Balmer Jump Research Articles

The rich O II recombination spectrum of the planetary nebula NGC 7009: new observations and atomic data

We present new spectrophotometric observations of the rich O II optical recombination line spectrum of the planetary nubula NGC 7009, obtained at a spectral resolution of about 1 Å (FWHM). New intermediate coupling quantal calculations of O II radiative recombination coefficients for the 3d-3p and 4f-3d transitions are presented. The effect of departure from pure LS-coupling is shown to be important. Excellent agreement is found between the observed relative intensities of the O II lines and those calculated from recombination theory allowing for intermediate coupling effects. C, N and O abundances based on our recombination line measurements are derived. In all cases, they are about a factor of 5 higher than the corresponding values deduced from collisionally excited lines, indicating that the discrepancy between the abundances derived from these two different types of emission lines, previously known to exist for C2+, is a common phenomenon, and is probably caused by the same physical process. The nature of this process is still not known. If the discrepancy is due to temperature fluctuations, the implied rms temperature fluctuation prameter t2 is about a factor of 2 larger than that derived by comparing the temperatures deduced from the [O III] forbidden line ratio and from the ratio of the nebular continuum Balmer discontinuity to Hβ. However, if we adopted the electron temperature derived from nebular contiuum Balmer discontinuity instead of that from the [O III]forbidden line ratio, the C and N abundances deduced from ultraviolet collisionally excited lines would come into agreement with those deduced from the optical recombination lines, although the abundance of oxygen deduced from the optical forbidden lines would still be a factor of 2 lower than the corresponding value obtained from the optical recombination lines. The O/H abundance ratio derived from our recombination line analysis of NGC 7009 is more than a factor of 2 higher than the solar oxygen abundance.

A First Search for Rapid Variations of Stellar and Circumstellar Balmer Discontinuity in Be Stars

AbstractSpectrophotometric observations of the Be star ηCen obtained in May, 1993 show smooth variations of both the stellar and circumstellar Balmer discontinuity (BD) on a time scale of hours. Simultaneous photometry and high-resolution spectroscopy suggest possible correlations with the large-amplitude brightness and line-profile variations.

Density condensations in planetary nebulae and the electron temperature

For planetary nebulae, the electron temperature derived from the [O III] line ratio is usually higher than that obtained from the Balmer discontinuity. This discrepancy is generally explained in terms of temperature fluctuations. It is shown here that the [O III] line ratio could lead to a lower value of the temperature if high-density condensations are present in the gas. An analytical formalism is given for the correction factors for ionic abundances in the presence of such condensations. Illustrative clump filling factors are deduced for several planetary nebulae recently observed by Liu & Danziger. We discuss the effect of dense clumps on the ionic and chemical abundances, partly based on photoionization model calculations. Accurate observations of [O II] 7325/3727 Å and [S II] 4070/6725 Å line ratios provide a diagnostic of the presence of dense clumps.

Anomalous Balmer continuum temperatures in the Orion Nebula

New long-slit spectra of the Orion Nebula in the near-ultraviolet were used to calculate the Balmer recombination temperature, T(Bac), from the Balmer discontinuity at 3646 A. The spatially resolved data show a decrease in temperature moving to the west of Theta(sup 1) Ori C, from 8400 K at a distance of 40 sec to a low of 2800 K at a distance of 220 sec. Such values are much lower than previously reported. The effect of scattered starlight on these results is calculated and shown to be less than 10%. Previous studies which found scattered light to be important at the discontinuity are in error. Such low temperatures and their impact on nebular physics and abundances are disconcerting and require further study.

An IUE survey of interstellar H I LY alpha absorption. 1: Column densities

view Abstract Citations (279) References (77) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An IUE Survey of Interstellar H i LY alpha Absorption. I. Column Densities Diplas, Athanassios ; Savage, Blair D. Abstract We measure Galactic interstellar neutral hydrogen column densities by analyzing archival interstellar Ly alpha absorption line data toward 554 B2 and hotter stars observed at high resolution with the IUE satellite. This study more than doubles the number of lines of sight with measures of N(H I) based on Ly alpha. We have included the scattered light background correction algorithm of Bianchi and Bohlin in our data reduction. We use the correlation between the Balmer discontinuity (c1) index and the stellar Ly alpha absorption in order to assess the effects of stellar Ly alpha contamination. Approximately 40% of the B stars with measured (c1) index, exhibit serious stellar Ly alpha contamination. One table contains the derived values of the interstellar N(H I) for 393 stars with at most small amounts of stellar contamination. Another lists the observed values of total N(H I) for 161 stars with suspected stellar Ly alpha contamination and/or uncertain stellar parameters. Publication: The Astrophysical Journal Supplement Series Pub Date: July 1994 DOI: 10.1086/192052 Bibcode: 1994ApJS...93..211D Keywords: Absorption Spectra; Astronomical Spectroscopy; H I Regions; Interstellar Gas; Lyman Alpha Radiation; Lyman Spectra; Ultraviolet Astronomy; Background Radiation; Gas Density; Hot Stars; Iue; Astrophysics; ISM: ABUNDANCES; STARS: EARLY-TYPE; ULTRAVIOLET: ISM full text sources ADS | data products SIMBAD (554) CDS (2) MAST (1) Related Materials (2) Catalog: 1997yCat..20930211D Part 2: 1994ApJ...427..274D

THE PHYSICAL DIAGNOSTICS AND IONIZATION STRUCTURE OF THE ORION NEBULA

ABSTRACT The most detailed study to date of the physical diagnostics and ionization structure of the core of M42 (the Orion Nebula) is presented using new long slit spectra and existing CCD imagery. Cospatial temperature and density diagnostics are presented for distances out to ~5 arcminutes from the Trapezium and clearly show gradients in Te and Ne. Temperatures are derived using S+, O+, N+, O++ and the Balmer Discontinuity, TBac. Values for T(S+) in the range 11,000-14,500 K are presented and suggest shock heating at the ionization front as predicted by some models. A gradient in TBac is derived with temperatures as low as 3000 K and is left open to interpretation. Density gradients for S+, O+ and Cl++ are also presented and are consistent with model predictions for an H II region in the champagne phase of evolution. Using imagery we have constructed a picture of the core of Orion, including previously unknown features. Ratio maps show a high ionization cavity centered on the Trapezium and a low ionization region to the southwest. Ripples and bends in the background molecular cloud are detected. Further to the southwest the ionization begins to increase with distance, which is interpreted as the molecular cloud curving back toward the observer. Additional features in the imagery include a bar-like structure to the northeast of the Trapezium and a high density knot and ellipse of material believed to be a Herbig Haro object. Our diagnostics provide unprecedented detail as to the variation of temperature fluctuations with position. The mean square temperature fluctuation, t2, for the ions O++ and N+ is similar on a global scale, with a value in the range 0.03-0.05 for an assumed constant recombination temperature. Using a gradient in TBac predicts t2 values of 0.01-0.15 with values increasing outward. The ion S+ gives unusually high values of t2 and is not a reliable indicator of fluctuations in the H+ zone since some of the S II emission arises from beyond the ionization front.

Mass loss, magnetic field and chemical inhomogeneities in the He-weak star HD 21699

HD 21699 is a He-weak star whose longitudinal magnetic field ranges from − 1.3 to +1.1 kG (Brown et al. 1985). Its effective temperature, Teff = 15900 K, and its gravity, log g = 4.0, were determined by Wolff (1990) from photometry of the Balmer discontinuity and from measurements of the Hγ line. HD 21699 was reported to exhibit remarkable variations in the C IV ultraviolet resonance lines at 1548 and 1551 Å (Shore et al. 1987). These variations occur with the same period, 2d49 (Percy 1985), as the magnetic variations and the photometric variations in the optical region. Similar UV line variations could be found only in two other He-weak stars, HD 5737 and HD 79158. They were interpreted as an evidence of the occurrence of outflowing winds controlled by the magnetic field. Furthermore, Shore et al. emphasized that no lines in IUE spectra other than C IV, and possibly Si IV, show any statistically significant variation. Recently, irregular brightening of a few hundredths of magnitude, superimposed over the periodic variations, were observed in the U photometric band (Vetö 1993); their origin is currently unknown.

The photosphere of Pleione

In this paper we report results concerning the photospheric Balmer discontinuity of Pleione. They show that after the Be → Be-shell phase transitions, the stellar photosphere keeps its characteristics unchanged.

T Tauri stars in blue

We present an atlas of 96 low dispersion spectra of T Tauri stars, both classical and weak-lined. The flux-calibrated spectra extend from Hβ to well blueward of the Balmer jump. Observed equivalent widths and rough blue veilings are presented. We model stars with excess emission as a combination of a weak-lined T Tauri star of a similar spectral type and a slab of hot hydrogen. We tabulate the physical parameters of the slab, and give both intrinsic and observed Balmer jumps. The region responsible for the excess blue continuum has a high density and small surface area, in agreement with previous work. This is consistent with the boundary layer hypothesis, but not unique to it. We suggest that the emission region may actually be the stellar photosphere, heated by accretion shocks at the footpoints of magnetic loops extending to the disk

The atmospheres of T Tauri stars. II - Chromospheric line fluxes and veiling

view Abstract Citations (30) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Atmospheres of T Tauri Stars. II. Chromospheric Line Fluxes and Veiling Batalha, C. C. ; Basri, Gibor Abstract A set of high-resolution calibrated spectra of southern T Tauri Stars (TTS), comprising objects with H-alpha equivalent widths ranging from 3 to 40 A are used to derive one-component plane-parallel chromospheric models. Our motivation is to discover how much emission activity can truly be ascribed to stellar activity, using real calibrated stellar data. We find that simple models can reproduce the Ca II line at 8542 A for most of the stars. The chromospheric enhancements are comparable to the ones found among active main-sequence stars of similar spectral type. These models cannot account for the veiling observed in TTS. Likewise, realistic TTS chromospheres are unable to create the observed Balmer jumps. The targets have a range of H-alpha strength that allow us to study possible correlations among narrow symmetric emission lines, and H-alpha which probes nonstellar regions. On the basis of their stellar vs systemic luminosities, Sz 77, Sz 82, and Sz 19 present signatures of disk accretion, in addition to the previously reported case of Sz 98. By correlating the fluxes of the Ca II IR line, H-alpha and the near-IR excess, we conclude that disk accretion may enhance the luminosity of typical chromospheric emission lines. Publication: The Astrophysical Journal Pub Date: July 1993 DOI: 10.1086/172926 Bibcode: 1993ApJ...412..363B Keywords: Chromosphere; Stellar Atmospheres; Stellar Spectra; T Tauri Stars; Emission Spectra; H Alpha Line; Spectral Line Width; Stellar Activity; Stellar Luminosity; Stellar Mass Accretion; Stellar Models; Astrophysics; STARS: PRE--MAIN-SEQUENCE; STARS: CHROMOSPHERES; LINE: FORMATION; RADIATIVE TRANSFER full text sources ADS | data products SIMBAD (6) Related Materials (1) Part 1: 1987ApJ...318..823F

Electron temperature determination from nebular continuum emission in planetary nebulae and the importance of temperature fluctuations

Electron temperatures derived from the observed Balmer discontinuity are presented for 14 planetary nebulae. Electron temperatures deduced from the Paschen discontinuity are also obtained for four of them. The new measurements, together with those published by others, are compared to temperatures deduced from the nebular-to-auroral forbidden line ratio of [O III], Te([O III]na). It is shown that, on average, Te(Bal) tends to be lower than the Te([O III]na) for the same object, indicating the presence of large temperature fluctuations. Except for a few planetary nebulae with t2 > 0.10, t2 = 0.03 is a representative value. Possible mechanisms that could be responsible for such large temperature fluctuations are discussed. The observations also reveal, for the first time, two extremely weak discontinuities, one of He I at λ3422 in NGC 7009 and another of He II at λ5694 in Me 2–1.

Classification algorithms applied to 10 588 low-resolution slitless spectra - A search for unusual and interesting objects

view Abstract Citations (4) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Classification Algorithms Applied to 10588 Low-Resolution Slitless Spectra: A Search for Unusual and Interesting Objects Beauchemin, M. ; Borra, E. F. Abstract We have performed a search for unusual objects among nearly 11000 low- resolution spectra taken from 23 grens plates of the CFHT-Laval survey (14.5 deg^2^;14 < V < 18). The classification method used is based on a hierarchical clustering analysis technique. In a first step, we exploit the highly effective and objective power of the method at isolating uncommon energy distributions. The clustering analysis also provides us with a tool to determine synoptic groups for common stars in the sample. This permits us to identify peculiar objects using a template matching technique. We furnish positions, magnitudes, grens energy distributions and finding charts for 4 objects having energy distribution significantly different from the rest of the population. We identify 32 other objects of interest, among them: 14 strong emission line objects, 8 suspected degenerate stars and 3 very blue spectra without a Balmer discontinuity. Publication: The Astronomical Journal Pub Date: April 1993 DOI: 10.1086/116539 Bibcode: 1993AJ....105.1587B Keywords: Algorithms; Peculiar Stars; Spectral Energy Distribution; Spectrum Analysis; Stellar Magnitude; Stellar Spectra; Cluster Analysis; Position (Location); Sky Surveys (Astronomy); Astronomy; SURVEYS; METHODS: ANALYTICAL full text sources ADS | data products SIMBAD (37) NED (25)

Pulsation Variables in the AF stars of the Case Low-Dispersion Survey

AbstractThe AF stars are those of spectral types A and F that have been discovered on objective-prism plates taken with the Burrell Schmidt in the Case Low-Dispersion Northern Sky Survey (Pesch and Sanduleak, 1983). In SA 57, this survey is complete to V = 16.5. In this field and also in the Lick Astrograph RR Lyrae survey field RR 7 (in the anticenter) the AF stars comprise (a) blue horizontal branch stars and RR Lyrae stars of the halo and (b) stars which have the higher surface gravities of main sequence stars. The two groups can be separated primarily by their differing Balmer jumps and Balmer line-widths. The latter group (which may well include blue stragglers of both Pop I and Pop II) extends some 10 kpc above the galactic plane and shows a wide range of metallicity. Photoelectric photometry of this AF star sample has allowed the detection field RR Lyrae stars of lower-amplitudes than could have been found by conventional blinking techniques; this has led to a significant increase in the number of RR Lyrae stars that are known in SA 57 and RR 7. The cooler main sequence and/or blue straggler AF stars lie in the zone of pulsational instability and one higher-amplitude δ-Scuti star was detected in field RR 7. It is suggested that these AF stars provide a good sample for studying the incidence of pulsation in the population of older stars that extends beyond the thin disk.

The Ionization and Thermal Structure of NGC 2392 and NGC 3242

In Liu and Danziger(1992b) we present Te derived from the Balmer discontinuities for 14 objects by long slit spectrophotometry. It is found that on the average the Te derived in this way are systematically lower than those derived the forbidden line ratios, indicating the presence of large temperature fluctuations(Peimbert 1967, 1971). The spectra obtained have been analyzed point-by-point along the slit for NGC 2392 and NGC 3242, for which t2 = 0.13 and 0.058, respectively, as measured on the summed spectra integrated along the slits.

Halo blue horizontal branch stars: spectroscopy in two fields

Results of spectroscopic observations at 2-A resolution of 44 stars in two fields are presented. 39 are identified as blue horizontal branch (BHB) stars, RR Lyrae or main-sequence A stars. The candidates were selected by colour from digitized UK Schmidt direct plates and by the presence of a strong Balmer discontinuity in their objective prism spectra. Velocities and distances are measured for each of the BHB candidates. At moderate spectral dispersion the distinction between main-sequence A stars and horizontal branch stars cannot be made on the basis of hydrogen linewidths alone

Luminosities, masses, and ages of B-type stars

Measurements of H-gamma lines and photometry of the Balmer discontinuity are combined with models of stellar atmospheres and interiors to derive values of T(eff) log g, mass, and M(bol) for main-sequence B-type stars in the Alpha Per and the Pleiades clusters and in the Orion, Sco-Gen, and I Lac associations. The masses as a function of T(eff) agree with the analyses of eclipsing binary orbits. The luminosities yield distances for the observed clusters and associations that are in good agreement with independent determinations. The observed main sequence is broader than predicted by standard models and suggests that mixing may play a role in extending the main-sequence lifetime. Stars within each cluster and association show a range of ages, a result that suggests that star formation occurred over a period of at least 1-3 x 10 to the 7th yr. 37 refs.

Balmer lines and Continuum Emission for two Flares of the dMe Star Gliese 729

It has been shown for solar flares (Donati-Falchi et al. 1985) that the continuum emission at the Balmer discontinuity (the blue “pseudo-continuum”) is a very sensitive tool to determine the electron density. In order to use the same interpretative scheme for stellar flares (Falchi et al. 1988), spectroscopic observations of various flare stars have been performed in June 1987 at ESO observatory (Chile). In this paper we report the analysis of spectra of the star n. 729 (V 1216 Sgr) of the Gliese catalog.

The extended atmosphere of Lambda Pavonis at the time of the emergence of H-emissions from minimum intensity

view Abstract Citations (4) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Extended Atmosphere of lambda Pavonis at the Time of the Emergence of H-Emissions from Minimum Intensity Sahade, Jorge ; Rovira, Marta ; Ringuelet, Adela E. ; Kondo, Yoji ; Cidale, Lydia Abstract A study of the Be star λ Pavonis, particularly of the changes in the Balmer discontinuity in the interval 1949 - 1982, is presented. Nearly simultaneous observations carried out with the ESO 1.5 m reflector at La Silla and with the IUE satellite correspond to an epoch when the H emission is starting to increase in intensity immediately after having reached its minimum strength. These observations suggest the presence of four distinct regions of line formation, with the material moving outward in the transition region. Publication: The Astrophysical Journal Pub Date: April 1988 DOI: 10.1086/166194 Bibcode: 1988ApJ...327..335S Keywords: B Stars; Balmer Series; Iue; Stellar Atmospheres; Stellar Spectra; Ultraviolet Spectra; Line Spectra; Radial Velocity; Stellar Envelopes; Astrophysics; STARS: BE; STARS: INDIVIDUAL CONSTELLATION NAME: LAMBDA PAVONIS; ULTRAVIOLET: SPECTRA full text sources ADS | data products SIMBAD (1) MAST (1) INES (1)

Ultraviolet and infrared excess emission in Be stars

Spectrophotometric observations of 26 Be stars are reported and analysed in the wavelength range λ3200 to λ8000 Å. Two observational features readily noted in these stars are: (i) a Balmer discontinuity smaller than that exhibited by normal B-type stars of similar spectral type; and (ii) an excess flux in the near-infrared region. A simple optically thin model is assumed which describes the small Balmer jump as due to free–bound emission in the Balmer continuum arising in a circumstellar envelope. The same model parameters also reproduce the near-infrared excess when Paschen free–free and Paschen free–bound emissions are invoked. The volume emission measures for the circumstellar region are calculated. It is found that the near-ultraviolet and the near-infrared volume emission measures are loosely correlated to each other. The value of the ratio of the Balmer emission measure, εUV, to the Paschen emission measure, εIR, is found to be equal to unity.

Spectrophotometry of bright Be stars

We present new measurements of the distribution of energy in the continuum for eight Be stars in the optical region (λλ3200-7600 A). The effective temperatures of these stars have been estimated from their observed fluxes. It is found that, in general, ‘pole-on’ stars show near-infrared excess emission. It is interesting to note that the Balmer jumps for stars having an infrared excess are systematically smaller than for those lacking the infrared excess.