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

Chandra observation of Cepheus A: The diffuse emission of HH 168 resolved

X-ray emission from massive stellar outflows has been detected in several cases. We present a Chandra observation of HH 168 and show that the soft X-ray emission from a plasma of 0.55 keV within HH 168 is diffuse. The X-ray emission is observed on two different scales: Three individual, yet extended, regions are embedded within a complex of low X-ray surface brightness. Compared to the bow shock the emission is displaced against the outflow direction. We show that there is no significant contribution from young stellar objects (YSOs) and discuss several shock scenarios that can produce the observed signatures. We establish that the X-ray emission of HH 168 is excited by internal shocks in contrast to simple models, which expect the bow shock to be the most X-ray luminous.Comment: 8 pages, 5 figures, accepted for publication in A&

K-alpha X-rays from cosmic ray oxygen

Equilibrium charge fractions are calculated for subrelativistic cosmic ray oxygen ions in the interstellar medium. These are used to determine the expected flux of K-alpha rays arising from atomic processes for a number of different postulated interstellar oxygen spectra. Relating these results to the diffuse X-ray background measured at the appropriate energy level suggests an observable line feature. If the flux of low energy cosmic ray oxygen is sufficiently large, K-alpha X-ray line emission from these nuclei will comprise a significant fraction of the total diffuse flux at approximately 0.6 keV. A satellite borne detector with a resolution greater than 30 percent could observe this feature if the subrelativistic interstellar cosmic ray oxygen spectrum is as large as certain theoretical estimates expressed in the text

X-ray spectra of the Crab pulsar and nebula

The spectrum of the Crab pulsar was measued from 2 to 50 keV as a function of pulse phase and a progressive hardening and subsequent softening of the spectrum across the pulse was found. The fraction of the pulsed flux which exhibits spectral variability is 0.14 and is concentrated solely in the region between the two peaks. A model is suggested in which the pulsed X-ray emission from the Crab pulsar consists of two components: one which has no spectral dependence with pulse phase and which is physically related to the double peaked gama ray pulse and, perhaps, the radio and optical pulses; and another component which exhibits spectral variability with pulse phase is confined to and comprises the interpeak emission, and which is only seen at X-ray energies. These results and studies of the binary X-ray pulsar Hercules X-1 suggest a phenomonological similarity. If the spectrally varying component in the Crab pulsar arises from a hot, magnetized plasma near the neutron star surface then higher energy spectral observations of this phase region might reveal spectral features which can be used to determine the surface field strength

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

Hercules X-1: Spectral Variability of an X-Ray Pulsar in a Stellar Binary System

A cosmic X-ray spectroscopy experiment onboard the Orbiting Solar Observatory 8 (OSO-8), observed Her x-1 continuously for approximately 8 days. Spectral-temporal correlations of the X-ray emission were obtained. The major results concern observations of: (1) iron band emission, (2) spectral hardening (increase in effective x-ray temperature) within the X-ray pulse, and (3) a transition from an X-ray low state to a high state. The spectrum obtained prior to the high state can be interpreted as reflected emission from a hot coronal gas surrounding an accretion disk, which itself shields the primary X-ray source from the line of sight during the low state. The spectral hardening within the X-ray pulse was indicative of the beaming mechanism at the neutron star surface. The hardest spectrum by pulse phase was identified with the line of sight close to the Her x-1 magnetic dipole axis, and the X-ray pencil beam become harder with decreasing angle between the line of sight and the dipole axis

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

Discovery of a Peculiar Dip from GX 301-2

We present temporal and spectral properties of a unique X-ray dip in GX 301-2 as seen with Rossi X-ray Timing Explorer in May 2010. The X-ray pulsation from the source gradually declined prior to the dip, disappears for one spin cycle during the dip and is abruptly restored in the spin cycle immediately after the dip. Moreover, the phase-integrated spectrum of the source becomes softer before and during the dip and it quickly hardens again following the dip. Our findings indicate the fact that the mechanism for pulsations gradually turned off briefly and underlying dim and softer emission likely from the accretion column became observable in the brief absence of high level emission due to wind accretion.Comment: Accepted for publication in A&A Letter

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

On the Completeness of Reflex Astrometry on Extrasolar Planets near the Sensitivity Limit

We provide a preliminary estimate of the performance of reflex astrometry on Earth-like planets in the habitable zones of nearby stars. In Monte Carlo experiments, we analyze large samples of astrometric data sets with low to moderate signal-to-noise ratios. We treat the idealized case of a single planet orbiting a single star, and assume there are no non-Keplerian complications or uncertainties. The real case can only be more difficult. We use periodograms for discovery and least-squares fits for estimating the Keplerian parameters. We find a completeness for detection compatible with estimates in the literature. We find mass estimation by least squares to be biased, as has been found for noisy radial-velocity data sets; this bias degrades the completeness of accurate mass estimation. When we compare the true planetary position with the position predicted from the fitted orbital parameters, at future times, we find low completeness for an accuracy goal of 0.3 times the semimajor axis of the planet, even with no delay following the end of astrometric observations. Our findings suggest that the recommendation of the ExoPlanet Task Force (Lunine et al. 2008) for "the capability to measure convincingly wobble semi-amplitudes down to 0.2 $\mu$as integrated over the mission lifetime," may not be satisfied by an instrument characterized by the noise floor of the Space Interferometry Mission, $\sigma_\mathrm{floor}\approx0.035\mu$as. An important, unsolved, strategic challenge for the exoplanetary science program is figuring out how to predict the future position of an Earth-like planet with accuracy sufficient to ensure the efficiency and success of the science operations for follow-on spectroscopy, which would search for biologically significant molecules in the atmosphere.Comment: v2: 16 pages, 4 figures; ApJ accepte