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At least 235 records · Page 13
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NASA NTRS1981

Steady flows in the solar transition region observed with SMM

Steady flows in the quiet solar transition region have been observed with the Ultraviolet Spectrometer and Polarimeter experiment on the Solar Maximum Mission (SMM) satellite. The persistent vertical motions seen at disk center have spatial rms amplitudes of 1.4 km/s in the C II line, 3.9 km/s in Si IV, and 4.2 km/s in C IV. The amplitudes of the more horizontal flows seen toward the limb tend to be somewhat higher. Plots of steady vertical velocity versus intensity seen at disk center in Si IV and C IV show two distinct branches.

Gebbie, K. B.↗
NASA NTRS1995

Excitation of the arched filaments near the Galactic Center

We discuss measurements of the far-infrared (FIR) fine structure lines from (S III) (33 microns), (Si II) (35 microns), (O III) (51, 88 microns), (OI) (63 microns), (C II) (158 microns), and the adjacent continua in a strip crossing two of the thermal radio filaments in the Galactic Center 'Arch'. The near spatial coincidence of the line and continuum emission maxima with the radio filaments demonstrates that any excitation mechanism must account for both the line and continuum emission. The peak FIR luminosity and (O III) emission pose difficulties for collisional excitation models; photoionization of molecular cloud edges by a random distribution of stars is the most plausible mechanism proposed.

Colgan, Sean W. J.↗
DOE OSTI2023

Gradient Amine Sorbents for Low Vacuum Swing CO 2 Capture at Ambient Temperature

The growing concerns over CO 2 emissions have led to the development of various methods for CO 2 capture. CO 2 capture by amine-based sorbents has been achieved by temperature (thermal) swing adsorption (TSA) process, pressure swing adsorption (PSA) process, and temperature pressure or vacuuming swing adsorption (TPSA or TVSA) process for CO 2 capture. Amine sorbents for energy-efficient TSA, PSA, VSA, TPSA or TVSA CO 2 capture should possess a capability which allow adsorbed CO 2 to desorb and amine sorbent to be regenerated at low temperatures or low vacuums. Development of such amine sorbents would significantly decrease energy consumption and sorbent degradation during the regeneration step. The objective of this project is to develop a low vacuum swing adsorption (VSA) process for the capture of CO 2 from air. The focus is on the development of amine sorbent which allows CO 2 to adsorb in the form of weakly adsorbed CO 2 . The weakly adsorbed CO 2 species can be collected from the sorbent by applying a low vacuum at ambient temperature. Specifically, no heating (i.e., thermal energy) is needed for regeneration of amine sorbents. This novel sorbent allows VSA to be operated at ambient temperature without a significant energy demand. This process eliminates the energy-intensive heating and cooling process in temperature swing adsorption (TSA) process. Ambient temperature operation could prolong the lifetime of sorbent and minimize the maintenance cost. Extensive sorbent studies with in-situ infrared spectroscopy have revealed the modifications of conventional amine sorbents with additives can increase the fraction of weakly adsorbed CO 2 . The amine sorbents with loaded CO 2 can be regenerated in part by the following approaches: (i) mild heating to temperatures below 70 °C, (ii) flowing purging gas over modified amine sorbents at temperatures below 40 °C, (iii) vacuuming at pressure below 0.2 atm and temperatures below 40 °C. The performance of these modified amine sorbents has been tested in a 60 grams VSA unit at ambient temperatures. The results show that weakly adsorbed CO 2 which can be desorbed from modified amine sorbents with purging gas at 100 cc/min can also be evacuated at a vacuum of 1 psia at room temperature. A preliminary techno- economic analysis indicates the major cost of the VSA process with modified amine sorbents stems from the operation of vacuum pumps. Thus, developing an excellent sealing with minimum leakage for the VSA unit is the most critical task for further the development of this VSA technology.

36 MATERIALS SCIENCE↗
NASA NTRS1994

Excitation of the E2 and W1 "Arched" Filaments Near the Galactic Center as Deduced from Far-Infrared Spectroscopy

We present measurements of the far-infrared (FIR) fine structure lines [S III] (33 microns), [Si II] (35 microns), [O III] (51, 88 microns), [O I] (63 microns) and [C II] (158 microns) and the adjacent continua along a scan crossing the E2 and W1 thermal radio filaments in the Galactic center 'Arc'. The deduced electron density and excitation vary along the scan by less than factors of two and three, respectively. The properties of the two filaments are similar: the line and continuum fluxes peak at the radio ridge, and the ridge/off-ridge contrast is greatest for the FIR continuum and ionized lines, lower for the single dish radio measurements (Sofue et al. 1986), and smallest for the low excitation lines. The spatial coincidence of the FIR and radio peaks demonstrates that any excitation mechanism for the radio continuum filaments must also account for the FIR line and continuum emission. The FIR luminosity of approx. 3 x 10(exp 5) Solar Luminosity per beam, and the association of [O III] emission with the filaments poses difficulties for shock and MHD models. Photoionization of molecular cloud edges by a random distribution of stars is the most likely excitation mechanism among those proposed. The continuum and the low excitation line fluxes are consistent with an origin in photodissociated molecular material adjacent to the photoionized gas.

Colgan, Sean W.J.↗
NASA NTRS2001

Beneficial effects of humic acid on micronutrient availability to wheat

Humic acid (HA) is a relatively stable product of organic matter decomposition and thus accumulates in environmental systems. Humic acid might benefit plant growth by chelating unavailable nutrients and buffering pH. We examined the effect of HA on growth and micronutrient uptake in wheat (Triticum aestivum L.) grown hydroponically. Four root-zone treatments were compared: (i) 25 micromoles synthetic chelate N-(4-hydroxyethyl)ethylenediaminetriacetic acid (C10H18N2O7) (HEDTA at 0.25 mM C); (ii) 25 micromoles synthetic chelate with 4-morpholineethanesulfonic acid (C6H13N4S) (MES at 5 mM C) pH buffer; (iii) HA at 1 mM C without synthetic chelate or buffer; and (iv) no synthetic chelate or buffer. Ample inorganic Fe (35 micromoles Fe3+) was supplied in all treatments. There was no statistically significant difference in total biomass or seed yield among treatments, but HA was effective at ameliorating the leaf interveinal chlorosis that occurred during early growth of the nonchelated treatment. Leaf-tissue Cu and Zn concentrations were lower in the HEDTA treatment relative to no chelate (NC), indicating HEDTA strongly complexed these nutrients, thus reducing their free ion activities and hence, bioavailability. Humic acid did not complex Zn as strongly and chemical equilibrium modeling supported these results. Titration tests indicated that HA was not an effective pH buffer at 1 mM C, and higher levels resulted in HA-Ca and HA-Mg flocculation in the nutrient solution.

Non-NASA Center↗
NASA NTRS1983

The 157 micron /C II/ emission from NGC 2024 - Core and halo components

Portions of the nebula NGC 2024 were mapped in the 157 micron (C II) line. Results show the presence of a compact component centered on the radio-recombination line regions and a more extended nebulosity. It is found that the central component emits 157 micron line and continuum fluxes at a level of 4 x 10 to the -16th W/sq cm and 5 x 10 to the -16th W/sq cm (micron), respectively. The continuum emission drops off more rapidly with distance from the peak, although line emission is approximately 30% of the peak value at a distance 18 min to the SE of the peak. In addition, related observations were carried out on M42.

Kurtz, N. T.↗
NASA NTRS2005

Research on Spectroscopy, Opacity, and Atmospheres

I propose to continue providing observers with basic data for interpreting spectra from stars, novas, supernovas, clusters, and galaxies. These data will include allowed and forbidden line lists, both laboratory and computed, for the first five to ten ions of all atoms and for all relevant diatomic molecules. I will eventually expand to all ions of the first thirty elements to treat far UV and X-ray spectra, and for envelope opacities. I also include triatomic molecules provided by other researchers. I have also made CDs with Partridge and Schwenke's water data for work on UV stars. The line data also serve as input to my model atmosphere and synthesis programs that generate energy distributions, photometry, limb darkening, and spectra that can be used for planning observations and for fitting observed spectra. The spectrum synthesis programs produce detailed plots with the lines identified. Grids of stellar spectra can be used for radial velocity-, rotation-, or abundance templates and for population synthesis. I am fitting spectra of bright stars to test the data and to produce atlases to guide observers. For each star the whole spectrum is computed from the UV to the far IR. The line data, opacities, models, spectra, and programs are freely distributed on CDs and on my Web site and represent a unique resource for many NASA programs. I am now in full production of new line lists for atoms. I am computing all ions of all elements from H to Zn and the first 5 ions of all the heavier elements, about 800 ions. For each ion I treat as many as 61 even and 61 odd configurations, computing all energy levels and eigenvectors. The Hamiltonian is determined from a scaled-Hartree-Fock starting guess by least squares fitting the observed energy levels. The average energy of each configuration is used in computing scaled-Thomas-Fermi-Dirac wavefunctions for each configuration which in turn are used to compute allowed and forbidden transition integrals. These are multiplied into the LS allowed and forbidden transition arrays. The transition arrays are transformed to the observed coupling to yield the allowed and forbidden line lists. Results are put on the web as they are finished. Provided I get funding,there will be more than 500 million lines. I will then compare ion by ion, to all the laboratory and computed data in the literature and make up a working line list for spectrum synthesis and opacity calculations with the best available data. As the laboratory spectrum analyses are improved, I will redo the calculations with the new energy levels. My original plan when I started the new calculations was to run through all the atoms using my old Cray programs from the 1980's that were limited to 1100 x 1100 arrays in the Hamiltonian for each J. Then I would go back and rerun the more complicated cases with 3000 x 3000 arrays so that I could include many more configurations and more configuration interactions. At present I am limited to 61 even and 61 odd configurations and I try to include everything up through n = 9. The current program runs on Alpha workstations. I decided to test the big program on Fe I and Fe II to see whether there was any great difference in the low configurations compared to those from the Cray program. Besides increasing the number of E1 lines by a factor of 6 to 7.7 million, there was an unexpected result: the electric quadrupole transitions were 10 times stronger than before because the transition integrals are weighted by r(exp 2) ---they become very large for high n, and because there are numerous configuration interactions that mix the low and high configurations. As a check I was able to reproduce Carstang's (1962) lower results by running his three configurations with my program. Since my model atom is still only a subset of a real Fe II ion, the true quadrupole A values are probably larger than mine. The magnetic dipole lines are affected by the mixing but the overall scale does not change. Because of this scovery I decided that there was no point in computing the small array cases. I have been running with as many configurations as I can and with thousands of parameters in the Hamiltonian. The computer runs take much longer to set up and produce than I had expected. I have concentrated on redoing the low iron group spectra, especially to get data for supernova modelers. I have done only Ca I -- Zn I, Ca II -- Zn II, CU I -- Cu XXIX, Zn I - Zn XXX, for practice at high stages of ionization, C I, C II, S I, and CL I and Ag I for people who were working on the laboratory spectra. Check my web site kurucz.harvard.edu for current additions. My latest calculations have been for carbon I and sulphur I, and silicon I is under way using the same elaborate approach as for C I, which took many months to do. These line lists greatly increase the number of lines in the ultraviolet, in the visible, and especially in the infrared. They will increase the opacity in A, F, and G stars. They will account for many unidentified lines in the sun.

Kurucz, Robert L.↗
NASA NTRS1993

Detection of the N II 122 and 205 micron lines - Densities in G333.6-0.2

Measurements of the G333.6-0.2 H II region which include the first detection of the N II 122 micron forbidden line in an astronomical force and the first measurement of the N II 205 micron forbidden line in a discrete source are presented. Also considered are fine structure lines of forbidden S III, forbidden Fe III, forbidden Si II, forbidden Ne III, forbidden O III, forbidden N III, forbidden O I, and forbidden C II from 19 to 206 microns. It is concluded that the N II 122 and 205 microns forbidden line pair in a discrete astronomical source was detected for the first time. The emission in transitions is produced largely by low-ioninzation, low-density material not easily probed by other lines. Other FIR line pairs generally originate in higher density regions closer to the exciting force.

Colgan, Sean W. J.↗
NASA NTRS1986

IUE absorption studies of broad- and narrow-line gas in Seyfert galaxies

The interstellar medium of a galaxy containing an active nucleus may be profoundly affected by the high energy (X-ray, EUV) continuum flux emanating from the central source. The energetic source may photoionize the interstellar medium out to several kiloparsecs, thereby creating a global H II region. The International Ultraviolet Explorer (IUE) satellite has attempted to observe in several Seyfert galaxies (NGC 3516, NGC 4151, NGC 1068, 3C 120) the narrow absorption lines expected from such global H II regions. Instead, in two of the galaxies (NGC 3516, NGC 4151) broad, variable absorption lines at C IV lambda 1550, N V lambda 1240, and Si IV lambda 1400 were found, as well as weaker absorption features at O I lambda 1302 and C II lambda 1335. These features swamp any possible global H II region absorption. Such broad absorption features have previously been observed in IUE data, but their origin is still not well understood.

Voit, G. M.↗
NASA NTRS1995

Wind Variability of B Supergiants: The Rapid Rotator HD 64760 (B0.5 Ib) - No. 1

We present the results of a 6 day time series of observations of the rapidly rotating B0.5 Ib star HD 64760. We point out several reasons why such intermediate luminosity B supergiants are ideal targets for wind variability studies and then present our results that show the following: continuous wind activity throughout the 6 day run with the wind never in steady state for more than a few hr; wind variability very near nu = 0 km sec(exp -1) in the resonance lines from the lower ionization stages (Al III and C II); a distinct correlation between variability in the Si III ; lambda(lambda)1300 triplets, the strong C III (lambda)1247 singlet, and the onset of extremely strong wind activity, suggesting a connection between photospheric and wind activity; long temporal coherence in the behavior of the strong absorption events; evidence for large-scale spatial coherence, implied by a whole scale, simultaneous weakening in the wind absorption over a wide range in velocities; and ionization variability in the wind accompanying the largest changes in the absorption strengths of the wind lines. In addition, modeling of the wind lines provides the following information about the state the wind in HD 64760. The number of structures on the portion of a constant velocity surface occulting the stellar disk at a particular time must be quite small, while the number on the entire constant velocity surface throughout the wind must be large. The escape probability at low velocity is overestimated by a normal beta approx. 1 velocity law, perhaps due to the presence of low-velocity shocks deep in the wind or a shallow velocity gradient at low velocity. Estimates of the ionization structure in the wind indicate that the ionization ratios are not those expected from thermal equilibrium wind models or from an extrapolation of previous O star results. The large observed q(N V)/q(Si IV) ratio is almost certainly due to distributed X-rays, but the level of ionization predicted by distributed X-ray wind models is inconsistent with the predicted mass-loss rate. Thus, it is impossible to reconcile the observed ionization ratios and the predicted mass-loss rate within the framework of the available models.

Massa, Derck↗
NASA NTRS1986

Space observations of comet Halley

Observations of comet Halley by the Solar Maximum Mission, the International Ultraviolet Explorer, the Pioneer Venus Orbiter, the Dynamics Explorer 1, and by sounding rockets are reviewed. Visual images taken through the wide-band blue filter showed an extended coma elongated approximately in the antisolar direction. Preperihelion observations show definite evidence for both a plasma and a dust tail. The ultraviolet observations indicate that an extensive hydrogen cloud extends to well over 10 billion km from the nucleus. The derived water production rate for the comet showed very large changes around the time of perihelion. The rocket observations showed strong H I, O I, and C I emissions, several CO fourth positive bands, and the C II emissions at 1335 A extending into the tail.

Brandt, J. C.↗
NASA NTRS1994

The Hubble Space Telescope quasar absorption line key project. 6: Properties of the metal-rich systems

We present an analysis of the properties of a sample of 18 metal-rich, low-redshift z(sub abs) much less than z(sub em) absorbers seen in low- and medium-resolution spectra obtained for the Quasar Absorption Line Key Project with the Hubble Space Telescope Faint Object Spectrograph (HST/FOS). For most of the C IV and Lyman-limit systems, observations in the optical wavelength range of the expected associated Mg II absorption are available. As at high redshift (z approximately 2), there are two subclasses of absorbers which are characterized by the presence or absence of MG II absorption. However, some low-redshift Mg II and Fe absorptions originate from regions optically thin to UV ionizing photons and thus, at low redshift, the low-ionization systems do not always trace high opacities, as is the case at high redshift. This implies that the mean ionization state of metal-rich, optically thin absorbing clouds falls with decreasing redshift, which is consistent with the hypothesis that the gas is photoionized by the metagalactic UV background radiation field. Two main constraints are derived from the analysis of the Lyman-limit sample, assuming photoionization models are valid. First, a low opacity to ionizing photons (tau(sub LL) approximately less than 1), as observed for several Mg II-Fe II systems at z approximately 0.5, sets limits on the ionization level of hydrogen, thus on the total hydrogen column density and the heavy element abundances, (Z/H) approximately -0.5 to -0.3. Second, the dimensions of individual Mg II clouds are smaller than at high redshift by a factor 3-10. At z approximately greater than 0.6, the O VI absorption doublet is detected in four of the five z(sub abs) much less than z(sub em) systems for which the O VI wavelength range has been observed, whereas the associated N V doublet is detected in only two cases. This suggests that the presence of a high-ionization O VI phase is a general property of z approximately 0.6-1 absorption systems, as is also probably the case at high redshift. These O VI absorbers can be ionized by the UV metagalactic field if their density is low, nH approximately less than 3 x 10(exp -4)/cc. The O VI phase would then be a homogeneous region of large extent, r approximately greater than 50 kpc. A detailed photoionization model of the z(sub abs) = 0.791 absorber toward PKS 2145+06 confirms the properties derived from the Mg II, C IV, O VI, and Lyman-limit samples. The galaxy causing this extensive metal-line absorption system has been identified, and its possible contribution to the UV ionizing flux does not substantially modify the value of the derived parameters. The heavy element abundances are about half the solar values. The O VI region has a density about 20 times lower than the Mg II clouds and a size of approximately 70 kpc. Alternatively, the high-ionization phase could be collisionally ionized and trace gas associated with a possible group of galaxies at the absorber redshift.

Bergeron, Jacqueline↗
NASA NTRS1994

Composition of interstellar clouds in the disk and halo. 2: Gamma(sup 2) Velorum

High-resolution observations of gamma(sup 2) Vel with the Goddard High-Resolution Spectrograph (GHRS) echelle on the Hubble Space Telescope reveal the presence of seven narrow absorption components, with LSR velocities between -23 and +9 km s(exp -1). Three of these show column density ratios N(S(++))/N(S(+)) and N(P(++))/N(P+)) of about 1 or more, and can be identified as H II regions, while the other four are H I regions, consistent with the O I profile and with the overall H(sup 0) column density of 5.9 x 10(exp 19) cm(exp -2), given the usual assumptions that S is undepleted while O has a depletion D(O) = -0.3 dex. The depletions of Fe, Si, and Mn, which could be measure accurately for two of the four H I regions (components 6 and 7), differ somewhat from the values of D(sub ws) found for slowly moving warm clouds in HD 93521; in particular, for the component at 4.0 km s(exp -1) (No. 6), abosolute of D exceeds absolute of D(sub ws) by 0.1-0.4 dex, while for that at 9.3 km s(exp -1) (No. 7), absolute of D equals absolute of D(sub ws) on the average. The observed ratio of Fe + Mg atoms to Si atoms in the grains of component 6 is 2.04 +/-0.10, consistent with an olivine grain composition; the Fe/Mg ratio is 1.5 +/- 0.2. The electron density in component 6, determined from the C II(sup *) feature, is 0.075 +/- 0.013 cm (exp -3), about two-thirds of that found for clouds of this velocity in HD 93521. In the two conspicuous H II regions, components 3 and 4, n(sub e), determined from the Si II(sup *) feature, is about 1 cm(exp -3). From the column density of S(+) + S(++) in these two components, the total H II path length is about 40 pc. With the radius of a wind-blown bubble around gamma(sup 2) Vel set equal to 60 pc, the effective Stromgren radius is about 100 pc, requiring that T approx. equal to 50,000 K for the Wolf-Rayet component of the gamma(sup 2) Vel binary. Since zeta Pup is a comparable source of ionizing radiation, this temperature is an upper limit. The profiles of the strongest H2 absorption features, from Copernicus archives, indicate that the absorbing molecules have a mean velocity identical with that of the strongest H II component (No. 4). We have no explanation for the possible presence of these H2 molecules in a region of ionized H. Alternatively, the H2 profiles can be explained by molecules in the two adjacent (in velocity) H I regions, components 2 and 5, provided their H I gas has densities and temperatures typical of normal cold clouds. The GHRS data show absorption by highly ionized atoms Si(3+) and C(3+), N(4+) in broad features, in addition to the narrow-line absorption by Si(3+) and C(3+) observed in the dominant H II components, Nos. 3 and 4. The broad C(3+) and N(4+) features have widths corresponding to T in the range (4-8) x 10(exp 5) K, consistent with the broad O(5+) line shown in Copernicus data. Despite some observational uncertainties, the ratios of column densities in the broad C(3+), N(4+), and O(5+) features agree to +/- 0.1 dex with theoretical values for warm gas, heating and evaporating by thermal conduction from an adjacent hot region. Outward evaporation from an isolated cloud in a hot ambient gas cannot be distinguished, on the basis of these data, from inward evaporation of a warm shell, compressed by an expanding, hot stellar-wind bubble. For several halo stars, the C IV/O VI ratio has a quite different average value, perhaps consistent with cooling of infalling hot gas instead of conductive heating and evaporation.

Fitzpatrick, Edward L.↗
NASA NTRS1991

Interstellar gas phase abundance of carbon, oxygen, nitrogen, copper, gallium, germanium, and krypton toward Zeta Ophiuchi

An analysis of weak (less than 10 mA) UV interstellar absorption line data obtained for the line of sight to the O9.5 IV star Zeta Oph is presented. Measurements of weak semiforbidden lines of N I, O I, Cu II, and a new UV detection of Na I are reported along with a small upper limit for C II. Interstellar detections of Ga II, Ge II, and Kr I are also presented. Ga, Ge, and Kr represent the heaviest elements detected in the ISM. A comparison of the derived column densities to cosmic abundances shows Ga to be depleted by about -1.2 dex while Ge is overabundant by +0.2 dex. Assuming Kr to be undepleted, a logarithmic cosmic abundance of Kr/H = 2.95 is obtained on the scale where H = 12.00.

Cardelli, Jason A.↗
NASA NTRS1995

Winds and accretion in delta Sagittae

The ten-year binary delta Sge (M2 Ib-II+B9.5 V) is a zeta Aur binary containing an abnormally cool component. Combining our analysis of the system as a visual binary with Batten's radial-velocity solution leads to the following properties: i = 40 deg, a = 51 mas = 8.83 A.U. = 1893 solar radius, hence d = 173 pc; M(sub B) = 2.9 solar mass and M(sub M) = 3.8 solar mass; and R(sub B) = 2.6 solar radius and R(sub M) = 152 solar radius. This interpretation of the orbit places the M supergiant on the asymptotic giant branch. We have collected ultraviolet spectra throughout the star's 1980-90 orbit, concentrated around the conjuction of 1990. The wind of the M giant appears in these as narrow shell lines of singly ionized metals, chiefly Fe II, with P-Cyg profiles at many phases, which show the slow variation in strength expected for the orbit but no pronounced atmospheric eclipse. The terminal velocity of the wind is 16-18 km/s, and its excitation temperature is approximately 10,000 K. Most of the broadening of the wind lines is caused by differential expansion of the atmosphere, with (unmeasurably) low turbulent velocities. Nontheless, the mass loss rate (1.1 +/- 0.4 X 10 (exp -8) solar mas/yr) is almost the same as found previously by Reimers and Schroder for very different assumptions about the velocity structure. Also seen in the spectrum throughout the orbit are the effects of a variable, high-speed wind as well as evidence for accretion onto the B9.5 star. This high-speed wind absorbs in species of all ionization stages observed, e. g., C II, Mg II, Al III, SI IV, C IV, and has a terminaal velocity in the range 200-450 km/s. We presume this wind originates at the B dwarf, not the M supergiant, and speculate that it comes from an accretion disk, as suggested by recent models of magnetically moderated accretion. Evidence for accretion is redshifted absorption in the same transitions formed in the high-speed wind, as well as broad emission lines of singly ionized metals. This emission seems to be scattered out of the continuum of the B star. Finally, we discuss Auger ionization by accretion luminosity as the cause of the high ionization in these high-speed flows as well as the source of the extended circumstellar clouds of Si(3+) and C(3+) ions in zeta Aur binaries.

Eaton, Joel A.↗
NASA NTRS1982

Chromospheres and coronae in the T Tauri stars

The Tauri stars exhibit strong far ultraviolet emission lines of C II, III, IV; OI; Si II, III; IV; and sometimes N V and He II. Surface fluxes of the lines for several T Tauri stars were computed, drawing upon both International Ultraviolet Explorer observations and published spectra. The surface fluxes are quite high. The FUV lines together account for 0.1% of the stellar luminosity. These results indicate the presence of active relatively dense chromospheres. The Tauri stars showing very strong visual emission spectra exhibit weakened high temperature FUV lines of C IV and N V, as well as He II which may be produced by X-rays. In the same stars, no X-ray emission was detected with Einstein. Among all the T Tauri stars the X-ray luminosities are deficient by factors of 100 to 1000 compared to the FUV lines. It is argued that the X-ray flux has not been heavily absorbed by circumstellar gas, as has been previously suggested, but that the X-rays are truly underluminous, perhaps due to the stellar wind. It is suggested that the birth and development of the chromosphere and corona may be occurring during the T Tauri stage of protostellar evolution.

Imhoff, C. L.↗
NASA NTRS2023

Primordial or Secondary? Testing Models of Debris Disk Gas with ALMA

The origin and evolution of gas in debris disks are still not well understood. Secondary gas production from cometary material or a primordial origin have been proposed. So far, observations have mostly concentrated on CO, with only a few C observations available. We overview the C and CO content of debris disk gas and test the state of the-art models. We use new and archival Atacama Large Millimeter/submillimeter Array (ALMA) observations of CO and C I emission, complemented by C II data from Herschel, for a sample of 14 debris disks. This expands the number of disks with ALMA measurements of both CO and C I by 10 disks. We present new detections of C I emission toward three disks: HD 21997, HD 121191, and HD 121617. We use a simple disk model to derive gas masses and column densities. We find that current state-of-the-art models of secondary gas production overpredict the C 0 content of debris disk gas. This does not rule out a secondary origin, but might indicate that the models require an additional C removal process. Alternatively, the gas might be produced in transient events rather than a steady-state collisional cascade. We also test a primordial gas origin by comparing our results to a simplified thermochemical model. This yields promising results, but more detailed work is required before a conclusion can be reached. Our work demonstrates that the combination of C and CO data is a powerful tool to advance our understanding of debris disk gas.

Radiative transfer↗
NASA NTRS1993

Galactic fine-structure lines - Morphologies of the warm ionized interstellar medium

Far-infrared ionic fine-structure emission lines associated with the large-scale structure of the Galactic ionized medium are interpreted in terms of a heuristic model in which three distinct source morphologies are considered: ionization-bounded H II regions, an extended, low-density medium, and externally ionized surfaces of neutral clouds. The model reveals that the overall luminosity of the Galaxy in the fine-structure lines of both C II and N II is due, in comparable proportions, to diffuse medium and the boundaries between ionized and neutral regions. Limits are set on the luminosity of the diffuse component in the plane of the Galaxy. Some individual lines of sight of the COBE FIRAS instrument are dominated, in these spectral lines, by H II regions, even with its 7 deg field of view. Average densities of both H II regions and photodissociation regions are estimated on a Galactic scale.

Petuchowski, S. J.↗