Astrometric Discovery of GJ 802b: In the Brown Dwarf Oasis?

The Stellar Planet Survey is an ongoing astrometric search for giant planets and brown dwarfs around a sample of about 30 M-dwarfs. We have discovered several low-mass companions by measuring the motion of our target stars relative to their reference frames. The lowest mass discovery thus far is GJ 802b, a companion to the M5-dwarf GJ 802A. The orbital period is 3.14 +/- 0.03 y, the system mass is 0.214 +/- 0.045 Msolar, and the semi-major axis is 1.28 +/- 0.10 AU or 81 +/- 6 mas. Imaging observations indicate that GJ 802b is likely to be a brown dwarf with the astrometrically determined mass 0.058 +/- 0.021 Msolar (one sigma limits). The remaining uncertainty in the orbit is the eccentricity that is now loosely constrained. We discuss how the system age limits the mass and the prospects to further narrow the mass range when e is more precisely determined.Comment: 13 pages, 6 figures, accepted for publication in ApJ on May 9, 200

GGD 27: X-rays from a Massive Protostar with an Outflow

We report the discovery of a cluster of Class I protostars in GGD 27. One of these protostars is the previously known, centrally located, GGD 27-ILL, which powers a massive bipolar outflow. We show that GGD 27-ILL, which is known to be the bright infrared (IR) source, IRAS 18162-2048, and a compact radio continuum source, is also the newly discovered hard X-ray source, GGD 27-X. The observations were made with the ACIS instrument on the Chandra X-ray Observatory. The X-rays from GGD 27-X are variable when compared with 4 years earlier, with an unabsorbed 2-10 keV X-ray luminosity in this observation of 1.5-12 × 10^31 erg s^–1 and a plasma temperature of ≥ 10^7 K. The X-rays are probably associated with the underlying B0 star (rather than outflowing material), providing a rare glimpse in hard X-rays of an optically obscured massive protostar with an outflow. The X-ray luminosity and spectrum appear to be consistent with stars of its type in other star formation regions. Several other variable X-ray sources are also detected in the IR cluster that contains GGD 27-X. We also discuss another nearby cluster. In each of the clusters there is an object that is X-ray hard, highly absorbed at low energies, in a blank optical/IR/radio field, and variable in X-ray intensity by a factor of ≥ 10 on a timescale of 4 years. These latter objects may arise from more recent episodes of star formation or may be "hidden" Class III sources

An Ultracool Star's Candidate Planet

We report here the discovery of the first planet around an ultracool dwarf star. It is also the first extrasolar giant planet (EGP) astrometrically discovered around a main-sequence star. The statistical significance of the detection is shown in two ways. First, there is a 2 x 10^-8 probability that the astrometric motion fits a parallax-and-proper-motion-only model. Second, periodogram analysis shows a false alarm probability of 3 x 10^-5 that the discovered period is randomly generated. The planetary mass is M2 = 6.4 (+2.6,-3.1) Jupiter-masses (MJ), and the orbital period is P = 0.744 (+0.013,-0.008) yr in the most likely model. In less likely models, companion masses that are higher than the 13 MJ planetary mass limit are ruled out by past radial velocity measurements unless the system radial velocity is more than twice the current upper limits and the near-periastron orbital phase was never observed. This new planetary system is remarkable, in part, because its star, VB 10, is near the lower mass limit for a star. Our astrometric observations provide a dynamical mass measurement and will in time allow us to confront the theoretical models of formation and evolution of such systems and their members. We thus add to the diversity of planetary systems and to the small number of known M-dwarf planets. Planets such as VB 10b could be the most numerous type of planets because M stars comprise >70% of all stars. To date they have remained hidden since the dominant radial-velocity (RV) planet-discovery technique is relatively insensitive to these dim, red systems.Comment: 1 30 page pdf file, 9 figures, accepted for publication in The Astrophysical Journa

X-rays from the Power Sources of the Cepheus A Star-Forming Region

We report an observation of X-ray emission from the exciting region of Cepheus A with the Chandra/ACIS instrument. What had been an unresolved X-ray source comprising the putative power sources is now resolved into at least 3 point-like sources, each with similar X-ray properties and differing radio and submillimeter properties. The sources are HW9, HW3c, and a new source that is undetected at other wavelengths "h10." They each have inferred X-ray luminosities >= 10^31 erg s^-1 with hard spectra, T >= 10^7 K, and high low-energy absorption equivalent to tens to as much as a hundred magnitudes of visual absorption. The star usually assumed to be the most massive and energetic, HW2, is not detected with an upper limit about 7 times lower than the detections. The X-rays may arise via thermal bremsstrahlung in diffuse emission regions associated with a gyrosynchrotron source for the radio emission, or they could arise from powerful stellar winds. We also analyzed the Spitzer/IRAC mid-IR observation from this star-formation region and present the X-ray results and mid-IR classifications of the nearby stars. HH 168 is not as underluminous in X-rays as previously reported.Comment: Accepted in the ApJ, 30 pages, 11 figures, in one .pdf fil

Astrometric Discovery of GJ 164B

We discovered a low-mass companion to the M-dwarf GJ 164 with the CCD-based imaging system of the Stellar Planet Survey (STEPS) astrometric program. The existence of GJ 164B was confirmed with Hubble Space Telescope NICMOS imaging observations. A high-dispersion spectral observation in V sets a lower limit of delta m> 2.2 mag between the two components of the system. Based upon our parallax value of 0.082 +/- 0.008, we derive the following orbital parameters: P = 2.04 +/- 0.03 y, a = 1.03 +/- 0.03 AU, and Mtotal = 0.265 +/- 0.020 MSun. The component masses are MA = 0.170 +/- 0.015 MSun and MB = 0.095 +/- 0.015 MSun. Based on its mass, colors, and spectral properties, GJ 164B has spectral type M6-8 V.Comment: pdf file 14 pages with 6 fig

Coronal X-ray emission from an intermediate-age brown dwarf

We report the X-ray detection of the brown dwarf (BD) companion TWA 5B in a $\simeq 12$ Myr old pre-main sequence binary system. We clearly resolve the faint companion (35 photons) separated from the X-ray luminous primary by 2 arcsec in a {\it Chandra} ACIS image. TWA 5B shows a soft X-ray spectrum with a low plasma temperature of only 0.3 keV and a constant flux during the 3 hour observation, of which the characteristics are commonly seen in the solar corona. The X-ray luminosity is 4$\times10^{27}$ erg s$^{-1}$ (0.1--10 keV band) or $\log L_X/L_{bol} = -3.4$. Comparing these properties to both younger and older BDs, we discuss the evolution of the X-ray emission in BDs. During their first few Myr, they exhibit high levels of X-ray activity as seen in higher mass pre-main sequence stars. The level in TWA 5B is still high at $t \simeq 12$ Myr in $\log L_X/L_{bol}$ while $kT$ has already substantially cooled

X-rays from Cepheus A East and West

We report the discovery of X-rays from both components of Cepheus A, East and West, with the XMM-Newton Observatory. HH 168 joins the ranks of other energetic Herbig Haro objects that are sources of T~10^6 K X-ray emission. The HH 168 effective temperature is T = 5.8 (+3.5,-2.3) x 10^6 K and its unabsorbed luminosity is 1.1 x 10^29 erg s^-1, making it hotter and less luminous than other representatives of its class. We also detect prominent X-ray emission from the complex of compact radio sources believed to be the power sources for Cep A. We call this source HWX and it is distinguished by its hard X-ray spectrum, T = 1.2 (+1.2,-0.5) x 10^8 K, and complex spatial distribution. It may arise from one or more protostars associated with the radio complex, the outflows, or a combination of the two. We detect 102 X-rays sources; many presumed to be pre-main sequence stars based upon the reddening of their optical/IR counterparts.Comment: 15 pages, 8 figures, data table not included because of size limit

The Carnegie Astrometric Planet Search Program

We are undertaking an astrometric search for gas giant planets and brown dwarfs orbiting nearby low mass dwarf stars with the 2.5-m du Pont telescope at the Las Campanas Observatory in Chile. We have built two specialized astrometric cameras, the Carnegie Astrometric Planet Search Cameras (CAPSCam-S and CAPSCam-N), using two Teledyne Hawaii-2RG HyViSI arrays, with the cameras' design having been optimized for high accuracy astrometry of M dwarf stars. We describe two independent CAPSCam data reduction approaches and present a detailed analysis of the observations to date of one of our target stars, NLTT 48256. Observations of NLTT 48256 taken since July 2007 with CAPSCam-S imply that astrometric accuracies of around 0.3 milliarcsec per hour are achievable, sufficient to detect a Jupiter-mass companion orbiting 1 AU from a late M dwarf 10 pc away with a signal-to-noise ratio of about 4. We plan to follow about 100 nearby (primarily within about 10 pc) low mass stars, principally late M, L, and T dwarfs, for 10 years or more, in order to detect very low mass companions with orbital periods long enough to permit the existence of habitable, Earth-like planets on shorter-period orbits. These stars are generally too faint and red to be included in ground-based Doppler planet surveys, which are often optimized for FGK dwarfs. The smaller masses of late M dwarfs also yield correspondingly larger astrometric signals for a given mass planet. Our search will help to determine whether gas giant planets form primarily by core accretion or by disk instability around late M dwarf stars.Comment: 48 pages, 9 figures. in press, Publ. Astron. Soc. Pacifi

Direct Detection of the Brown Dwarf GJ 802B with Adaptive Optics Masking Interferometry

We have used the Palomar 200" Adaptive Optics (AO) system to directly detect the astrometric brown dwarf GJ 802B reported by Pravdo et al. 2005. This observation is achieved with a novel combination of aperture masking interferometry and AO. The dynamical masses are 0.175$\pm$0.021 M$_\odot$ and 0.064$\pm$0.032 M$_\odot$ for the primary and secondary respectively. The inferred absolute H band magnitude of GJ 802B is M$_H$=12.8 resulting in a model-dependent T$_\mathrm{eff}$ of 1850 $\pm$ 50K and mass range of 0.057--0.074 M$_\odot$.Comment: 4 Pages, 5 figures, emulateapj format, submitted to ApJ

Masses of Astrometrically-Discovered and Imaged Binaries: G 78-28AB and GJ 231.1BC

The Stellar Planet Survey (STEPS) is an ongoing astrometric search for giant planets and brown dwarfs around a sample of ~30 M-dwarfs. We have discovered several low-mass companions by measuring the motion of our target stars relative to their reference frames. The highest mass discovery thus far is G 78-28B, a companion to the M-dwarf G 78-28A. The orbital period is 4.18 +/- 0.03 y, the system mass is 0.565 +/- 0.055 Msolar, and the semi-major axis is 2.19 +/- 0.10 AU. Imaging observations with the Keck laser guide star adaptive optics (LGSAO) and the Palomar AO instruments resolved the system and also yielded JHK-band delta magnitudes. We use the orbital solution, light ratios, and mass-luminosity relationships to derive component masses of MA = 0.370 +/- 0.034 Msolar and MB = 0.195 +/- 0.021 Msolar. G 78-28B is of type M4 V based upon its colors and mass. We also discovered GJ 231.1C, a companion to GJ 231.1B, with STEPS and imaged the companion with LGSAO and Palomar AO, but the orbital period is longer than our observing baseline; thus the system parameters are less constrained. In GJ 231.1BC the masses are MB = 0.25 +/- 0.06 Msolar and MC =0.12 +/- 0.02 Msolar. The inferred spectral type of GJ 231.1C is M5 V. We demonstrate the results of the current state of mass estimation techniques with our data.Comment: 25 pages, 8 figures, accepted for Ap