Nebular Surface Research Articles

Nebular C iv λ1550 Imaging of the Metal-poor Starburst Mrk 71: Direct Evidence of Catastrophic Cooling

We use the Hubble Space Telescope Advanced Camera for Surveys to obtain the first spatially resolved, nebular imaging in the light of C iv λ λ1548, 1551 by using the F150LP and F165LP filters. These observations of the local starburst Mrk 71 in NGC 2366 show emission apparently originating within the interior cavity around the dominant super star cluster (SSC), Knot A. Together with imaging in He ii λ4686 and supporting Space Telescope Imaging Spectrograph far-ultraviolet spectroscopy, the morphology and intensity of the C iv nebular surface brightness and the C iv/He ii ratio map provide direct evidence that the mechanical feedback is likely dominated by catastrophic radiative cooling, which strongly disrupts adiabatic superbubble evolution. The implied extreme mass loading and low kinetic efficiency of the cluster wind are reasonably consistent with the wind energy budget, which is probably enhanced by radiation pressure. In contrast, the Knot B SSC lies within a well-defined superbubble with associated soft X-rays and He ii λ1640 emission, which are signatures of adiabatic, energy-driven feedback from a supernova-driven outflow. This system lacks clear evidence of C iv from the limb-brightened shell, as expected for this model, but the observations may not be deep enough to confirm its presence. We also detect a small C iv-emitting object that is likely an embedded compact H ii region. Its C iv emission may indicate the presence of very massive stars (>100 M ⊙) or strongly pressure-confined stellar feedback.

Erratum to Thermal and hydrostatic structure of the protoplanetary nebula exposed to stellar radiation and stellar wind from the central star

A model for a nebula exposed to the radiation and stellar wind from the central star has been examined. T Tauri stars commonly have disks and stellar wind, though we have no theoretical model on nebular thermal structure under the wind. The aims of this paper are to propose a theoretical nebula model, and to perform mathematical analysis on the geometrical structure and temperature distribution of a passive nebula. Its geometrical surface is determined by the dynamical pressure of the stellar wind. The nebular surface is assumed to be a black body surface, heated by the incident stellar photon flux. We obtain temperature from the equation of energy balance between the stellar radiation upon this nebular surface and the black body radiation from it. The temperature distribution in passive disks is insensitive to the wind strength and is almost identical even if the wind strength changes by five orders of magnitude. The nebula temperature is not expressed by a simple power law function of the distance from the central star. This is an important difference between our results and those of previous works. Since the nebula surrounding a T Tauri star is influenced by stellar wind, our model may be more appropriate than any other simple single power law temperature for passive protoplanetary nebulae.

Timescales for the evolution of oxygen isotope compositions in the solar nebula

We review two models for the origin of the calcium-, aluminum-rich inclusion (CAI) oxygen isotope mixing line in the solar nebula: (1) CO self-shielding, and (2) chemical mass-independent fractionation (MIF). We consider the timescales associated with formation of an isotopically anomalous water reservoir derived from CO self-shielding, and also the vertical and radial transport timescales of gas and solids in the nebula. The timescales for chemical MIF are very rapid. CO self-shielding models predict that the Sun has Δ 17O SMOW ∼ −20‰ (Clayton, 2002), and chemical mass-independent fractionation models predict Δ 17O SMOW ∼0‰. Preliminary Genesis results have been reported by McKeegan et al. (McKeegan K. D., Coath C. D., Heber, V., Jarzebinski G., Kallio A. P., Kunihiro T., Mao P. H. and Burnett D. S. (2008b) The oxygen isotopic composition of captured solar wind: first results from the Genesis. EOS Trans. AGU 89(53), Fall Meet. Suppl., P42A-07 (abstr)) and yield a Δ 17O SMOW of ∼ −25‰, consistent with a CO self-shielding scenario. Assuming that subsequent Genesis analyses support the preliminary results, it then remains to determine the relative contributions of CO self-shielding from the X-point, the surface of the solar nebula and the parent molecular cloud. The relative formation ages of chondritic components can be related to several timescales in the self-shielding theories. Most importantly the age difference of ∼1–3 My between CAIs and chondrules is consistent with radial transport from the outer solar nebula (>10 AU) to the meteorite-forming region, which supports both the nebular surface and parent cloud self-shielding scenarios. An elevated radiation field intensity is predicted by the surface shielding model, and yields substantial CO photolysis (∼50%) on timescales of 0.1–1 My. An elevated radiation field is also consistent with the parent cloud model. The elevated radiation intensities may indicate solar nebula birth in a medium to large cluster, and may be consistent with the injection of 60Fe from a nearby supernova and with the photoevaporative truncation of the solar nebula at KBO orbital distances (∼47 AU). CO self-shielding is operative at the X-point even when H 2 absorption is included, but it is not yet clear whether the self-shielding signature can be imparted to silicates. A simple analysis of diffusion times shows that oxygen isotope exchange between 16O-depleted nebular H 2O and chondrules during chondrule formation events is rapid (∼minutes), but is also expected to be rapid for most components of CAIs, with the exception of spinel. This is consistent with the observation that spinel grains are often the most 16O-rich component of CAIs, but is only broadly consistent with the greater degree of exchange in other CAI components. Preliminary disk model calculations of self-shielding by N 2 demonstrate that large δ 15N enrichments (∼ +800‰) are possible in HCN formed by reaction of N atoms with organic radicals (e.g., CH 2), which may account for 15N-rich hotspots observed in lithic clasts in some carbonaceous chondrites and which lends support to the CO self-shielding model for oxygen isotopes.

Thermal and hydrostatic structure of the protoplanetary nebula exposed to stellar radiation and stellar wind from the central star

Abstract A model for a nebula exposed to the radiation and stellar wind from the central star has been examined. T Tauri stars commonly have disks and stellar wind, though we have no theoretical model on nebular thermal structure under the wind. The aims of this paper are to propose a theoretical nebula model, and to perform mathematical analysis on the geometrical structure and temperature distribution of a passive nebula. Its geometrical surface is determined by the dynamical pressure of the stellar wind. The nebular surface is assumed to be a black body surface, heated by the incident stellar photon flux. We obtain temperature from the equation of energy balance between the stellar radiation upon this nebular surface and the black body radiation from it. The temperature distribution in passive disks is insensitive to the wind strength and is almost identical even if the wind strength changes by five orders of magnitude. The nebula temperature is not expressed by a simple power law function of the distance from the central star. This is an important difference between our results and those of previous works. Since the nebula surrounding a T Tauri star is influenced by stellar wind, our model may be more appropriate than any other simple single power law temperature for passive protoplanetary nebulae.

Chemical abundances in seven galactic planetary nebulae

An observational study of chemical abundances in the galactic planetary nebulae NGC 1535, NGC 2438, NGC 2440, NGC 3132, NGC 3242, NGC6302, and NGC 7009 based on long-slit spectra of high signal-to-noise ratio in the 3100 to 6900 A range is presented. We determined the N, O, Ne, S, and Cl abundances from collisionally excited lines and the He and O ++ abundances from recombination lines. The O ++ /H + estimates derived from recombination lines are about a factor of four and two higher than those derived from forbidden lines for NGC 7009 and NGC 3242, respectively. Spatial profiles of O ++ /H + abundance from OII permitted lines and from [OIII] forbidden lines were obtained for the planetary nebula NGC 7009. The differences between O ++ /H + derived from recombination and from forbidden lines present smooth variations along the nebular surface of NGC 7009, with the differences decreasing from the center to the edges of the nebula. If these abundance differences are explained by the presence of electron temperature fluctuations, quantified by the parameter t 2 , a value of about t 2 = 0.09 is required for NGC 3242 and NGC 7009.

Electron temperature fluctuations in planetary nebulae

An observational study of the spatial variation of the electron temperature and density in 10 galactic planetary nebulae is presented. The data consist of long-slit spectra of high signal-to-noise ratio in the 3100 to 6900 A range. Electron temperatures were determined from the [Oiii](λ 4959 + λ 5007)/λ 4363 and [Nii](λ 6548 + λ 6583)/λ 5755 ratios and from the Balmer discontinuity. Electron densities were estimated from the [Sii]λ 6716/λ 6731, [Cliii]λ 5517/λ 5537, and [Ariv]λ 4711/λ 4740 ratios. Electron temperature variations of low amplitude were found across the nebular surface in the planetary nebulae studied. The temperature distribution across each nebula presents a variance relative to the mean corresponding to 0.0003 ≤ t 2 (Bal) ≤ 0.0078, 0.0003 ≤ t 2 (Nii) ≤ 0.0097, and 0.0011 ≤ t 2 (Oiii) ≤ 0.0050. A systematic spatial variation of electron density has been detected in most of objects (NGC 1535, NGC 2438, NGC 2440, NGC 3132, NGC 3242, NGC 6302, NGC 6563, and NGC 7009). The remaining objects (NGC 6781 and NGC 6853) have not shown any significant electron density dependence on position. NGC 2438, NGC 6563, NGC 6781, and NGC 6853 are in general the most diffuse and probably evolved objects studied here, with low mean densities in the range Ne(Sii) ≈ 95–158 cm −3 . An anti-correlation between temperature and density was found for NGC 2438 and NGC 3132, with the electron temperature increasing with the decrease of electron density and a correlation between temperature and density was found for NGC 2440, NGC 3242, NGC 6302, and NGC 7009, with the electron temperature increasing with the increase of electron density. These relationships seem to be associated with the structure of the nebula. The nebulae in which the correlation between temperature and density is present are ring shaped. The anti-correlation between temperature and density is found in bipolar planetary nebulae that are denser in the centre of the nebula.

Integrated spectrum of the planetary nebula NGC 7027

We present deep optical spectra of the archetypal young planetary nebula (PN) NGC 7027, covering a wavelength range from 3310 to 9160 A. The observations were carried out by uniformly scanning a long slit across the entire nebular surface, thus yielding average optical spectra for the whole nebula. A total of 937 emission features are detected. The extensive line list presented here should prove valuable for future spectroscopic analyses of emission line nebulae. The optical data, together with the archival IUE and ISO spectra, are used to probe the temperature and density structures and to determine the elemental abundances from lines produced by different excitation mechanisms. Electron temperatures have been derived from the hydrogen recombination Balmer jump (BJ), from ratios of He  optical recombination lines (ORLs) and from a variety of diagnostic ratios of collisionally excited lines (CELs). Electron densities have been determined from the intensities of high-order H  Balmer lines and of He  Pfund lines, as well as from a host of CEL diagnostic ratios. CEL and ORL diagnostics are found to yield compatible results. Adopting respectively electron temperatures of Te = 12 600 and 15 500 K for ions with ionization potentials lower or higher than 50 eV and a constant density of Ne = 47 000 cm −3 , elemental abundances have been determined from a large number of CELs and ORLs. The C 2+ /H + , N 2+ /H + ,O 2+ /H + and Ne 2+ /H + ionic abundance ratios derived from ORLs are found to be only slightly higher than the corresponding CEL values. We conclude that whatever mechanism is causing the BJ/CEL temperature discrepanies and the ORL/CEL abundance discrepancies that have been observed in many PNe, it has an insignificant effect on this bright young compact PN. The properties of the central star are also discussed. Based on the integrated spectrum and using the energy-balance method, we have derived an effective temperature of 219 000 K for the ionizing star. Finally, we report the first detection in the spectrum of this bright young PN of Raman-scattered O  features at 6830 and 7088 A, pointing to the existence of abundant neutral hydrogen around the ionized regions.

Rocket andFar Ultraviolet Spectroscopic ExplorerObservations of IC 405: Differential Extinction and Fluorescent Molecular Hydrogen

We present far-ultraviolet spectroscopy of the emission/reflection nebula IC 405 obtained by a rocket-borne long-slit spectrograph and the Far Ultraviolet Spectroscopic Explorer (FUSE). Both data sets show a rise in the ratio of the nebular surface brightness to stellar flux (S/F*) of approximately 2 orders of magnitude toward the end of the far-UV bandpass. Scattering models using simple dust geometries fail to reproduce the observed S/F* for realistic grain properties. The high spectral resolution of the FUSE data reveals a rich fluorescent molecular hydrogen spectrum ≈1000'' north of the star that is clearly distinguished from the steady continuum. The S/F* remains roughly constant at all nebular pointings, showing that fluorescent molecular hydrogen is not the dominant cause of the rise. We discuss three possible mechanisms for the blue dust: differential extinction of the dominant star (HD 34078), unusual dust-grain properties, and emission from nebular dust. We conclude that uncertainties in the nebular geometry and the degree of dust clumping are most likely responsible for the rise. As an interesting consequence of this result, we consider how IC 405 would appear in a spatially unresolved observation. If IC 405 were observed with a spatial resolution of less than 0.4 pc, for example, an observer would infer a far-UV flux that was 2.5 times the true value, giving the appearance of a stellar continuum that was less extinguished than radiation from the surrounding nebula, an effect that is reminiscent of the observed ultraviolet properties of starburst galaxies.

Chemical abundances of planetary nebulae from optical recombination lines - I. Observations and plasma diagnostics

We have obtained deep optical spectra of medium resolution for a sample of 12 Galactic planetary nebulae (PNe). Optical recombination lines (ORLs) from carbon, nitrogen and oxygen have been detected in 11 of them and neon ORLs in nine of them. All spectra were obtained by scanning a long slit across the nebular surface, yielding relative line intensities for the entire nebula that are suitable for comparison with integrated line fluxes measured in other wavelength regions using space-borne facilities, such as the Infrared Space Observatory (ISO) and the International Ultraviolet Explorer (IUE). For 11 PNe, ISO infrared spectra between 2.4 and 197 µm are available, most of them taken by ourselves, plus a Kuiper Airborne Observatory (KAO) infrared spectrum of NGC 6210. IUE ultraviolet (UV) spectra are available for all nebulae except one in our sample. The UV, optical and infrared spectra have been combined to study nebular thermal and density structures and to determine elemental abundances. We have determined UV to optical extinction curves towards these PNe by examining observed fluxes of H I and He II recombination lines, radio free‐free continuum flux density, and UV to optical nebular continua. For 11 PNe in our sample, the derived optical reddening curves are found to be consistent with the standard Galactic extinction law for a total-toselective extinction ratio, R ≡ A(V)/EB‐V = 3.1. However, the optical extinction curve towards Hu 1‐2 yields R = 2.0. The UV extinction towards Hu 1‐2 and NGC 6572 is also found to be much steeper than the standard Galactic reddening law. In contrast, the UV extinction curve along the sight lines towards NGC 6210 is found to be much shallower, although in the latter case the uncertainties involved are quite large. Electron temperatures and densities have been derived using a variety of diagnostic ratios of collisionally excited lines (CELs) in the UV, optical and infrared. The results show clear stratifications, both in temperature and density. Lines emitted by ions formed in regions of higher ionization degree yield higher temperatures than lines arising from regions of lower ionization degree, while densities deduced from ratios of infrared diagnostic CELs of low critical densities, such as the [O III] 88-µm/52-µm ratio, are systematically lower than those derived from UV and optical diagnostic lines, which in general have much higher critical densities than the infrared fine-structure lines. Electron temperatures have also been derived from the ratio of the nebular continuum Balmer discontinuity to H 11 for 11 PNe. For four of these, the Balmer jump temperatures are more than 1000 K lower than values derived from the [O III] optical collisionally excited diagnostic line ratio. With a difference of 3580 K, NGC 40 has the lowest Balmer jump temperature relative to the [O III] optical forbidden-line temperature. High-order Balmer line decrements

A deep survey of heavy element lines in planetary nebulae - II. Recombination-line abundances and evidence for cold plasma

In our Paper I, we presented deep optical observations of the spectra of 12 Galactic planetary nebulae (PNe) and three Magellanic Cloud PNe, carrying out an abundance analysis using the collisionally excited forbidden lines. Here, we analyse the relative intensities of faint optical recombination lines (ORLs) from ions of carbon, nitrogen and oxygen in order to derive the abundances of these ions relative to hydrogen. The relative intensities of four high-l C ii recombination lines with respect to the well-known 3d–4f λ4267 line are found to be in excellent agreement with the predictions of recombination theory, removing uncertainties about whether the high C2+ abundances derived from the λ4267 line could be due to non-recombination enhancements of its intensity. We define an abundance discrepancy factor (ADF) as the ratio of the abundance derived for a heavy element ion from its recombination lines to that derived for the same ion from its ultraviolet, optical or infrared collisionally excited lines (CELs). All of the PNe in our sample are found to have ADFs that exceed unity. Two of the PNe, NGC 2022 and LMC N66, have O2+ ADFs of 16 and 11, respectively, while the remaining 13 PNe have a mean O2+ ADF of 2.6, with the smallest value being 1.8. Garnett and Dinerstein found that for a sample of about 12 PNe the magnitude of the O2+ ADF was inversely correlated with the nebular Balmer line surface brightness. We have investigated this for a larger sample of 20 PNe, finding weak correlations with decreasing surface brightness for the ADFs of O2+ and C2+. The C2+ ADFs are well correlated with the absolute radii of the nebulae, although no correlation is present for the O2+ ADFs. We also find both the C2+ and O2+ ADFs to be strongly correlated with the magnitude of the difference between the nebular [O iii] and Balmer jump electron temperatures (ΔT), corroborating a result of Liu et al. for the O2+ ADF. ΔT is found to be weakly correlated with decreasing nebular surface brightness and increasing absolute nebular radius. There is no dependence of the magnitude of the ADF upon the excitation energy of the ultraviolet, optical or infrared CEL transition used, indicating that classical nebular temperature fluctuations – i.e. in a chemically homogeneous medium – are not the cause of the observed abundance discrepancies. Instead, we conclude that the main cause of the discrepancy is enhanced ORL emission from cold ionized gas located in hydrogen-deficient clumps inside the main body of the nebulae, as first postulated by Liu et al. for the high-ADF PN, NGC 6153. We have developed a new electron temperature diagnostic, based upon the relative intensities of the O ii 4f–3d λ4089 and 3p–3s λ4649 recombination transitions. For six out of eight PNe for which both transitions are detected, we derive O2+ ORL electron temperatures of ≤300 K, very much less than the O2+ forbidden-line and H+ Balmer jump temperatures derived for the same nebulae. These results provide direct observational evidence for the presence of cold plasma regions within the nebulae, consistent with gas cooled largely by infrared fine-structure transitions; at such low temperatures, recombination transition intensities will be significantly enhanced due to their inverse power-law temperature dependence, while ultraviolet and optical CELs will be significantly suppressed.

Coronagraphic Imaging with the Hubble Space Telescope and the Space Telescope Imaging Spectrograph11Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5‐26555. This study is part of the STIS IDT protoplanetary disk key project.

ABSTRACTThe unfiltered Space Telescope Imaging Spectrograph (STIS) CCD in tandem with focal plane wedges and a Lyot stop provides a simple white‐light coronagraph with a bandpass of 0.2–1.0 μm, which has been used since late 1998 to image nebulosity around stars in the ranges 0.34≤V≤14 and -0.03≤B− V≤1.65. The residual starlight seen in STIS coronagraphic images includes diffraction spikes due to the Hubble Space Telescope (HST) secondary support structure, lacks distinct Airy rings, and varies smoothly with radius from the star. The point‐spread function (PSF) shape is a strong function of the source spectral energy distribution: we find that the PSF template needs to differ from the occulted source color by Δ(B− V)≤0.08 mag. Optimal PSF removal is achieved for contemporary template observations matched to HST orbital phase of the science observations. Use of noncontemporary PSF templates can degrade the limiting contrast by up to a factor of 10–12 at r≤2′′. These systematic effects are negligible when the nebular surface brightness is comparable to the residual starlight, and they become progressively more important as surface brightness decreases. STIS has been used to detect circumstellar envelopes and protoplanetary disks with surface brightnesses spanning 10−4 to 10−7 per HST resolution element per Fstar at 2″ from the star, debris disks with FIR/Fstar≥0.001, and emission‐line nebulosity associated with bipolar outflows. The limiting contrast for single‐orbit integrations with the star placed at a location where the coronagraphic wedge is 1 .″0 wide is 10−8 per HST resolution element per Fstar for 6≤V≤8 stars. Deeper imagery can be obtained by placing the star off the active detector area. When the star is 5″ from the detector, a limiting contrast for a single‐orbit integration of 2.5 × 10-10 per HST resolution element per Fstar is reached. At this contrast level, background objects and the stellar color variability prevent further improvement by co‐adding data from multiple orbits.

Temperature fluctuations and chemical homogeneity in the planetary nebula NGC 4361

We have obtained deep long-slit spectra of NGC 4361 between λλ4000 and 4975 at a spectral resolution of 1.5 A. The spectra were used to map the C II, C III and C IV recombination lines and the resulting C2+/H+, C3+/H+ and C4+/H+ ionic abundances across the nebular surface. Our results confirm the high recombination-line C abundance derived by Torres-Peimbert, Peimbert & Pena – the C2+/H+ and C3+/H+ ionic abundances derived from recombination lines are respectively a factor of ∼15 and ∼5 higher than the corresponding values derived from the UV collisionally excited C III]λ1908 and C IVλ1549 lines. The recombination-line C abundance is found to be constant across the nebula within the measurement uncertainties (≲0.2 dex), as opposed to the model proposed by Torres-Peimbert, Peimbert & Pena, which predicts a C-rich inner zone with a C/H abundance ratio two orders of magnitude higher than in the outer regions. We have also detected the N IIIλ4379 and O IVλ4632 recombination lines from NGC 4361. The N3+/H+ abundance ratio derived from the λ4379 line is at least a factor of 10 higher than the upper limit from the UV collisionally excited N IV]λ1487 line. There is clear evidence of temperature variations across the nebular surface, with an amplitude of ±1000 K around a mean value of 18000 K, probably produced by shock heating. The observed temperature fluctuations are, however, too small to have a significant effect on the ionic abundances derived from collisionally excited lines. Observations with higher spatial and spectral resolution might quantify better the amplitude of temperature fluctuations and their effects on abundance determination. We have obtained the first reliable measurement of the abundance of fluorine for a planetary nebula. From the [F IV]λ4060 line, a F/H abundance ratio of 4.5×10−8 is found. The [F/O] and C/O abundance ratios derived for NGC 4361 are roughly in agreement with the correlation found between these two quantities among red giant and AGB stars.

Masses of the central stars of Planetary Nebulae

The aim of this study is to derive the masses of the central stars (CSPN) for a large sample of the planetary nebulae (PN). These masses, M∗, are derived from the observed PN positions in three diagnostic diagrams and their comparison with evolutionary tracks of model PN. Two of the diagrams, namely LZan(H) versus TZan(H) and Mv versus Rneb, have already been used in numerous studies. The third one, SHβ versus SV, has recently been introduced by Górny, Stasińska & Tylenda (1996, hereinafter GST96). Here SHβ is the nebular surface brightness in Hβ and Sv is defined as Fv/(πθ2), where Fv is the stellar flux in the V band and θ is the observed nebular angular radius.

Voyager 2 observations of NGC 7023 - Dust scattering shortward of 1600 A

We report the first observational determination of the nebular-to-stellar flux ratio out to wavelengths as short as 1000 A for the reflection nebula NGC 7023, using a combination of spectroscopic data from Voyager 2 and from the Hopkins Ultraviolet Telescope (HUT) on the Astro 1 mission. After accounting for contributions by H2 fluorescence to the nebular flux, the wavelength variation of the ratio of the residual dust scattered flux to that observable from the embedded star is consistent with a reduction of the dust albedo by 25 percent over the wavelength interval 1300-1000 A. This is most easily interpreted, if the particles responsible for the FUV rise in the extinction curve are low-albedo, not necessarily zero-albedo, grains. The direct proportionality between the nebular surface brightness, measured at 22 arcsec offset by HUT, and the nebular flux suggests that the phase function of scattering does not vary in the 2000-1000 A range.

SPATIALLY EXTENDED 10 MU-M EMISSION FROM THE INFRARED REFLECTION NEBULA GSS 30.

ABSTRACT A surface brightness map and point-wise photometry at 10 μm of the infrared reflection nebula GSS30 were made with the Wyoming 2.3 m telescope. Extended emission is detected as far as 10 arc seconds north of the illuminating star, IRS 1. The photometry shows that the nebular surface brightness is a factor of three more than that expected from thermal emission from dust grains. If the 10 μm flux is due to scattering, then grains with a radius of 1.6 μm are required. Alternatively, the 10 μm flux is consistent with the flux expected from the unidentified IR emission features at 7.7, 8.6, and 11.3 μm.

Photometric study of NGC 2023 in the 3500 A to 10000 A region - Confirmation of a near-IR emission process in reflection nebulae

A surface brightness study of the reflection nebula NGC 2023 covering the 3500-10,000 A wavelength region performed with uvby photoelectric photometry and BVRI imaging with a CCD detector is reported along with VRI photometry of a cluster of embedded red stars. The nebular radiation in the 3500-5500 A region is dust-scattered starlight originating in the star HD 37903. The embedded red stars are probably pre-main sequence stars. The nebular surface brightness in R and I exceeds that expected on the basis of a reasonable radiative transfer model by factors of two and more than three, respectively. The excess radiation is extended across the nebula in a manner similar to the scattered light. The extended red emission may be interpreted as the high-frequency extension of extended emission discovered by Sellgren, Werner, and Dinerstein (1983) in the 2-5 micron region in NGC 2023 and two other reflection nebulae.

The scattering phase function of interstellar grains - The case of the reflection nebula NGC 7023

view Abstract Citations (79) References (57) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The scattering phase function of interstellar grains : the case of the reflection nebula NGC 7023. Witt, A. N. ; Walker, G. A. H. ; Bohlin, R. C. ; Stecher, T. P. Abstract IUE observations of the reflection nebula NGC 7023 and the illuminating star, HD 200775, in the spectral range from 1300 to 3100 A are combined with ground-based measurements of the nebular brightness distribution at 3500, 4100, 4700, and 5500 A to determine the scattering properties of the nebular dust grains. Total nebular fluxes are derived from existing data obtained by satellite and ground-based observations. The relevance of far-IR fluxes from the nebula is considered, and models are derived that are applicable to NGC 7023 both with respect to the ratio of nebular to stellar fluxes and with respect to the nebular surface brightness distribution. It is shown that the average grain albedo in the UV is about 0.54 and that the albedo increases to a level of about 0.6 at 1400 A after reaching a minimum of approximately 0.4 near 2200 A. The results suggest that isotropically scattering particles of high albedo make a significant contribution to interstellar scattering in the far-UV. Publication: The Astrophysical Journal Pub Date: October 1982 DOI: 10.1086/160360 Bibcode: 1982ApJ...261..492W Keywords: Cosmic Dust; Interstellar Matter; Interstellar Radiation; Nebulae; Scattering Functions; Ultraviolet Astronomy; Brightness Distribution; Iue; Luminosity; Astrophysics full text sources ADS | data products SIMBAD (2) MAST (1) INES (1)

The Photoelectric Photometry of Planetary Nebulae.

view Abstract Citations (68) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Photoelectric Photometry of Planetary Nebulae. Liller, William Abstract The surface brightnesses of fourteen planetary nebulae and the lower limits of the magnitudes of their exciting stars have been determined photoelectrically Both narrow-band interference filters as well as conventional glass and gelatin filters were used to sort out the desired nebular and central-star radiations. The nebular surface brightnesses are in excellent accord with the results of Liller and Aller, but all indications are that the photographic magnitudes compiled by Berman are systematically too bright However, the central-star magnitudes listed by Berman are generally in accord with the photoelectric observations reported here. Publication: The Astrophysical Journal Pub Date: September 1955 DOI: 10.1086/146082 Bibcode: 1955ApJ...122..240L full text sources ADS | data products SIMBAD (17)

The photoelectric photometry of 18 planetary nebulae.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The photoelectric photometry of 18 planetary nebulae. Liller, William Abstract Because the photographic plate is not well- suited for measuring the brightness of extended objects, the magnitudes of planetary nebulae have never been determined with high precision. The magnitudes of the central stars of these objects are also difficult to measure, since they can never be removed from the abnormally high background fog produced by the surrounding nebulae. Therefore, it is possible to know only approximately the physical conditions which exist in planetary nebulae. The brightnesses of eighteen planetary nebulae and the colors and magnitudes of ten of the exciting stars have been determined photoelectrically in order to improve the basic data needed for the interpretation of these objects. The photometer was designed by the writer and was used with both the ~~-inch Cassegrain reflector of the McMath-Hulbert Observatory and with the 24- 36 inch Curtis Schmidt telescope. Two techniques were used to measure the nebular surface brightness. In the first method a multi-layer interference filter with a half-width of 35 A, and a maximum transmission of 81 per cent at ~4985 almost completely isolated the usually strong forbidden EOIII~ radiations in the green. Small corrections made necessary by the presence of HP and the nebular continuum were calculated from earlier measures by Aller, Page, and others. To determine the amount of light from the central star passing through this filter, the spectral region from X5I00 to N6ooo, where the nebular radiations are few and weak, was isolated with conventional filters. From estimates of the colors of the central stars, I was then able to calculate the amount of radiation from the central star passing through the green interference filter. This quantity, when subtracted from the initial measurement, gave finally the deflection from the light of the green EOIIIJ lines. A comparison of these measures with solar-type stars of known magnitude permitted evaluation of the surface brightness in ergs cm-2 sec-'. A second method, describer by Aller and the writer in the following paper, involves the photoelectric comparison of the nebular spectrum with those of appropriate comparison stars. A comparison of the surface brightnesses derived by the two independent techniques shows that the nebular brightnesses have been determined with an average probable error of ~0.07 mag. The photoelectric results indicate that the earlier photographic measurements of the nebular magnitudes made by Berman are systematically too bright by an average of 0.7 mag. The deflections of the central stars made through the yellow filter and additional measures made through a conventional interference filter centered at X4220 permitted estimates to be made of the colors and magnitudes of the central stars. The magnitudes of these objects published by Berman are in good agreement with these values. Because of the narrowness of the violet interference filter and the generally low intensities of the exciting stars, it was not possible to derive reliable colors for these objects with the present equipment. Publication: The Astronomical Journal Pub Date: June 1954 DOI: 10.1086/106989 Bibcode: 1954AJ.....59..186L full text sources ADS |