arXiv subjects

Debra A. Fischer

Publications and source records attributed to Debra A. Fischer.

154 records · Page 9Linked to original sources

arXiv2004

A Neptune-Mass Planet Orbiting the Nearby M Dwarf GJ 436

We report precise Doppler measurements of GJ 436 (M2.5V) obtained at Keck Observatory. The velocities reveal a planetary companion with orbital period of 2.644 d, eccentricity of 0.12 (consistent with zero) and velocity semi-amplitude of $K =18.1$ \ms. The minimum mass (\msini) for the planet is 0.067 \mjup = 1.2 M$_{\rm NEP}$ = 21 M$_{\rm EARTH}$, making it the lowest mass exoplanet yet found around a main sequence star and the first candidate in the Neptune mass domain. GJ 436 (Mass = 0.41 \msune) is only the second M dwarf found to harbor a planet, joining the two--planet system around GJ 876. The low mass of the planet raises questions about its constitution, with possible compositions of primarily H and He gas, ice/rock, or rock--dominated. The implied semi--major axis is $a$ = 0.028 AU = 14 stellar radii, raising issues of planet formation, migration, and tidal coupling with the star. GJ 436 is $>3$ Gyr old, based on both kinematic and chromospheric diagnostics. The star exhibits no photometric variability on the 2.644-day Doppler period to a limiting amplitude of 0.0004 mag, supporting the planetary interpretation of the Doppler periodicity. Photometric transits of the planet across the star are ruled out for gas giant compositions and are also unlikely for solid compositions. As the third closest known planetary system, GJ 436 warrants follow--up observations by high resolution optical and IR

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arXiv2004

Multiple Companions to HD 154857 and HD 160691

Precise Doppler measurements from the AAT/UCLES spectrometer reveal two companions to both HD 154857 and HD 160691. The inner companion to HD 154857 has a period of 398 d, an eccentricity of 0.51, and a minimum mass of 1.8 M_Jupiter An outer companion has a period much longer than 2 years and is currently detected only as a linear trend of 14 m/s per year. The inner companion to HD 160691, previously announced from AAT data, has a period of 645 d, an eccentricity of 0.20, and a minimum mass of 1.7 M_Jupiter. For the outer planet, whose orbit is less well constrained, a two Keplerian fit yields a period of 8.2 yr, an eccentricity of 0.57, and a minimum mass of 3.1 M_Jupiter. With these orbital parameters, its maximum separation from the star of 0.4 arcsec makes it a viable target for direct imaging.

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arXiv2004

Detection of a NEPTUNE-mass planet in the $ρ^{1}$ Cancri system using the Hobby-Eberly Telescope

We report the detection of the lowest mass extra-solar planet yet found around a Sun-like star - a planet with an \msini of only 14.21 $\pm$ 2.91 Earth masses in an extremely short period orbit (P=2.808 days) around $ρ^{1}$ Cancri, a planetary system which already has three known planets. Velocities taken from late 2003-2004 at McDonald Observatory with the Hobby-Eberly Telescope (HET) revealed this inner planet at 0.04 AU. We estimate an inclination of the outer planet $ρ^{1}$ Cancri d, based upon {\it Hubble Space Telescope} Fine Guidance Sensor (FGS) measurements. This inclination suggests an inner planet of only 17.7 $\pm$ 5.57 Earth masses, if coplanarity is assumed for the system.

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arXiv2003

A Planet in a Circular Orbit with a 6 Year Period

Precision Doppler velocity measurements from the 3.9--m Anglo-Australian Telescope reveal a planet with a 6 year period orbiting the G5 dwarf HD 70642. The a = 3.3 AU orbit has a low eccentricity (e = 0.1), and the minimum (\msini) mass of the planet is 2.0 \mjup. The host star is metal rich relative to the Sun, similar to most stars with known planets. The distant and approximately circular orbit of this planet makes it a member of a rare group to emerge from precision Doppler surveys.

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arXiv2003

Early-type Stars: Most Favorable Targets for Astrometrically Detectable Planets in the Habitable Zone

Early-type stars appear to be a difficult place to look for planets astrometrically. First, they are relatively heavy, and for fixed planetary mass the astrometric signal falls inversely as the stellar mass. Second, they are relatively rare (and so tend to be more distant), and for fixed orbital separation the astrometric signal falls inversely as the distance. Nevertheless, because early-type stars are relatively more luminous, their habitable zones are at larger semi-major axis. Since astrometric signal scales directly as orbital size, this gives early-type stars a strong advantage, which more than compensates for the other two factors. Using the Hipparcos catalog, we show that early-type stars constitute the majority of viable targets for astrometric searches for planets in the habitable zone. We contrast this characteristic to transit searches, which are primarily sensitive to habitable planets around late-type stars.

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arXiv2002

A Planet at 5 AU Around 55 Cancri

We report precise Doppler shift measurements of 55 Cancri (G8V) obtained from 1989 to 2002 at Lick Observatory. The velocities reveal evidence for an outer planetary companion to 55 Cancri orbiting at 5.5 AU. The velocities also confirm a second, inner planet at 0.11 AU. The outer planet is the first extrasolar planet found that orbits near or beyond the orbit of Jupiter. It was drawn from a sample of ~50 stars observed with sufficient duration and quality to detect a giant planet at 5 AU, implying that such planets are not rare. The properties of this jupiter analog may be compared directly to those of the Jovian planets in our Solar System. Its eccentricity is modest, e=0.16, compared with e=0.05 for both Jupiter and Saturn. Its mass is at least 4.0 jupiter masses (M sin i). The two planets do not perturb each other significantly. Moreover, a third planet of sub-Jupiter mass could easily survive in between these two known planets. Indeed a third periodicity remains in the velocity measurements with P = 44.3 d and a semi-amplitude of 13 m/s. This periodicity is caused either by a third planet at a=0.24 AU or by inhomogeneities on the stellar surface that rotates with period 42 d. The planet interpretation is more likely, as the stellar surface is quiet, exhibiting log(R'_{HK}) = -5.0 and brightness variations less than 1 millimag, and any hypothetical surface inhomogeneity would have to persist in longitude for 14 yr. Even with all three planets, an additional planet of terrestrial--mass could orbit stably at ~1 AU. The star 55 Cancri is apparently a normal, middle-aged main sequence star with a mass of 0.95 solar masses, rich in heavy elements ([Fe/H] = +0.27). This high metallicity raises the issue of the relationship between its age, rotation, and chromosphere.

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arXiv2002

Radial Velocities for 889 Late-type Stars

We report radial velocities for 844 FGKM-type main sequence and subgiant stars and 45 K giants, most of which had either low-precision velocity measurements or none at all. These velocities differ from the standard stars of Udry et al. by 0.035 km/s (RMS) for the 26 FGK standard stars in common. The zero-point of our velocities differs from that of Udry et al.: (V_present - V_Udry) = +0.053 km/s. Thus these new velocities agree with the best known standard stars both in precision and zero-point, to well within 0.1 km/s. Nonetheless, both these velocities and the standards suffer from three sources of systematic error, namely, convective blueshift, gravitational redshift, and spectral type mismatch of the reference spectrum. These systematic errors are here forced to be zero for G2V stars by using the Sun as reference, with Vesta and day sky as proxies. But for spectral types departing from solar, the systematic errors reach 0.3 km/s in the F and K stars and 0.4 km/s in M dwarfs. Multiple spectra were obtained for all 889 stars during four years, and 782 of them exhibit velocity scatter less than 0.1 km/s. These stars may serve as radial velocity standards if they remain constant in velocity. We found 11 new spectroscopic binaries and report orbital parameters for them.

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arXiv2002

Crossing the Brown Dwarf Desert Using Adaptive Optics: A Very Close L-Dwarf Companion to the Nearby Solar Analog HR 7672

We have found a very faint companion to the active solar analog HR 7672 (HD 190406; GJ 779; 15 Sge). Three epochs of high resolution imaging using adaptive optics (AO) at the Gemini-North and Keck II Telescopes demonstrate that HR 7672B is a common proper motion companion, with a separation of 0.79" (14 AU) and a 2.16 um flux ratio of 8.6 mags. Using follow-up K-band spectroscopy from Keck AO+NIRSPEC, we measure a spectral type of L4.5+/-1.5. This is the closest ultracool companion around a main sequence star found to date by direct imaging. We estimate the primary has an age of 1-3 Gyr. Assuming coevality, the companion is most likely substellar, with a mass of 55-78 Mjup based on theoretical models. The primary star shows a long-term radial velocity trend, and we combine the radial velocity data and AO imaging to set a firm (model-independent) lower limit of 48 Mjup. In contrast to the paucity of brown dwarf companions at <~4 AU around FGK dwarfs, HR 7672B implies that brown dwarf companions do exist at separations comparable to those of the giant planets in our own solar system. Its presence is at variance with scenarios where brown dwarfs form as ejected stellar embryos. Moreover, since HR 7672B is likely too massive to have formed in a circumstellar disk as planets are believed to, its discovery suggests that a diversity of physical processes act to populate the outer regions of exoplanetary systems.

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arXiv2001

Ten Low Mass Companions from the Keck Precision Velocity Survey

Ten new low mass companions have emerged from the Keck precision Doppler velocity survey, with minimum (msini) masses ranging from 0.8 mjup to 0.34 msun. Five of these are planet candidates with msini < 12 mjup, two are brown dwarf candidates with msini ~30 mjup, and three are low mass stellar companions. Hipparcos astrometry reveals the orbital inclinations and masses for three of the (more massive) companions, and it provides upper limits to the masses for the rest. A new class of extrasolar planet is emerging, characterized by nearly circular orbits and orbital radii greater than 1 AU. The planet HD 4208b appears to be a member of this new class. The mass distribution of extrasolar planets continues to exhibit a rapid rise from 10 mjup toward the lowest detectable masses near 1 msat.

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arXiv2000

The Very Low Mass Component of the Gliese 105 System

Multiple-epoch, multicolor images of the astrometric binary Gliese 105A and its very low mass companion Gliese 105C have been obtained using the Hubble Space Telescope's Wide Field Planetary Camera 2 (WFPC2) and Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). The optical and near-infrared colors of Gl 105C strongly suggest a spectral type of M7V for that star. Relative astrometric measurements spanning 3 yr reveal the first evidence of Gl 105C's orbital motion. Previous long-term astrometric studies at Sproul and McCormick Observatories have shown that the period of Gl 105A's perturbation is approximately 60 yr. To satisfy both the observed orbital motion and Gl 105A's astrometric period, Gl 105C's orbit must have an eccentricity of approximately 0.75 and a semimajor axis of approximately 15 AU. Measurements of Gl 105A's radial velocity over 12 yr show a linear trend with a slope of 11.3 m/s/yr, which is consistent with these orbital constraints and a nearly face-on orbit. As no other faint companions to Gl 105A have been detected, we conclude that Gl 105C is probably the source of the 60-yr astrometric perturbation.

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