S. H. Pravdo

Magnetic Flaring in the Pre-Main-Sequence Sun and Implications for the Early Solar System

To address the role of energetic processes in the solar nebula, we provide a detailed characterization of magnetic flaring in stellar analogs of the pre-main-sequence Sun based on two 0.5 day observations of the Orion Nebula cluster obtained with the Chandra X-Ray Observatory. The sample consists of 43 stars with masses between 0.7 and 1.4 M☉ and ages from less than 0.3 to 10 Myr. We find that the X-ray luminosities measured in the 0.5-8 keV band are strongly elevated over main-sequence levels with an average = 30.3 ergs s-1 and = -3.9. The X-ray emission is strongly variable within our exposures in nearly all solar analogs; about 30 flares with 29.0 ergs s-1 < log LX(peak) < 31.5 ergs s-1 on timescales from 0.5 to more than 12 hr are seen during the Chandra observations. Analogs of the ≤1 Myr old pre-main-sequence Sun exhibited X-ray flares that are 101.5 times more powerful and 102.5 times more frequent than the most powerful flares seen on the contemporary Sun. Radio observations indicate that acceleration of particles to relativistic energies is efficient in young stellar flares. Extrapolating the solar relationship between X-ray luminosity and proton fluence, we infer that the young Sun exhibited a 105-fold enhancement in energetic protons compared to contemporary levels. Unless the flare geometries are unfavorable, this inferred proton flux on the disk is sufficient to produce the observed meteoritic abundances of several important short-lived radioactive isotopes. Our study thus strengthens the astronomical foundation for local proton spallation models of isotopic anomalies in carbonaceous chondritic meteorites. The radiation, particles, and shocks produced by the magnetic reconnection flares seen with Chandra may also have flash-melted meteoritic chondrules and produce excess 21Ne seen in meteoritic grains.

Discovery of X-rays from the protostellar outflow object HH2

Herbig–Haro (HH) objects have been known for 50 years to be luminous condensations of gas in star-forming regions, but their underlying physical nature is still being elucidated. Previously suggested models encompass newborn stars, stellar winds clashing with nebular material, dense pockets of interstellar gas excited by shocks from outflows, and interstellar ‘bullets’ (ref. 6). Recent progress has been made with the jet-induced shock model, in which material streams out of young stellar objects and collides with the surrounding interstellar medium. A clear prediction of this model is that the most energetic Herbig–Haro objects will emit X-rays, although they have not hitherto been detected. Here we report the discovery of X-ray emission from one of the brightest and closest Herbig–Haro objects, HH2, at a level consistent with the model predictions. We conclude that this Herbig–Haro object contains shock-heated material located at or near its leading edge with a temperature of about 106 K.

Chandra X-Ray Observatory Study of the Orion Nebula Cluster and BN/KL Region

About 1000 X-ray emitting young pre–main-sequence (PMS) stars distributed in mass from ~0.05 M☉ brown dwarfs to a ~50 M☉ O star are detected in an image of the Orion Nebula obtained with the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory. This is the richest field of sources ever obtained in X-ray astronomy. Individual X-ray luminosities in the Orion Nebula cluster range from the sensitivity limit of 2 × 10^(28) ergs s^(-1) to ~10^(32) ergs s^(-1). ACIS sources include 85%–90% of V < 20 stars, plus a lower but substantial fraction of deeply embedded stars with extinctions as high as A_V ≃ 60. The relationships between X-ray and other PMS stellar properties suggest that X-ray luminosity of lower-mass PMS stars depends more on mass, and possibly stellar rotation, than on bolometric luminosity, as widely reported. In a subsample of 17 unabsorbed stars with mass ≃ 1 M☉, X-ray luminosities are constant at a high level around L_x ≃ 2 × 10^(30) ergs s^(-1) for the first ≃ 2 Myr while descending the convective Hayashi track, but diverge during the 2–10 Myr phase with X-ray emission plummeting in some stars but remaining high in others. This behavior is consistent with the distribution of X-ray luminosities on the zero-age main sequence and with current theories of their rotational history and magnetic dynamos. The sources in the Becklin-Neugebauer/Kleinman-Low region of massive star formation are discussed in detail. They include both unabsorbed and embedded low-mass members of the Orion Nebula cluster, the luminous infrared Source n, and a class of sources without optical or infrared counterparts that may be new magnetically active embedded PMS stars. Several X-ray sources are also variable radio emitters, an association often seen in magnetically active PMS stars. Faint X-ray emission is seen close to, but apparently not coincident with, the Becklin-Neugebauer object. Its nature is not clear.

The Near-Earth Asteroid Tracking (NEAT) Program: An Automated System for Telescope Control, Wide-Field Imaging, and Object Detection

The Near-Earth Asteroid Tracking (NEAT) system operates autonomously at the Maui Space Surveillance Site on the summit of the extinct Haleakala Volcano Crater, Hawaii. The program began in 1995 December and continues with an observing run every month. Its astrometric observations result in discoveries of near-Earth objects (NEOs), both asteroids (NEAs) and comets, and other unusual minor planets. Each six-night run NEAT covers about 10% of the accessible sky, detects thousands of asteroids, and detects two to five NEAs. NEAT has also contributed more than 1500 preliminary designations of minor planets and 26,000 detections of main-belt asteroids. This paper presents a description of the NEAT system and discusses its capabilities, including sky coverage, limiting magnitude, and detection efficiency. NEAT is an effective discoverer of NEAs larger than 1 km and is a major contributor to NASAs goal of identifying all NEAs of this size. An expansion of NEAT into a network of three similar systems would be capable of discovering 90% of the 1 km and larger NEAs within the next 10–40 yr, while serving the additional role of satellite detection and tracking for the US Air Force. Daily updates of NEAT results during operational periods can be found at JPLs Web site (http://huey.jpl.nasa.gov/~spravdo/neat.html). The images and information about the detected objects, including times of observation, positions, and magnitudes are made available via NASAs SkyMorph program.

MASSES OF ASTROMETRICALLY DISCOVERED AND IMAGED BINARIES: G78-28AB 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 G78-28B, a companion to the M dwarf G78-28A. The orbital period is 4.18 ± 0.03 yr, the system mass is 0.565 ± 0.055 M_☉, and the semimajor 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 M_A = 0.370 ± 0.034 M_☉ and M_B = 0.195 ± 0.021 M_☉. G78-28B is of type M4 V based on 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 M_B = 0.25 ± 0.06 M_☉ and M_C = 0.12 ± 0.02 M_☉. 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.

An Orbital Light-Curve Model for GX 301–2

We present the results of X-ray observations of GX 301-2 with instruments aboard the ASCA and RXTE spacecraft, as well as analysis of archival data from BATSE/CGRO and the All-Sky Monitor on RXTE. We discuss evidence that (1) GX 301-2 has changed its spin state, perhaps entering a stochastic spin state similar to 1974-1984; (2) epochal changes occur in the constant and flaring components of the orbital light curve—a 25% increase in the overall luminosity is coincident with the changed spin state and is due to an increase in the constant component accompanied by a decrease in the near-apastron flare; and (3) the orbital light curve, in general, and timing of the preperiastron flare, in particular, can be understood as a latency effect in the GX 301-2 accretion disk.

CHANDRA X-RAY OBSERVATIONS OF YOUNG CLUSTERS. I. NGC 2264 DATA

We present results of a Chandra observation of a field in NGC 2264. The observations were taken with the ACIS-I camera with an exposure time of 48.1 ks. We present a catalog of 263 sources, which includes X-ray luminosity, optical and infrared photometry, and X-ray variability information. We found 41 variable sources, 14 of which have a flarelike light curve, and two of which have a pattern of a steady increase or decrease over a 10 hr period. The optical and infrared photometry for the stars identified as X-ray sources are consistent with most of these objects being pre–main-sequence stars with ages younger than 3 Myr.

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 ~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 yr, the system mass is 0.214 ± 0.045 M☉, and the semimajor 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 M☉ (1 σ 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 of further narrowing the mass range when e is more precisely determined.

H 1504 + 65 - An extraordinarily hot compact star devoid of hydrogen and helium

The optical identification of the bright soft X-ray source H 1504 + 65, originally discovered by the HEAO I A-2 low energy detectors, is reported. The optical counterpart exhibits a steep, virtually featureless continuum from optical wavelengths through the ultraviolet. The spectrum is remarkably featureless except for a broad O VI adsorption feature at 3494.7 A and C IV absorption at 3934 A and emission at 4658 A. The soft X-ray data, which are not consistent with any published models for degenerate stars having predominantly H or He compositions, suggest that H 1504 + 65 is a metal-rich, near-degenerate object of very high temperature. Qualitative considerations indicate that it is definitely hotter than PG 1159 - 035 and similar objects. A temperature of roughly 160,000 K is suggested. 33 references.

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 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 Δm > 2.2 mag between the two components of the system. Based on our parallax value of 82 ± 8 mas, we derive the following orbital parameters: P = 2.04 ± 0.03 yr, a = 1.03 ± 0.03, and Mtotal = 0.265 ± 0.020 M☉. The component masses are MA = 0.170 ± 0.015 M☉ and MB = 0.095 ± 0.015 M☉. Based on its mass, colors, and spectral properties, GJ 164B has spectral type M6-M8 V.