Strong Halpha Emission Research Articles

The brown dwarf population in the Chamaeleon I cloud

We present the results of a multiband survey for brown dwarfs in the Chamaeleon I dark cloud with the Wide Field Imager (WFI) camera at the ESO/MPG 2.2-m telescope on La Silla (Chile). The survey has revealed a substantial population of brown dwarfs in this southern star forming region. Candidates were selected from R, I and H-alpha imaging observations. We also observed in two medium-band filters, M855 and M915, for the purpose of spectral type determination. The former filter covers a wavelength range containing spectral features characteristic of M-dwarfs, while the latter lies in a relatively featureless wavelength region for these late-type objects. A correlation was found between spectral type and (M855-M915) colour index for mid- to late M-type objects and early L-type dwarfs. With this method, we identify most of our object candidates as being of spectral type M5 or later. Our results show that there is no strong drop in the number of objects for the latest spectral types, hence brown dwarfs may be as abundant as low-mass stars in this region. Also, both kind of objects have a similar spatial distribution. We derive an index $\alpha = 0.6 \pm 0.1$ of the mass function in this region of dispersed star formation, in good agreement with the values obtained in other star forming regions and young clusters. Some of the brown dwarfs have strong H-alpha emission, suggesting mass accretion. For objects with published infrared photometry, we find that strong H-alpha emission is related to a mid-infrared excess, indicative of the existence of a circumstellar disk.

Evidence for high accretion rates in weak-line T Tauri stars?

We have discovered T Tauri stars which show startling spectral variability between observations seperated by 20 years. In spectra published by Bouvier & Appenzeller (1992) these objects showed very weak H-alpha emission, broad CaII absorption and so called ``composite spectra'', where the spectral type inferred from the blue region is earlier than that inferred from the red. We present here new spectroscopy which shows that all four stars now exhibit strong H-alpha emission, narrow CaII emission and a spectral type which is consistent at all wavelengths. We propose a scheme to understand these changes whereby the composite spectra of these stars can be explained by a period of active accretion onto the central, young star. In this scheme the composite spectrum consists of a contribution from the stellar photosphere and a contribution from a hot, optically thick, accretion component. The optically thick nature of the accretion flow explains the weakness of the H-alpha emission during this phase. Within this scheme, the change to a single spectral type at all wavelengths and emergence of strong H-alpha emission are consistent with the accretion columns becoming optically thin, as the accretion rate drops. There is a strong analogy here with the dwarf novae class of interacting binaries, which show similar behaviour during the decline from outbursts of high mass-transfer rate. The most important consequence of this interpretation is that these objects bring into question the association of Weak-Line T Tauri stars (WTTs) with non-accreting or discless objects. In light of this result we consider the justification for this paradigm.

2MASS 1315−2649: A High Proper-Motion L Dwarf with Strong Hα Emission

In Hall (2002) I reported that 2MASSI J1315309-264951 is an L dwarf with strong H-alpha emission. Two spectroscopic epochs appeared to show that the H-alpha was variable, decreasing from 121 Angstroms to 25 Angstroms EW, which I interpreted as a flare during the first observation. Gizis (2002) independently discovered this object, and his intermediate spectroscopic epoch shows H-alpha with 97 Angstroms EW. A new fourth epoch of spectroscopy again shows a very large H-alpha EW (124 Angstroms), confirming this object to be a persistent, strong H-alpha emitter. Whether the H-alpha is steady (like 2MASS 1237+6526) or from continuous strong flaring (like PC0025+0447) remains unclear. Imaging confirms that 2MASS 1315-2649 has a high proper motion (0.71/year), corresponding to a transverse velocity of ~76 km/s at its distance of ~23 pc. Thus 2MASS 1315-2649 is consistent with being >~2 Gyr old and therefore relatively massive. If that is so, the correlation of H-alph activity with mass found by Gizis et al. (2000) would seem to support the continuous strong flaring scenario, though it does not rule out a brown dwarf binary accretion scenario.

A PHOTOMETRIC AND SPECTROSCOPIC SURVEY OF AGB STARS IN M31

ABSTRACT Asymptotic giant-branch (AGB) stars are identified and classified in five 7' X 7' fields spaced along M31's SW semi-major axis using a four band photometric system. An investigation of the AGB luminosity functions and red giant-branch widths reveals significant differences between the star forming histories of the five fields. The distance modulus of M31 is derived using carbon stars (C-stars) and found to be consistent with both a value obtained from Cepheids and with values in the literature. The ratio of AGB C- to M-stars (C/M ratio) in the five fields is found to increase with galactocentric distance and it is shown that photometric incompleteness is not responsible for this effect. This is the first clear demonstration of a varying C/M ratio in an external galaxy. The C/M ratios appear to be insensitive to star-forming history differences but sensitive to metallicity differences between the fields. Previous observations are used to define a relationship between the C/M ratio and metallicity, and this is used to obtain estimates of the field metallicities. These estimates are found to be consistent with a previous measurement of M31's metallicity gradient. The C/M ratios measured in M31 indicate that the composition of M31's interstellar medium may be position dependent, and evidence is cited in favour of this. Follow up spectroscopy was obtained in two of the five fields, and is used to show that the photometric system did an excellent job of discriminating between M-, S- and C-stars. Of the 48 C-stars for which spectra were obtained, 7 have strongly enhanced 13C bands (J-stars), 2 have strong H-alpha emission, while 3 are found to exhibit enhanced Li absorption (Li-stars). Both the J- and Li-stars are fainter than predicted by current theoretical models, while the colours of the H-alpha stars suggest they may be in the terminal phases of their evolution. The C2 and CN bandstrengths of the C-stars are measured, and no correlation between these bandstrengths and either Mbol or (V-I) is found. It is suggested that this lack of correlation is due to an age spread. The spectra of the first confirmed S-star in M31 is presented, and two evolutionary pathways are suggested to account for this star's high luminosity.

H-alpha imaging of galaxies in the Bootes void

We report H-alpha and continuum imaging of 12 galaxies in the Bootes void. The H-alpha emission in ten galaxies is due to star formation; one galaxy is probably a narrow emission line Active Galactic Nuclei (AGN), and one a Seyfert 1. Star formation rates range from 3-55 solar mass/yr, with the most active galaxies producing stars at almost three times the rate found in normal field disk systems. Eleven galaxies have strong H-alpha emission in the nucleus. The morphology of several of the galaxies is disturbed. There are at least five spirals, three of which are probably one-armed spirals, and three edge-on disk systems. Three galaxies not identifiable as spirals have H-alpha emission from regions outside the nuclear point source. 1510+4727, which has the highest rate of star formation in our sample, does not have the strong far-infrared emission expected from a galaxy experiencing significant star formation. The galaxies are quite luminous M(sub B) = -21 to -23 mag), and are not the type predicted to be found in voids.

Interaction of NGC 2276 with the NGC 2300 group - Fabry-Perot observations of the H-alpha velocity field

We report kinematic observations of H-alpha emission from the spiral galaxy NGC 2276 obtained with a Fabry-Perot Camera. The 'bow shock' appearance and enhanced star formation in NGC 2276 have been attributed by Mulchaey et al. (1993) to a ram-pressure interaction with the dense IGM detected in ROSAT observations of the NGC 2300 group of galaxies. Along the 'bow shock' limb of the galaxy, we observe strong H-alpha emission and significant kinematic perturbations located immediately interior to an abrupt decrease in the scale length of the optical disk. Although ram-pressure forces may be important in the evolution of the outer gaseous disk, the peculiar kinematics and the truncation in the stellar disk are difficult to explain in a ram-pressure model; a more likely cause is tidal interaction, probably with the elliptical galaxy NGC 2300.

The Ly-alpha/H-alpha ratio in high-redshift radio galaxies

The first spectroscopic detection of H-alpha emission from radio galaxies at z greater than 2 are presented. Strong H-alpha emission is detected at z = 2.429 in B3 0731 + 438, and H-alpha is directed at z = 2.428 in 0406 - 244 at a significant level of greater than 6 sigma. The resulting Ly-alpha/H-alpha ratios for 0731 + 438 and 0406 - 244 are 3.9 and 3.2 with 3 sigma uncertainties of 1.5 for each. A range of possible extinctions is derived depending on the reddening-free Ly-alpha/H-alpha ratio assumed and the extinction curve employed. The most important result of this study is the demonstration that the Ly-alpha/H-alpha ratio in distant galaxies can now be measured with relative ease.

Radial velocities of carbon stars - CO versus optical measurements

view Abstract Citations (20) References (61) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radial Velocities of Carbon Stars: CO versus Optical Measurements Barnbaum, Cecilia Abstract Measurements of the optical radial velocities of 34 optically bright carbon stars in the solar neighborhood are compared with the velocities of circumstellar CO radio emission lines taken from the literature. Twelve stars of the 34 show a large offset, greater than 5 km/s, between the CO and optical velocities. This group distinguishes itself by several characteristics: IRAS and 2-micron colors, variability class (most are Miras), large amplitude of optical velocity variation over several epochs, strong H-alpha emission, and relatively large mean outflow velocity of the circumstellar matter. The optical velocities for this group are consistently redshifted from the CO mean, whereas the optical velocities for the other carbon stars are distributed on either side of their mean CO velocity. It is argued that this redshift bias in optical velocity is related to the shock structure or pulsational mode of these stars. A mean amplitude of optical velocity variation of 8.0 km/s and 2.4 km/s for the Mira and SR variables, respectively, is reported. Publication: The Astrophysical Journal Pub Date: February 1992 DOI: 10.1086/170976 Bibcode: 1992ApJ...385..694B Keywords: Asymptotic Giant Branch Stars; Carbon Monoxide; Carbon Stars; Radial Velocity; Stellar Mass Ejection; Visible Spectrum; Center Of Mass; Emission Spectra; Line Spectra; Solar Neighborhood; Astrophysics; RADIO LINES: MOLECULAR: CIRCUMSTELLAR; STARS: CARBON; STARS: MASS LOSS; STARS: VARIABLES: OTHER RR LYRAE; TECHNIQUES: RADIAL VELOCITIES full text sources ADS | data products SIMBAD (34)

Emission-line galaxies in the third list of the Case Low-Dispersion Northern Sky Survey

Observations of 47 galaxies in the third Case list are reported. Thirty-five of the galaxies in the sample were selected for the presence of emission lines on the objective prism plates. At the higher spectral dispersion of the data, significant line emission was found in 46 of the 47 galaxies. Twenty-six galaxies are found to be undergoing significant bursts of star formation. Ten additional galaxies may be starburst galaxies with low-excitation spectra. Two galaxies are probably type Seyfert 2. The most distant object, CG 200, at a redshift of 0.144, has a strong broad H-alpha emission line, and is probably a Seyfert 1. Seventeen of the galaxies have been detected by IRAS. Eight of the IRAS galaxies have H-II-region-type spectra and eight have low-ionization starburst spectra. The galaxies represent a mixture of types, ranging from intrinsically faint dwarf galaxies with Mb equalling -16 mag, to powerful galaxies with MB equalling -23 mag. Galaxies CG 234 and CG 235 are interacting, as are galaxies CG 269 and CG 270.

The optical counterpart of the X-ray source H0253+193 - A distant, high-luminosity RS Canum Venaticorum system

Optical and near-infrared photometry in the VRIJHK and CO (2.35 microns) bands and optical spectroscopy and polarimetry reveal that the optical counterpart of the X-ray source H0253+193 is most likely a RS CVn binary system. The results show a visual extinction of 11.3 mag to the star, with an IR excess possibly due a cooler binary companion or circumstellar material. It is suggested that the I-band polarization of 8.1 + or - 1.8 may be induced in the starlight by passage through the molecular cloud. The lack of strong H-alpha emission in the optical spectrum argues against the possibility of a classical T Tauri star, and the IR excess argues against a naked T Tauri star. The wide energy spectrum found in the dereddened optical and near-infrared photometry supports the binary star model.

Optical identification of 2S 1417-62

The 17 s X-ray pulsar 2S 1417-62 has been identified with a V of approximately 16.9 mag OB star on the basis of an X-ray position obtained with the Einstein X-Ray Observatory and spectrophotometry with the 4-m telescope at Cerro Tololo Inter-American Observatory. Strong H-alpha emission is observed as well as significant changes in X-ray luminosity. The optical counterpart may be either a Be or a B I star, but the H-alpha emission strength and galactic position suggest that a Be star is more likely. A Be star identification also further supports the limit of approximately 15 days or more on the system binary period obtained by Kelley et al. (1981) from pulse timing studies.

NARROW- AND INTERMEDIATE-BAND HΑ AND O I Λ7774 PHOTOMETRY ANDRETICON SPECTROSCOPY OF SX CASSIOPEIAE.

ABSTRACT Observations of the 36.57-day eclipsing binary SX Cas made on eight nights during February and March 1979 with pairs of intermediate- and narrow-band filters centered on the wavelengths of the Balmer H-alpha line and the O I 7774 A triplet are discussed. In addition, a reticon spectrum covering a wavelength range of 4750 A to 7700 A was obtained in September 1979, showing strong H-beta and H-alpha emission lines as well as absorption features of the K-giant component of the system. From the photometry, strong H-alpha emission is found at all the orbital phases sampled. However, no appreciable O I 7774 A line emission is detected. It is noted that the strong H-alpha emission is consistent with the presence of significant amounts of circumstellar and circumbinary gas. From the observations, it appears that the H-alpha emitting region is centered chiefly on the primary component, but the gas is partially eclipsed by the larger cool giant during primary eclipse. The H-alpha-emitting region therefore appears larger than the dimensions of the stellar components. In addition, the apparent nonrepeatability of the H-alpha emission strength from one orbital cycle to the next implies that relatively rapid changes in the extent or quantity of gas occur in the system.

An optical search for ionized hydrogen in globular clusters. II

view Abstract Citations (30) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An optical search for ionized hydrogen in globular clusters. II. Hesser, J. E. ; Shawl, S. J. Abstract Results of an optical search for ionized hydrogen in 31 globular clusters, including four X-ray clusters, are reported which consist of photoelectric H-alpha line profiles obtained with a single-etalon Fabry-Perot interferometer. A previous error regarding NGC 6441 is corrected, showing that this cluster has a radial velocity of about plus 13 km/s rather than -70 km/s and does not exhibit strong H-alpha emission. No ionized hydrogen is detected in any of the other clusters either. Stringent upper limits of no more than 1 solar mass are placed on the ionized hydrogen in the four X-ray clusters (NGC 1851, 6441, 6624, and 7078), and 47 Tuc is estimated to contain no more than 0.03 solar mass of ionized hydrogen in its core relative to a comparable volume of its halo. It is concluded that the mass of interstellar matter expected to accumulate in globular star clusters as a natural consequence of stellar evolutionary processes is not present in the amounts predicted by the best available theoretical models. Publication: The Astrophysical Journal Pub Date: November 1977 DOI: 10.1086/182558 Bibcode: 1977ApJ...217L.143H Keywords: Globular Clusters; Hydrogen Ions; Interstellar Matter; Stellar Mass Ejection; Astronomical Catalogs; Electrophotometers; H Alpha Line; Radial Velocity; Stellar Evolution; X Ray Sources; Astronomy full text sources ADS | data products SIMBAD (31) NED (2)