Short Wavelength Spectrometer Research Articles

Infrared Spectroscopy of NGC 4151: Probing the Obscured Ionizing Active Galactic Nucleus Continuum

The Infrared Space Observatory (ISO) Short-Wavelength Spectrometer (SWS) infrared spectroscopic observations of the nucleus of Seyfert galaxy NGC 4151, which are described in a companion paper, are used in conjunction with a compilation of UV to IR narrow emission line data to determine the spectral shape of the obscured extreme-UV continuum that photoionizes the narrow-line-emitting gas in the active galactic nucleus (AGN). We present a new method for determining the best-fitting photoionizing continuum and emission-line cloud model from a heterogeneous set of emission-line data. For NGC 4151, we find a best-fit model that reproduces the observed line fluxes to within a factor of 2 on average, and that is consistent with the observed geometry of the optical narrow-line region (NLR). Our model consists of a clumpy, optically thick (ionization-bounded) gas distribution, with a hydrogen gas density of ~1000 cm-3 and a volume-filling factor of 6.510-4. Our best-fitting spectral energy distribution (SED) falls sharply beyond the Lyman limit and then rises sharply again toward 100 eV. In particular, it does not display the blue signature of a hot accretion disk. We find that this SED, which best reproduces the NLR line emission on the 100-500 pc scale, does not have enough UV photons to produce the observed broad-line region (BLR) recombination emission from the inner 1 pc. This suggests that the BLR is photoionized by the intrinsic continuum source, which does have a strong UV component (perhaps a big blue bump), but that this UV component is absorbed by material located between the NLR and BLR. Our analysis suggests that the absorber consists of ~51019 cm-2 of neutral hydrogen. Such an absorber has been independently detected by UV absorption lines. Using our new method, we confirm our previous conclusion that a big blue bump is present in the SED of the Seyfert 2 galaxy Circinus.

New Insights into the Large 1992 July 15–17 Flare on AU Microscopii: The First Detection of Posteruptive Energy Release on a Red Dwarf Star

Extreme Ultraviolet Explorer (EUVE) observations of the long-duration 1992 July 15-17 flaring event on the red dwarf star AU Mic are discussed. The flare was detected in the EUVE Deep Survey (DS) Lexan/B band (65-190 Å) and was made up of a large impulsive event corresponding to an increase in the extreme ultraviolet (EUV) emission by a factor of 15 relative to quiescence, followed by a slow exponential decay interspersed with another smaller impulsive event and further decay. As observed in the EUV, the flare lasted for more than half a day. A similar exponential decay of the flux in the Fe XVIII λ93.9 line was detected in the EUVE short wavelength (SW) spectrometer following the first powerful impulsive phase. The flare decay in the 65-190 Å band and in the Fe XVIII line during this prolonged event lasted a factor of 10 or more longer than would have been the case had the plasma cooling been due only to radiative losses at typical flare plasma densities. An analysis of the simultaneous DS Lexan/B data and the Fe XVIII line flux has allowed us to determine the approximate temporal behavior of the flaring plasma temperature (d log T/dt ≈ -0.5 day-1) and of the emission measure of the EUV source. The total energy emitted in the 1-2000 Å range during the first 12 hours or so of the flaring event was 3 × 1035 ergs. Solar analogs of processes providing similar long-duration soft X-ray radiation are discussed: (1) a coronal mass ejection (CME); (2) a system of giant postflare arches, which can form in powerful solar flares after a CME; (3) typical soft X-ray flaring loops. A general expression describing the energy balance in the EUV source is derived for both CMEs and giant arch systems. As a result, we conclude that the long-duration EUV emission in the AU Mic flare most resembles the solar scenario of emission from a system of giant coronal arches forming after a CME. Such a system on AU Mic would have a size of 1-2 stellar radii and would be energized by reconnection in a vertical current sheet. In this picture, a substantial part of the radiative loss is compensated for by additional postflare energy input, in a similar way as is thought to take place in the posteruptive phases of the most powerful solar flares. Apparently, we are faced here with a new kind of surface activity on late-type stars, which is intermediate between impulsive flares on red dwarf stars and the long-duration, powerful events seen on subgiant components of the RS CVn binaries.

An ISO View on Interstellar and Cometary Ice Chemistry

The ISO-SWS instrument offering a large wavelength coverage and a resolution well adapted to the solid phase has changed our knowledge of the physical-chemical properties of ices in space. The discovery of many new ice features was reported and the comparison with dedicated laboratory experiments allowed the determination of more accurate abundances of major ice components. The presence of CO2 ice has recently been confirmed with the SWS (Short Wavelength Spectrometer) as a dominant ice component of interstellar grain mantles. The bending mode of CO2 ice shows a particular triple-peak structure which provides first evidence for extensive ice segregation in the line-of-sight toward massive protostars. A comparison of interstellar and cometary ices using recent ISO data and ground-based measurements has revealed important similarities but also indicated that comets contain, beside pristine interstellar material, admixtures of processed material. The investigation of molecules in interstellar clouds is essential to reveal the link between dust in the interstellar medium and in the Solar System.

The stellar content of the Wolf-Rayet galaxy NGC 5253 from ISO-SWS spectroscopy

Wolf-Rayet galaxies are a subset of blue emission-line galaxies, whose spectra show the signature of large numbers of WR stars, with ages in the range 1-10 x 106 yr. Mid infra-red observations are well suited to the study of their hot, massive stars since this spectral region contains many fine-structure nebular lines which depend very sensitively on stellar content. The observations of ISO have opened-up this window, which we are exploiting through a Guest Observer program with the Short Wavelength Spectrometer (sws), that includes NGC 5253, a nearby (4 Mpc) WR galaxy. NGC 5253 contains several young super-star clusters, no older than a few million years and has been the focus of many recent studies which have shown that the extinction is very high and patchy in the UV and optical. ISO observations of NGC 5253 are particularly important since the interstellar extinction at mid-IR wavelengths is low and because of its remarkably hot and young stellar population.

ISOSWS Spectral Variations of Oxygen-Rich Miras

In conjunction with an extensive ground-based monitoring program of 32 oxygen-rich Mira variables, a subset of this sample has been monitored with phase using ISO's Short Wavelength Spectrometer (ISOSWS) and ground-based mid-infrared spectrometers. Some of the six sources will be presented here in 2.5–45 μm ISOSWS spectra and 7.5–13.5 μm ground-based spectra using CGS3. Discussion of the spectral features of SiO, CO, H2O, and silicates will be presented. These data are considered in the context of recent demonstrations of variations in the spectral features with phase of the Miras.

The 14 μm Band of Carbon Stars

We have studied the absorption bands around 14 μm in the spectra of 11 carbon stars with mass-loss rates ranging from 10−8 to 10−4 M⊙ yr−1, based on data obtained with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). All stars clearly show a C2H2 absorption band peaking at 13.7 μm, while the contribution from HCN molecules is small in this wavelength region. A simple plane-parallel LTE model with two layers at different temperatures is used to derive the C2H2 abundances in the outer photosphere and in the circumstellar envelope. We find that (1) the column density of the hot-layer, placed at about 3R* with a temperature of 1400 K is roughly the same for all stars regardless of the mass-loss rate, and (2) the contribution of cool molecules in the circumstellar envelope increases with the dust mass-loss rate, (3) the abundance of C2H2 in the two layers is about the same, i.e. no obvious depletion of C2H2 molecules seems to occur in the circumstellar envelope.

30 Doradus as a Nearby Infrared Guide to Starbursts

We are exploring the properties of obscured starburst galaxies, using observations of atomic fine structure lines taken with the Short Wavelength Spectrometer aboard ISO. However, it is important to ascertain how well our starburst models can recover the properties of the stellar populations in more distant starbursts. For this purpose, we use observations of a nebular “shell” in the 30 Doradus region, to show that our models reliably predict the presence of the very massive stars observed directly in the 30 Doradus region.

ISO spectroscopy of nearby starburst galaxies

We present spectroscopic mid-IR observations of prominent starburst galaxies obtained with the Short Wavelength Spectrometer onboard the Infrared Space Observatory. The wavelength range accessible by ISO-SWS (2.5–45 μm) contains a large number of emission lines of atomic and molecular hydrogen and several other atomic species. In this paper we discuss the interpretation of the [Ne III] 15.5 μm/[Ne II] 12.8 μm line-ratio, the faint [OIV]25.9 μm line and the pure rotational lines of molecular hydrogen observed in our target galaxies.

Neon abundances in three Wolf-Rayet stars observed with the ISO Short Wavelength Spectrometer

We are motivated to study the Ne/He abundance ratio at the surfaces of Wolf-Rayet stars by evolutionary models, which have predicted enhancements by more than an order of magnitude over cosmic levels during the WC stage (e.g., Maeder 1991). This should occur early in the WC phase, as core-hydrogen is exhausted and the CNO bi-cycle gives way to the following chain of reactions for the Galactic stars: The latest evolutionary models, which include the effects of interior mixing due to stellar rotation, indicate that the enhancement may be somewhat more gradual, but that the Ne/He ratio is essentially unchanged in its peak value, coinciding with complete H-depletion (e.g., Meynet, these Proceedings).

ITAL]Infrared Space Observatory[/ITAL] Observations of Molecular Hydrogen in HH 54:Measurement of a Nonequilibrium Ratio of Ortho- to Para-H[TINF]2[/TINF

We have detected the S(1), S(2), S(3), S(4), and S(5) pure rotational lines of molecular hydrogen toward the outflow source HH 54 using the Short Wavelength Spectrometer on board the Infrared Space Observatory. The observed H2 line ratios indicate the presence of warm molecular gas with an H2 density of at least 10(exp 5) cm(exp -3) and a temperature approximately 650 K in which the ratio of ortho- to para-H2 is only 1.2 +/- 0.4, significantly smaller than the equilibrium ratio of 3 expected in gas at that temperature. These observations imply that the measured ratio of ortho- to para-H2 is the legacy of an earlier stage in the thermal history of the gas when the gas had reached equilibrium at a temperature approximately less than 90 K. Based upon the expected timescale for equilibration, we argue that the nonequilibrium ratio of ortho- to para-H2 observed in HH 54 serves as a chronometer that places a conservative upper limit of approximately 5000 yr on the period for which the emitting gas has been warm. The S(2)/S(1) and S(3)/S(1) H2 line ratios measured toward HH 54 are consistent with recent theoretical models of Timmermann for the conversion of para- to ortho-H2 behind slow, C-type shocks, but only if the preshock ratio of ortho- to para-H2 was approximately less than 0.2.

Radiative Transfer Models of Emission and Absorption in the H[TINF]2[/TINF]O 6 Micron Vibration-Rotation Band toward Orion-BN-KL

We report the spectrum of Orion-BN/KL between 5.3 and 7.2 μm observed with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory. H2O lines of the ν2=1-0 bending mode with λ 6.3 μm (the P-branch) are observed in emission. Radiative transfer models including the (0, 0, 0), (0, 1, 0), and (0, 0, 1) vibrational levels show that this effect is produced in a natural way by H2O absorption of 6 μm continuum photons followed by spontaneous de-excitation to the ground state (resonant scattering). The intensities of the absorption lines can be explained with an H2O column density of a few 1017 cm-2 in gas with a temperature of ~150 K and n(H2)~106 cm-3, although these parameters depend on the assumed location of the absorbing shell. Since the observed intensity of the H2O emission lines are larger than those seen in absorption, we suggest that rovibrational excitation of H2O by collisions in denser and hotter gas, such as found in shocks, also contribute to the P-branch emission of the 6 μm H2O band.

The [ITAL]ISO[/ITAL]–SWS 2.4–45.2 Micron Spectrum toward Orion IR[CLC]c[/CLC

The complete infrared spectrum from 2.4 to 45.2 μm toward the prototypical massive star-forming region Orion IRc2 is presented, obtained with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO) at a resolving power λ/Δλ ≈ 1300-2500. A wealth of emission and absorption features is found, including H2 vibration-rotation lines, the full set of H2 pure rotational lines (0,0) S(1)-S(17), H recombination lines, ionic fine-structure lines, PAH emission features, and absorption and emission bands by interstellar ices and gas-phase molecules, including CO2, CH4, and SO2. Particularly interesting is the detection of strong emission and absorption lines in the H2O ν2 bending mode at 6.2 μm and the observation of highly excited pure rotational lines of H2O in absorption at 25-45 μm. The origin of these lines in each of the physical components included in the ISO-SWS beam (H II region, PDR, quiescent ridge, shocked low-velocity plateau) is briefly discussed.

The H[CLC]e[/CLC][TSUP]+[/TSUP]/H[TSUP]+[/TSUP] Abundance in the Orion Nebula from [ITAL]Infrared Space Observatory[/ITAL] Measurements

Using the Short Wavelength Spectrometer on the Infrared Space Observatory, we measured in the Orion Nebula several components of the He I Brα transition, the first detections to the best of our knowledge in an H II region, and the H I Brα line. A value of He+/H+=0.085±0.003 is inferred from these data and is typical of previous values found for Orion. The IR method to obtain He+/H+ is very insensitive to extinction. The measurement of the weak He I 2.855 μm (5p 3Po→4s 3S) flux permits an assessment of Te in the He+ region. Its flux relative to the flux of He I 4.0490 μm (5g 3G→4f 3Fo, 5g 1G→4f 1Fo blend) is roughly ∝Te (for Te range applicable to most H II regions and planetary nebulae) and depends little on Ne. From our measured ratio, using Ne = 104 cm-3, Te is slightly less than 104 K. We stress that many IR measurements of a hydrogen line are likely to also include one or more significant helium line contributions. Attributing all of the flux to the stronger H I line will result in underestimates for abundance ratios relative to H+. This has usually not been taken into account in the past when deriving abundances of other ions/elements relative to H. Errors in the velocity and line width inferred may also occur by failure to consider the blended He component. Because the strongest He I lines will generally be on the blue side of their counterpart H I line, interpreting a blended feature as only H I will cause a spurious bias to lower velocities.

ISO observations of interstellar ices

The first spectroscopic results from ISO (Infrared Space Observatory, Kessler et al. 1996) have revealed a wealth of interesting features and in particular absorption signatures of a wide variety of solid state molecular species. We present here some new ISO data obtained with SWS (Short Wavelength Spectrometer) toward the young deeply embedded object associated with RAFGL 7009S. Signatures of H 2O, CO, CO 2 ices can be readily identified but also some much less abundant species such as 13 CO 2, H 2 CO and CH 4, all occurring in the near and mid-infrared region between 3 and 16 μm. The detection of CO 2 at 4.27 and 15.2 μm confirms its presence in the IRAS-LRS spectra of several heavily absorbed sources. The very high extinction toward RAFGL 7009S makes it an excellent case to study other weak solid state absorption features, commonly measured in laboratory experiments.

First Results and Discoveries with the ISO Short-Wavelength Spectrometer

AbstractThe Short-Wavelength Spectrometer(SWS) is one of the four instruments on-board of ESA’s Infrared Space Observatory(ISO), launched on 15 Nov. 1995. It covers the wave-length range of 2.38-45.2μm with a spectral resolution ranging from 1000-2000. An overview is given of the in-orbit performance and a summary of the main scientific results and discoveries obtained in the first 18 months of operation of the ISO-SWS.

Infrared Spectroscopy of Dust in the Diffuse Interstellar Medium toward Cygnus OB2 No. 12

Observations made with the short-wavelength spectrometer of the Infrared Space Observatory are used to investigate the composition of interstellar dust in the line of sight to Cygnus OB2 No. 12, commonly taken as representative of the diffuse (low-density) interstellar medium. Results are compared with data for the Galactic center source Sgr A*. Nondetections of the 3.0 and 4.27 μm features of H2O and CO2 ices in Cyg OB2 No. 12 confirm the absence of dense molecular material in this line of sight, whereas the presence of these features in Sgr A* indicates that molecular clouds may contribute as much as 10 mag of visual extinction toward the Galactic center. The spectrum of Cyg OB2 No. 12 is dominated by the well-known 9.7 μm silicate feature; detection of a shallow feature near 2.75 μm indicates that the silicates are at least partially hydrated, with composition possibly similar to that of terrestrial phyllosilicates such as serpentine or chlorite. However, the 2.75 μm feature is not seen in the Galactic center spectrum, suggesting that silicates in this line of sight are less hydrated or of different composition. The primary spectral signatures of C-rich dust in the diffuse ISM are weak absorptions at 3.4 μm (the aliphatic C=H stretch) and 6.2 μm (the aromatic C=C stretch). We conclude, based on infrared spectroscopy, that the most probable composition of the dust toward Cyg OB2 No. 12 is a mixture of silicates and carbonaceous solids in a volume ratio of approximately 3:2, with the carbonaceous component primarily in an aromatic form such as amorphous carbon.

New Wavelength Determinations of Mid‐Infrared Fine‐structure Lines byInfrared Space ObservatoryShort Wavelength Spectrometer

We report accurate new wavelengths for 29 mid-infrared ionic fine-structure lines, based on observations with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). Our results originate from observations of NGC 7027, NGC 6543, NGC 6302, the Circinus galaxy, Sgr A West, and W51 IRS 2. The obtained accuracies (λ/Δλ) range from 3 × 104 to 1 × 105, depending on instrumental mode and uncertainty in radial velocities.

ISO-SWS Spectral Characterization of Stars

Spectral classification in the infrared is necessary to complete the spectral classification in the visual and to understand the physics and evolution of stars. A major contribution was made by the IRAS Low Resolution Spectrometer, which allowed a characterization of the sources according to both spectral features and general shape of the energy distribution in the 8–22 µ,m (IRAS Science Working group, 1986). However, spectra obtained with the ISO Short Wavelength Spectrometer (SWS) (de Graauw et al. 1996, Kessler et al. 1996) provide the basis for a more complete data-base. Compared with LRS, SWS spectra cover a wider flux range and a wider spectral range 2.4–44.5 µm at a significantly higher spectral resolution. This work is part of an extensive ISO dedicated time program STARTYP1/2, which aims to provide a comprehensive spectral classification scheme of over 700 of the brightest representative infrared sources.

Detection of thermal water vapor emission from W Hydrae

We have detected four far-infrared emission lines of water vapor toward the evolved star W Hydrae, using the Short Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO). This is the first detection of thermal water vapor emission from a circumstellar outflow.

Abundant, hot water toward young stellar objects

The large amount of water vapour in the Earth's atmosphere makes it extremely difficult to observe water in interstellar space, except under unusual conditions (e.g., masers). The Short Wavelength Spectrometer (SWS; de Graauw et al. 1996) on board the Infrared Space Observatory (ISO) provides an unique opportunity to study the ro-vibrational fundamental bands of water in absorption against bright infrared continuum sources. Such absorption line data have several advantages over emission observations. For example, only a pencil beam to the infrared source is probed, which minimizes the complications due to the small-scale physical and chemical heterogeneity of star-forming regions. In addition, lines from all rotational levels are present within a vibrational band, so that the complete spectrum yields a direct estimate of the level populations and excitation temperature along the line of sight. Other molecules, such as CO, HCN and C2H2, can be observed with the same technique, leading to accurate relative abundances. The main limitation of the ISO-SWS grating is its low spectral resolving power, λ/Δ λ ≈ 1350 at 6 μm, since the lines are intrinsically much narrower. This limits the sensitivity, so that only high column densities can be detected, and complicates the interpretation because detectable lines are often saturated. The low spectral resolution also blends the lines, making it difficult to determine the equivalent widths unambiguously. The ISO-SWS is therefore particularly sensitive to molecules in warm gas with relatively large line widths (Δ V > 3 km s−1).