric measurements. Target stars should be located at small distances so that a possible planet would cause a detectable astrometric signal. Moreover, a suitable target star and a suitable reference star have to be found within about 10 arcseconds of each other to ensure the highest accuracy and effectiveness, which obviously requires some trade-off in the final target list. Possible strategies and preparatory observations for the assembly of a suitable target list for the astrometric planet search with PRIMA/VLTI will be discussed.

K giants as astrometric reference stars for the Space Interferometry Mission

Wide angle astrometry with the Space Interferometry Mission needs a set of several thousand grid stars distributed uniformly over the sky. The requirements for candidate grid stars are quite stringent: the photocenters of these stars have to be astrometrically stable to within a few microarcseconds, which makes binary stars unacceptable as grid stars. We search the most precise and comprehensive astrometric catalogs available today--the Hipparcos, Tycho-1 and Tycho-2 Catalogues--for possible grid stars, and discuss the properties of samples of K giants derived from these catalogs. Furthermore, we present results of a precise radial velocity study of a small proxy sample of Hipparcos K giants. We demonstrate that it is possible to find K giants with radial velocity variations smaller than a few tens of meters per second on timescales of several months. It is thus possible to detect stellar companions in samples of candidate grid stars by means of a radial velocity survey. We discuss the results of Monte Carlo simulations that address the consequences of the measurement accuracy of a radial velocity survey on the portion of undetected binary systems.