G. Neugebauer

THE INFRARED ASTRONOMICAL SATELLITE (IRAS) MISSION

The Infrared Astronomical Satellite (IRAS) consists of a spacecraft and a liquid helium cryostat that contains a cooled IR telescope. The telescopes focal plane assembly is cooled to less than 3 K, and contains 62 IR detectors in the survey array which are arranged so that every source crossing the field of view can be seen by at least two detectors in each of four wavelength bands. The satellite was launched into a 900 km-altitude near-polar orbit, and its cryogenic helium supply was exhausted on November 22, 1983. By missions end, 72 percent of the sky had been observed with three or more hours-confirming scans, and 95 percent with two or more hours-confirming scans. About 2000 stars detected at 12 and 25 microns early in the mission, and identified in the SAO (1966) catalog, have a positional uncertainty ellipse whose axes are 45 x 9 arcsec for an hours-confirmed source.

Infrared standard stars

The results of an observational program aimed at setting up a network of faint near-infrared standards of sufficient accuracy are reported. The network covers both northern and southern hemispheres and includes standards red enough to provide at least a limited check on color transformations. The standards are set up at J (1.2 micron), H (1.6 micron), K (2.2 microns), and L (3.5 microns), and their H2O and CO molecular absorption indices are determined. The problem of color transformations between observatories is discussed briefly. All magnitudes presented are transformed to the natural system defined by the CIT observations.

DISCOVERY OF A SHELL AROUND ALPHA-LYRAE

IRAS observations of Alpha Lyrae reveal a large infrared excess beyond 12 microns. The excess over an extrapolation of a 10,000 K blackbody is a factor of 1.3 at 25 microns, 7 at 60 microns, and 16 at 100 microns. The source of 60 microns emission has a diameter of about 20 arcsec. This is the first detection of a large infrared excess from a main-sequence star without significant mass loss. The most likely origin of the excess is thermal radiation from solid particles more than a millimeter in radius, located approximately 85 AU from Alpha Lyr and heated by the star to an equilibrium temperature of 85 K. These results provide the first direct evidence outside of the solar system for the growth of large particles from the residual of the prenatal cloud of gas and dust.

The multiplicity of T Tauri stars in the star forming regions Taurus-Auriga and Ophiuchus-Scorpius : A 2.2 micron speckle imaging survey

We present the results of a magnitude limited (K ≤ 8.5 mag) multiplicity survey of T Tauri stars (TTS) in two nearby star forming regions (SFR), Taurus-Auriga and Ophiuchus-Scorpius. Each of the 69 stars in the sample was observed at K(2.2 µm) with an infrared array camera on the Hale 5-m Telescope at Palomar Observatory and imaged using two-dimensional speckle interferometric techniques. Thirty three companion stars were found, of which 15 were new detections. A subset of 64 observations was sensitive to all companion stars in the projected linear range 14 to 225 AU and the magnitude difference range 0.0 to 2.0 mag. We used this subset and region to study the multiplicity of TTS; the frequency of companion stars within this region is 34 ± 9%, independent of SFR. We discovered a distinction between the classical TTS (CTTS) and the weak-lined TTS (WTTS) based on the binary star frequency as a function of separation; the WITS dominate the binary star distribution at the closer separations and the CTTS populate the wider separations. The cross over occurred near 100 AU, the size typically quoted for a circumstellar disk. We suggest that all TTS begin as CTTS and become WTTS when accretion has ceased, and that the nearby companion stars act to shorten the accretion timescale in multiple star systems. Integrated over all magnitude differences the binary star frequency in the projected linear separation range 14 to 225 AU for TTS (59 ± 16%) is a factor of 3.5 greater than that of the solar-type main sequence stars (17 ± 3%). Given the limited angular separation range that we are sensitive to, i.e., both the spectroscopic and wide binaries are missed, the rate at which binaries are detected suggests that most, if not all, TTS stars have companions. We propose that the observed overabundance of companions to TTS with respect to their older counterparts on the main sequence is an evolutionary effect; in this scheme triple and higher order TTS, which are observed at higher frequencies than for the solar-type main sequence stars, are disrupted by close encounters with another star or system of stars.

The First Measurement of Spectral Lines in a Short-Period Star Bound to the Galaxy's Central Black Hole: A Paradox of Youth

We have obtained the first detection of spectral absorption lines in one of the high-velocity stars in the vicinity of the Galaxys central supermassive black hole. Both Brγ (2.1661 μm) and He I (2.1126 μm) are seen in absorption in S0-2 with equivalent widths (2.8 ± 0.3 and 1.7 ± 0.4 A) and an inferred stellar rotational velocity (220 ± 40 km s^(-1)) that are consistent with that of an O8-B0 dwarf, which suggests that it is a massive (~15 M_☉) young (less than 10 Myr) main-sequence star. This presents a major challenge to star formation theories, given the strong tidal forces that prevail over all distances reached by S0-2 in its current orbit (130-1900 AU) and the difficulty in migrating this star inward during its lifetime from farther out where tidal forces should no longer preclude star formation. The radial velocity measurements (v_z) = -510 ± 40 km s^(-1)) and our reported proper motions for S0-2 strongly constrain its orbit, providing a direct measure of the black hole mass of 4.1(±0.6) × 10^6 (Ro/8kpc)^3 M_☉. The Keplerian orbit parameters have uncertainties that are reduced by a factor of 2-3 compared to previously reported values and include, for the first time, an independent solution for the dynamical center; this location, while consistent with the nominal infrared position of Sgr A^*, is localized to a factor of 5 more precisely (±2 mas). Furthermore, the ambiguity in the inclination of the orbit is resolved with the addition of the radial velocity measurement, indicating that the star is behind the black hole at the time of closest approach and counterrevolving against the Galaxy. With further radial velocity measurements in the next few years, the orbit of S0-2 will provide the most robust estimate of the distance to the Galactic center.

Infrared cirrus - New components of the extended infrared emission

Extended sources of far-infrared emission superposed on the zodiacal and galactic backgrounds are found at high galactic latitudes and near the ecliptic plane. Clouds of interstellar dust at color temperatures as high as 35 K account for much of this complex structure, but the relationship to H I column density is not simple. Other features of the extended emission show the existence of warm structures within the solar system. Three bands of dust clouds at temperatures of 150-200 K appear within 10 deg on both sides of the ecliptic plane. Their ecliptic latitudes and derived distances suggest that they are associated with the main asteroid belt. A third component of the 100-micron cirrus, poorly correlated with H I, may represent cold material in the outer solar system or a new component of the interstellar medium.

The IRAS Bright Galaxy Sample. IV. Complete IRAS observations

Total flux densities, peak flux densities, and spatial extents at 12, 25, 60, and 100 microns are presented for the 330 sources in the IRAS Bright Galaxy Sample. The flux density ratios S{sub nu} (60 microns)/S{sub nu} (100 microns) and S{sub nu} (12 microns)/S{sub n} (25 microns) are found to correlate with both the infrared luminosity and the ratio of IR to visible flux. The relation between these two flux density ratios is shown to follow that found previously, with different slopes appearing for the warmer and colder galaxies in the sample. The results suggest that single photon heating of small grains (often the dominant source of 12 and 25 micron radiation from galaxies) significantly affects the emission of some galaxies at 60 microns, and that optical depth effects may alter the emergent radiation at 12 and 25 microns. 31 refs.

Absolute Calibration and Characterization of the Multiband Imaging Photometer for Spitzer. I. The Stellar Calibrator Sample and the 24 μm Calibration

We present the stellar calibrator sample and the conversion from instrumental to physical units for the 24 μm channel of the Multiband Imaging Photometer for Spitzer (MIPS). The primary calibrators are A stars, and the calibration factor based on those stars is MJy sr^−1 (DN s^−1)^−1, with a nominal uncertainty of 2%. We discuss the data reduction procedures required to attain this accuracy; without these procedures, the calibration factor obtained using the automated pipeline at the Spitzer Science Center is lower. We extend this work to predict 24 μm flux densities for a sample of 238 stars that covers a larger range of flux densities and spectral types. We present a total of 348 measurements of 141 stars at 24 μm. This sample covers a factor of 460 in 24 μm flux density, from 8.6 mJy up to 4.0 Jy. We show that the calibration is linear over that range with respect to target flux and background level. The calibration is based on observations made using 3 s exposures; a preliminary analysis shows that the calibration factor may be 1% and 2% lower for 10 and 30 s exposures, respectively. We also demonstrate that the calibration is very stable: over the course of the mission, repeated measurements of our routine calibrator, HD 159330, show a rms scatter of only 0.4%. Finally, we show that the point-spread function (PSF) is well measured and allows us to calibrate extended sources accurately; Infrared Astronomy Satellite (IRAS) and MIPS measurements of a sample of nearby galaxies are identical within the uncertainties.

A lunar occultation and direct imaging survey of multiplicity in the Ophiuchus and Taurus star-forming regions

We present an IR lunar occultation and direct imaging search for companions in the Ophiuchus star-forming region and update a similar search of the Taurus region. The search is sensitive to companions in the angular separation range 0.005-10 sec. In Ophiuchus, we surveyed 35 young star targets; this sample contains at least 10 binaries, two triples, and one quadruple. Ten of the companion stars are newly discovered. In Taurus, the survey now includes 47 systems among which there are at least 22 binaries and four triples. Only two companion stars are newly identified because there is strong overlap with prior work. All the triples and quadruple are hierarchical. The observed binary frequency in Ophiuchus, in the 3-1400 AU range of separations, is at least 1.1 +/- 0.3 that of the nearby solar-like stars. This value is a lower bound because we make no corrections for incompleteness. In Taurus, in the same range of separations, the observed binary frequency is at least 1.6 +/- 0.3 that of the nearby solar-like stars. This value extends Ghez et al.s (1993) and Leinerts et al.s (1993) determination of an excess binary frequency to 3 AU separation. We used the weak-line T Tauri star/T Tauri star (WT/TT) type and the K-L color index to distinguish between systems with and without inner disks. We find no convincing difference in the binary frequency or distribution of separations of the systems with and without inner disks. The 1.3 mm continuum emission of the single systems exceeds that of the multiples suggesting that their extensive outer disks are more massive. The specific angular momenta of the binaries overlap those of molecular cloud cores measured by Goodman et al. (1993).

The 24 Micron Source Counts in Deep Spitzer Space Telescope Surveys

Galaxy source counts in the infrared provide strong constraints on the evolution of the bolometric energy output from distant galaxy populations. We present the results from deep 24 μm imaging from Spitzer surveys, which include ≈5 × 10^4 sources to an 80% completeness of ≃ 60 μJy. The 24 μm counts rapidly rise at near-Euclidean rates down to 5 mJy, increase with a super-Euclidean rate between 0.4 and 4 mJy, and converge below ~0.3 mJy. The 24 μm counts exceed expectations from nonevolving models by a factor of ≳10 at S_ν ~ 0.1 mJy. The peak in the differential number counts corresponds to a population of faint sources that is not expected from predictions based on 15 μm counts from the Infrared Space Observatory. We argue that this implies the existence of a previously undetected population of infrared-luminous galaxies at z ~ 1-3. Integrating the counts to 60 μJy, we derive a lower limit on the 24 μm background intensity of 1.9 ± 0.6 nW m^(-2) sr^(-1) of which the majority (~60%) stems from sources fainter than 0.4 mJy. Extrapolating to fainter flux densities, sources below 60 μJy contribute 0.8^(+0.9)_(-0.4) nW m^(-2) sr^(-1) to the background, which provides an estimate of the total 24 μm background of 2.7^(+1.1)_(-0.7) nW m^(-2) sr^(-1).