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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

The ultraviolet spectrum of Herbig-Haro object 2H

IUE spectra of Herbig-Haro object 2H are presented. The spectra show a strong 'excess' UV continuum and prominent emission lines of C, N, O, Si, Mg, and possibly Al. The continuum, F(lambda), exhibits a turnover shortward of about 1450 A, confirming for the first time the H0 two-photon nature of the emission source. A possible absorption feature near 1680 A, which could result from a new grain or molecular constituent in these protostellar objects is also noted. Recently computed models of steady shocks into partially ionized gas reproduce the two-photon spectral shape, but its observed intensity relative to H-beta and the Balmer continuum is anomalously high. It is suggested that a range of shock velocities, 70-100 km/s, or nonsteady, 'truncated' shocks may be responsible. Future high-sensitivity UV observations of HH objects may be used to probe grain extinction curves in star-forming regions.

Brugel, E. W.↗
NASA NTRS1983

Ultraviolet continuum and H2 fluorescent emission in Herbig-Haro objects 43 and 47

IUE short wavelength spectra are presented for the low excitation Herbig-Haro objects HH 43 and HH 47. In the former, several emission lines in the Lyman band of H2 from an excited state are observed which are due to fluorescence from the H Ly-alpha line pumping a lower state (that is in turn excited by a low-velocity shock wave). No evidence of highly ionized gas emission is found in the UV spectra, and both objects exhibit a UV continuum which peaks in the vicinity of 1500 A and is probably caused by H two-photon emission enhanced by low velocity shock collisional excitation.

Schwartz, R. D.↗
NASA NTRS1981

An X-ray active region in Orion - X-rays from a Herbig-Haro object

The Imaging Proportional Counter on the Einstein X-ray Observatory has observed an approximately 1.0 deg square region in Orion centered near right ascension (1950) = 5 hours 34 min, declination = -6 deg 47 arcmin. This is an area containing a dark cloud with evidence for recent star formation and several Herbig-Haro objects. Some 25 stellar X-ray sources are found, and several are identified with stars which have X-ray luminosities at the upper end of the distribution of X-ray luminosity versus stellar type. In addition there is evidence for X-ray emission associated with HH 1 which can provide support for models which include strong stellar winds interacting with the interstellar medium.

Pravdo, S. H.↗
NASA NTRS1979

Interstellar bullets - H2O masers and Herbig-Haro objects

The paper considers the energy input to H2O masers in a dense molecular cloud and analyzes the response time scales for both collisional and radiative pump models. Specific mechanisms for magnetic energy input are given. The evolution of dense H2O maser clouds is studied in the environment of a young pre-main-sequence star that has developed an appreciable supersonic wind. It is shown that compact clouds can be ejected at high velocity to distances of order 1 pc. It is suggested that these clouds (referred to as interstellar bullets) might explain the phenomena of Herbig-Haro objects and high-velocity gas motions observed near compact infrared objects.

Norman, C.↗
NASA NTRS1983

The exciting stars of Herbig-Haro objects

Accurate astrometry and infrared photometry are presented for 16 objects that are believed to represent the exciting stars for Herbig-Haro (HH) objects in their vicinities. These identifications are made on the bases of steeply rising energy distributions, proper motions of HH objects, alignment of sources with HH objects, or far-infrared emission. Exciting stars for the following HH objects are newly discovered or recognized: HH 17; HH 31; HH 32; HH 33 and 40; HH 34; HH 38 and 43; HH group 14 in NGC 2264; RNO 40; RNO 43; and GGD 33a, 33b, and 35. Geometrical alignments of some sources with HH objects support the view that young stars are capable of channeling kinetic energy into small solid angles, as does the fact that some sources are slightly displaced from the apices of fan nebulae (HH 17, R Mon, Haro 6-10). These exciting stars span a wide range of bolometric luminosity, although all are likely to be very young.

Cohen, M.↗
NASA NTRS1982

The ultraviolet continuous and emission-line spectra of the Herbig-Haro objects HH 2 and HH 1

Recent studies of the continuous spectrum of Herbig-Haro (HH) objects at optical and near-infrared wavelengths and the observation of continuous radiation in the ultraviolet have shown an unexpectedly steep increase of the flux toward shorter wavelengths. The present investigation provides the results of ultraviolet observations of HH 2. The obtained data are compared with the HH 1 data. It is found that HH 2 has an ultraviolet continuous and emission-line spectrum which is similar to that of HH 1. The UV line spectrum of HH 2H indicates an even somewhat larger ionization than does the HH 1 spectrum. As in HH1, the UV emission-line spectrum shows a much higher degree of ionization than that derived from the optical spectrum. Consequently, the same difficulty arises as in the case of HH 1. The complete UV plus optical spectrum cannot be explained by a single plane-parallel shock-wave model.

Boehm-Vitense, E.↗
NASA NTRS1981

The ultraviolet spectrum of Herbig-Haro Object 1

Continuum and emission line fluxes for HH 1 in the range of 1200-1300 A have been determined using IUE spectra, and it is found that the luminosity of HH 1 in this wavelength interval is slightly larger than 1 solar luminosity, or 20 times greater than for the wavelength range 3200-11,000 A. If averaged over intervals of 100 A, it is shown that the UV continuum rises almost monotonically toward shorter wavelengths over the entire interval 1300-3000 A. The UV continuum energy distribution cannot be explained by a T Tauri stellar continuum that is scattered by dust. Emission line identifications and flux measurements strongly confirm the statement that the high-ionization emission lines are too strong to be explained by shock-wave models derived from the emission line spectrum in the interval 3700-11,000 A. A possible explanation for this fact is discussed.

Boehm, K. H.↗
NASA NTRS1981

Spectropolarimetry of Herbig-Haro objects and the exciting star of HH 30

Spectrophotometric and spectropolarimetric observations are presented of three HH objects: HH 11, 30, and 43. Although null polarization results are obtained for HH 11 and 43, the unusually strong continuum of HH 30 is linearly polarized by 3%. The emission lines in this nebula are unpolarized. The electric vector position angle appropriate to the continuum polarization, and the presence of Fe II emission lines apparently from the photosphere of a T Tauri-like star, indicate HL Tau as the exciting/illuminating star for HH 30. Evidence is adduced for a highly anisotropic distribution of circumstellar obscuration around this star.

Cohen, M.↗
NASA NTRS1982

Ultraviolet spectra of Herbig-Haro objects and of the environment of the Cohen-Schwartz star

Observations in both spectral regions were used in order to determine the continuous energy distribution and the emission line fluxes for H-H 2. The continuous spectrum is similar to that in H-H 1 and F lambda increases rapidly towards shorter wavelengths. This statement is found to be qualitatively correct for all obvious choices of the ultraviolet extinction curve if we use the E(B-V) value determined by the use of the S II method. The origin of the continuum remains enigmatic. The emission line spectrum of H-H 2 shows an even somewhat higher degree of ionization than the spectrum of H-H 1, indicating an even larger discrepancy between ionization information from optical data and that contained in the UV spectra. The immediate environment of the Cohen-Schwartz star emits a continuous spectrum similar to that of a H-H-object but increasing even more steeply towards shorter wavelengths.

Boehm, K. H.↗
NASA NTRS1984

Far-infrared observations of southern Herbig-Haro objects

Ground-based and airborne infrared observations are presented of the vicinities of several HH objects. New photographs of these HH nebulae have made it possible to locate the exciting stars of HH 46/47 and HH 57 from their infrared peak positions. Bolometric luminosities of the exciting stars lie in the range 4-12, consistent with those for low-mass accreting protostars.

Cohen, M.↗
NASA NTRS1982

Infrared observations of low-mass star formation in Orion - HH objects

The results of a preliminary analysis of IR data on Herbig-Haro objects in the Orion nebula are reported. The observations were made with the high angular resolution IR photometry equipment on the NASA Kuiper Airborne Observatory and the NASA facility on Mauna Kea, HI. Data were taken in the 1-200 microns region with 40, 6, and 8 arcsec resolution. Attention was focused on NGC 1999 (HH1-3) and M78 (HH24-25) and the determination of absolute luminosities of the exciting stars. Measurements were also made of the IR energy distribution in the thermally emitting dust clouds and the point sources. Herbig-Haro objects featured compact and far IR sizes and large visual extinction, in addition to a steeply rising energy distribution up to 50-100 microns, where the luminosity emitted was concentrated.

Harvey, P. M.↗
NASA NTRS1982

IUE observations of Herbig-Haro and related objects

The I.U.E. spectra of the Herbig-Haro objectives H-H 1, H-H 2H, H-H 2G and H-H 32 A are considered. In order to obtain a better understanding of the physics of these intriguing objects and of the formation of the ultraviolet continua and emission lines in H-H objects, the UV spectra of the Cohen-Schwartz star, the T Tauri star AS 353A, and of the reflection nebula NGC 1999 (which is illuminated by the young object V 380 Orionis), are also examined.

Boehm, K. H.↗
NASA NTRS1980

Red and nebulous objects in dark clouds - A survey

A search on the NGS-PO Sky Survey photographs has revealed 150 interesting nebulous and/or red objects, mostly lying in dark clouds and not previously catalogued. Spectral classifications are presented for 55 objects. These indicate a small number of new members of the class of Herbig-Haro objects, a significant number of new T Tauri stars, and a few emission-line hot stars. It is argued that hot, high-mass stars form preferentially in the dense cores of dark clouds. The possible symbiosis of high and low mass stars is considered. A new morphology class is defined for cometary nebulae, in which a star lies on the periphery of a nebulous ring.

Cohen, M.↗
NASA NTRS1978

A study of the Taurus dark cloud complex

A near-infrared survey has been conducted of nearly 20 square degrees of the Taurus dark cloud complex. Additional observations have been made of selected objects detected in this survey. These observations show that recently formed stars are spread throughout the cloud and that these stars are primarily T Tauri stars or T Tauri-like stars. Two luminous objects are identified embedded in the reflection nebulae IC 359 and IC 2087. A new Herbig-Haro object is also described. The reddening law of the dark cloud material is discussed; it does not appear to be unusual in the infrared. Comparison of the young stellar population in Taurus with that in Ophiuchus suggests that the star formation mechanisms in the two regions are qualitatively different

Elias, J. H.↗
NASA NTRS1976

CO observations of NGC 1579 /S222/ and S239

The 2.6-mm lines of (C-12)O and (C-13)O have been observed toward the small galactic diffuse nebulae NGC 1579 (S222) and S239. The H92-alpha recombination line has also been detected from NGC 1579. Toward NGC 1579, evidence was observed for self-absorption in the (C-12)O line. For both regions, high-velocity wings and line broadening are observable in the (C-12)O line; the features observed toward S239 suggest ordered free-fall collapse of a localized region of the cloud onto a newborn stellar cluster. Examination of the available data suggests that NGC 1579 is both a reflection nebula (illuminated by LKH-alpha 101) and an obscured H II region excited by a star of spectral type near B1, while S239 is a reflection nebula or a Herbig-Haro object. The present studies of these regions show that line widening in CO is at least as important an indicator of star-formation activity as enhanced line emission.

Knapp, G. R.↗
NASA NTRS1979

Shock waves in the interstellar medium

Calculations of the optical and UV emission-line intensities and column densities of ions observable in the optical and UV are presented for plane-parallel steady-state shock waves in the interstellar medium. The range of shock velocities considered is from 50 to 200 km/s, and preshock densities are between 1 and 300 per cu cm. The ionization state of the preshock gas is found to be an important parameter, and several sets of elemental abundances corresponding to various amounts of depletion onto grains are used. The results are compared with observations of the Cygnus Loop, the Vela supernova remnant, and Herbig-Haro objects.

Raymond, J. C.↗