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A catalog of co-added IRAS fluxes for the pre-main-sequence objects in the Herbig-Bell catalog (HBC) is presented. This catalog doubles the number of HBC stars with detected IRAS fluxes and provides improved flux values for the previously known sources. Noise level are given for all HBC fields in each band, permitting upper limits to be estimated for all undetected sources.
Echelle spectroscopy data were taken of the quasar 0000-263 on the 4m telescope at CTIO in Chile. Three nights of red and three nights of blue data were obtained (about four hours data per night). The resolution is typically 0.4 A at 5000 A (lamda/d(lamda) = 12500) and has good signal-to-noise over the wavelength range 4600 A to 8000 A. To keep the statistics manageable the data were not rebinned, although the instrument was stable enough for each night's data on the object to be co-added. Arc spectra taken between each exposure were also co-added within the nights. During profile fitting the model was convolved with a Gaussian whose width was equal to nearby arc lines, so any broadening generated by the co-adding of data will not affect the final results. Since OI and HI have the same ionization potential (13.6 eV), (O/H) is approximately (OI/HI). Unfortunately, in this data only the 1302 A OI transition is visible, and it is blended with HI in the Lyman Alpha forest. However, OI and NI coexist (the ionization potential for NI is 14.5 eV) and so the NI triplet can constrain the redshift and turbulent width of the OI. When a fit was made to the regions containing OI and NI absorption three systems were apparent with similar redshifts and velocity widths. These three systems were fitted with tied redshifts and turbulent widths, while the other lines in the region were fitted as Lyman-Alpha. The saturated Lyman-Alpha profile was then fitted with three components at the same redshifts as these observed lines, with the line widths tied. Finally, metallicity ratios for the individual lines and their totals were calculated and compared to solar values. The results for the separate clouds from low to high redshift and for the total are presented.
Development of a sensor incorporating a CCD and photocathode for operation in the electron bombarded mode in both a magnetic and an electrostatic focussed, gated configuration is described. Initial format size is 100 x 160 pixels with eventual growth to 400 x 400 pixels. The sensor is part of a digital camera system which includes the low level video conditioning electronics, a camera controller, a high speed buffer memory, and digital recording and display electronics. The memory uses CMOS/SOS and has a capacity of 1.6 Mbits with operating rates of 48 Mbits/sec. Individual frames are co-added to provide wide dynamic range and photometric precision better than 1%. A 4-bit video quantization is used to increase the photon counting detection rate before coincidence losses become serious.
Results are reported for measurements of the quadrupole S1 line of the (3, 0) absorption band of H2 in spectra of Titan, which were performed at an effective resolution of 0.09 A by scanning with a PEPSIOS spectrometer over a range of 0.85 A centered at the expected position of the investigated line and sampling in 80 contiguous channels. No statistically significant H2 (3, 0) S1 feature is found in the spectra, but a three-sigma upper limit of 3 mA is set for the equivalent width of the S1 line that might be present in a co-added spectrum. It is concluded that these measurements do not provide any evidence for the presence of H2 in the atmosphere of Titan.
The International Ultraviolet Explorer (IUE) Satellite was used to collect interstellar extinction curves for 58 moderately reddened lines of sight. Spectral types of the background stars ranged from O7 to A0. All luminosity classes are represented, but early B main-sequence stars predominate. E(B-V) reddening values ranged from 0.14 to 1.13 mag, with an average of 0.63 mag. These extinction curves, less some 10 curves judged of lower quality, were co-added in order to reduce the random and fixed-pattern noise of the instrument. The resulting averaged curve was examined for evidence of small-scale structure, with negative results. The 2-sigma limits were found to be 0.09 mag for wavelengths between 1150 A and 2000 A, and 0.16 mag for wavelengths between 2000 A and 2300 A. Because of mismatch errors in weak stellar lines, it is unlikely that much improvement in these results is possible. The implications of these negative results are discussed with particular reference to the MgO theory of interstellar grains and diffuse bands.
Using the Infrared Astronomy Satellite (IRAS) data, an investigation was begun of the far infrared properties of Crab-like supernova remnants and other synchrotron nebulae. Both the co-added scanning data and, where available, pointed observations were examined. To date infrared emission was found from two Crab-like remnants: G24.7+0.6 in which the infrared source near the center of the object was shown to be a compact HII region with EM greater than or approximately 10 to the 7th pc/cm(6); and G0.9+0.1 where a marginal detection of a 25 micron source coincident with the remnant core was used to set limits on the energetics of this synchrotron nebula. Further work, in progress under a second year of this program, should yield additional information concerning the distribution of initial pulsar spin periods and the evolution of synchrotron nebulae.
An observational and data reduction technique for fixed pattern noise (FPN) and random noise (RN) in fully extracted IUE high-dispersion spectra is described in detail, along with actual empirical values of signal-to-noise ratio (S/N) achieved. A co-addition procedure, involving SWP and LWR cameras observations of the same spectrum at different positions in the image format, provides a basis to disentangle FPN from RN, allowing each average amplitude, within a given wavelength interval, to be estimated as a function of average flux number. Empirical curves, derived with the noise algorithm, make it possible to estimate the S/N in individual spectra at the wavelengths investigated. The average S/N at the continuum level in well-exposed stellar spectra varies from 10 to 20, for the orders analyzed, depending on position in the spectral format. The co-addition procedure yields an improvement in S/N by factors ranging from 2.3 to 2.9. Direct measurements of S/N in narrow, line-free wavelength intervals of individual and co-added spectra for weak-lined stars yield comparable, or in some cases somewhat higher, S/N values and improvement factors.
The molecular cloud associated with compact radio and bright optical H II regions is studied from observations of (C-12)O, (C-13)O, HCN J = 1-0 emission. The far-infrared and millimeter molecular line emission from the S254-S258 molecular cloud is analyzed, using co-added survey data from IRAS, to investigate the coupling between the dust and gas components and the internal energetics associated with molecular clouds. The dust component is analyzed by fitting model intensities, derived from a dust mass column density temperature relation, to the observed far-infrared emission. Such modeling is shown to provide a more complete description of the cold dust component which comprises most of the dust mass than can be determined from the standard single temperature analysis. Local heating of the dust component by newborn massive stars associated with the H II regions of the cloud is inferred from the spatial distribution of temperatures of grains which make the largest contributions to the observed 60 and 100 micron emission.
A large body of work has accumulated in recent years which throws into disarray the traditional assumption that S0 systems are inert, non-starforming galaxies with uniform, old stellar populations. The copious 21-cm data have been recently reviewed and assessed by Wardle and Knapp (1986). This work showed that roughly a third of the several hundred observed S0's contain detectable amounts of neutral hydrogen (HI). More recently, Pogge and Eskridge (1987) have shown that a significant fraction of HI-rich systems also exhibit H alpha emission. Thronson et al. (1989) report detection of CO line emission from two thirds of the S0's in their sample. Both of these last papers, however, report on fairly small data sets (approx. 20 objects each). From co-added Infrared Astronomy Satellite (IRAS) data, Knapp et al. (1989) report that roughly two thirds of a sample of several hundred S0's are detected at 60 and 100 microns. Work by Bally and Thronson (1989) and Walsh et al. (1989) has shown that, while a large number of S0's follow a relation between radio continuum and far infrared radiation (FIR) emission similar to that found for spirals, significant numbers of both radio-bright, and FIR-bright S0's exist. Clearly, a large number of factors are involved in determining the state of the interstellar medium in S0 galaxies. The class is probably heterogeneous, suggesting that large data samples are required to sort out various sorts of objects. The 21 cm and FIR samples are the two largest currently available. It is therefore of interest to compare the two and see where this leads. The following results are already clear: HI and FIR flux data can be used to isolate strong candidates for systems which have gained their HI gas via accretion; a rough power-law relationship exists for galaxies which are undergoing relatively normal star-forming activity; a heterogeneous class of galaxies with strong FIR emission compared to their HI emission exists. Further work is required to determine the various physical processes responsible for this last class.
The dominant galaxy in each of approx. 40 clusters was studied using co-added Infrared Astronomy Satellite (IRAS) survey data, and 11 of these galaxies were observed for CO (J=1 to 0) emission with the 12 m telescope at Kitt Peak. Half of the galaxies in the sample are in clusters reported to have cooling flows while the other half are not. Six of the galaxies appear to have been detected by IRAS at fairly low flux levels, in addition to one previously known strong detection; all seven have reported cooling flows. No detectable CO emission (to 2 to 3 mK) was found in any of the 11 galaxies observed. Assuming accretion rates of approx. 100 Solar Mass yr(-1), the star formation rates and efficiencies in these galaxies must be quite high in order to render the CO undetectable. At the same time, the infrared luminosities of these galaxies is unremarkable, suggesting that the correlation between star formation efficiency and infrared luminosity found for spirals may not hold for cooling flows.
A recent survey by Roby (1987) discovered that the relatively weak high excitation lines of N 1 near 7468 and 8680 A were undetectable in the majority of HgMn stars, leading to upper limits on the N abundance of roughly ten times below that of the solar N abundance. Standard stars with similar temperatures (10,000 to 13,000 K) did exhibit these same N lines and were found to have roughly solar N abundances. The N abundances were redetermined in two HgMn stars and four standard stars using the strong, low excitation lines of N 1 found in the ultraviolet. The observational data consisted of high quality, high resolution, co-added International Ultraviolet Explorer spectra which was previously collected and reduced. Examination of the spectra plus considerations of signal/noise and severe line blending led to the choice of three promising N 1 lines located at 1742.7, 1745.3 and 1411.9 A. The atomic data for these lines were previously calculated using the best laboratory measurements found in a search of the relevant literature. The chosen N lines turned out to be blended significantly with Fe 2 lines. To obtain the abundances for N synthetic spectra were computed to match the observed spectra. The synthetic spectra were calculated using lie-blanketed model atmospheres, stellar parameters, and abundances for the other elements based upon previous work by S. Adelman. The N abundance was then adjusted to give the best fit of the observed line profiles. In the 2 HgMn stars, the N lines were again found to be undetectable, but the stronger intrinsic strength of the new lines yield more stringent upper limits than those obtained previously. Model atmosphere and abundances were updated in two stars where new results were reported. An additional N 1 line and an additional standard star were added to the program. A line opacity model was developed for a missing feature adjacent to the N 1 line at 1745.2 A, leading to a 25 percent improvement in the determined from this line.
The IRAS data base is used to locate young stellar object candidates in and near the IC 1396 complex located in the Cepheus OB2 association. Co-added survey data are used to identify all sources with a flux density Snu(100) greater than 10 Jy and with Snu(100) greater than Snu(60). The 15 sources located at the positions of globules and dark clouds are further analyzed using the inscan slices to assess the source profiles.
Fluxes of 453 IRAS point sources selected for proximity to dense gas have been accurately determined by co-adding IRAS data. From this set, 258 sources are detected in all four bands: 12, 25, 60, and 100 microns. As distance from dense molecular material increases, these sources exhibit systematic trends in color and a markedly decreasing luminosity. These trends are broadly consistent with expected evolutionary changes in these stars, with younger objects found closer to dense gas. These data suggest a subtle variation on the criterion for identifying the youngest objects detectable by IRAS, in addition to that of steeply rising spectra: a color temperature which is uniform across all four IRAS bands in the range 40-50 K. These characteristics are used as a criterion to select point sources as candidate very young pre-main-sequence stars.
Traditionally elliptical galaxies were thought to be essentially devoid of interstellar matter. However, recent advances in instrumental sensitivity have caused a renaissance of interest in dust and gas in - or associated with - elliptical galaxies. In particular, the technique of co-adding IRAS survey scans has led to the detection of more than half of all ellipticals with BT less than 11 mag. in the Revised Shapley-Ames catalog, indicating the presence of 10(exp 7) - 10(exp 8) solar mass of cold interstellar matter (Jura et al. 1987). In addition, CCD multi-color surface photometry shows dust patches in about 30 percent of the cases studied to date (e.g., Veron-Cetty & Veron 1988). Thorough study of the gas and dust in ellipticals is important to (1) determine its origin (mass-loss from late-type stars, merging collisions with other galaxies or accretion inflows from cooling X-ray gas), and (2) investigate the 3-D shape of ellipticals, as can be derived from the orientation of the dust lanes and the 2-D velocity field of the gas. An important result of our comprehensive CCD imaging program is that a relevant fraction (approximately 40 percent) of the sample objects exhibits dust patches within extended H-alpha+(NII) line-emitting filaments. This common occurrence can be easily accounted for if the dust and gas have an external origin, i.e., mergers or interactions with gas-rich galaxies. Evidence supporting this suggestion: (1) the ionized gas is usually dynamically decoupled from the stellar velocity field (see, e.g., Sharples et al. 1983, Bertola & Bettoni 1988); (2) it is shown in a companion paper (Goudfrooij et al. 1992) that internal stellar mass loss alone can not account for the dust content of elliptical galaxies.
Motivated by the possibility of detecting strong absorption features toward extragalactic background objects, we present optimally extracted and co-added IUE spectra of QSOs, BL Lac objects, and Seyfert galaxies. These spectra together with those presented (in a similar work) by Kinney et al. form an essentially complete sample of the SWP, LWP, and LWR observations of QSOs, BL Lac objects, and Seyfert galaxies with redshifts z greater than 0.05 available in the IUE merged observation log as of 1991 December.
An International Ultraviolet Explorer (IUE) observing strategy that has yielded co-added spectra with enhanced S/N ratios for several A and B stars was established. New observations by Roby and Adelman using the same technique were added two new Hg-Mn stars into this sample. A long-term study of elemental abundances in this uniform, high-quality set of IUE spectra for 13 stars was begun. The first stages of this project are reported: abundances for N, Cr, Mn, Fe, Co, and Ni. The study of the Fe-peak elements show that our data set can provide accurate abundances and that abundances obtained from UV and optical spectra often are in good agreement. The groundwork for selfconsistent abundance analyses of more exotic elements in our long term project was provided.
We introduce the method of 'network synthesis,' which allows the detection of very weak gamma-ray transient signals in the data of the Ulysses gamma-ray burst (GRB) experiment from repeating sources. It consists of defining a grid of alpha, delta values, and for each BATSE detection of a burst from a soft gamma repeater, predicting the arrival time of the burst at Ulysses and co-adding the Ulysses data rephased so that the burst signals are aligned in time and produce a detectable pulse. We demonstrate that this method identifies the position of the soft repeater SGR 1806-20, and apply it to the repeater B1900+14. We show that the counterpart to this burst source is probably in or in the vicinity of the Galactic supernova remnant G42.8+0.6.
Archival high-dispersion International Ultraviolet Explorer (IUE) spectra have been used to search for circumstellar gas absorption features in alpha PsA (A3 V), a nearby (6.7 pc) proto-planetary system candidate. Recent sub-millimeter mapping observations around the region of alpha PsA indicate a spatially resolved dust disk like the one seen around Beta Pic. To determine how closely this putative disk resembles that of Beta Pic, we have searched for signatures of circumstellar gaseous absorption in all the available IUE high-dispersion data of alpha PsA. Examination of co-added IUE spectra shows weak circumstellar absorptions from excited levels in the resonance multiplet of Fe II near 2600 A. We also conclude that the sharp C I feature near 1657 A, previously identified as interstellar absorption toward alpha PsA, likely has a circumstellar origin. However, because the weakness of these absorption features, we will consider the presence of circumstellar gas as tentative and should be verified by using the Goddard High-Resolution Spectrograph aboard the Hubble Space Telescope. No corresponding circumstellar absorption is detected in higher ionization Fe III and Al III. Since the collisionally ionized nonphotospheric Al III resonance absorption seen in Beta Pic is likely formed close to the stellar surface, its absence in the UV spectra of alpha PsA could imply that, in contrast with Beta Pic, there is no active gaseous disk infall onto the central star. In the alpha PsA gaseous disk, if we assume a solar abundance for iron and all the iron is in the form of Fe II, plus a disk temperature of 5000 K, the Fe II UV1 absorption at 2611.8743 A infers a total hydrogen column density along the line of sight through the circumstellar disk of N(H) approximately equals 3.8 x 10(exp 17)/cm.