| 1. |
|
|
| 2. |
- Aldenius, Maria, et al.
(författare)
-
Accurate laboratory ultraviolet wavelengths for quasar absorption-line constraints on varying fundamental constants
- 2006
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 370:1, s. 444-452
-
Tidskriftsartikel (refereegranskat)abstract
- The most precise method of investigating possible space-time variations of the fine-structure constant, alpha equivalent to (1/hc)(e(2)/4 pi epsilon(0)), using high-redshift quasar absorption lines is the many-multiplet (MM) method. For reliable results this method requires very accurate relative laboratory wavelengths for a number of UV resonance transitions from several different ionic species. For this purpose laboratory wavelengths and wavenumbers of 23 UV lines from Mg I, Mg II, Ti II, Cr II, Mn II, Fe II and Zn II have been measured using high-resolution Fourier transform (FT) spectrometry. The spectra of the different ions (except for one Fe II line, one Mg I line and the Ti II lines) are all measured simultaneously in the same FT spectrometry recording by using a composite hollow cathode as a light source. This decreases the relative uncertainties of all the wavelengths. In addition to any measurement uncertainty, the wavelength uncertainty is determined by that of the Ar II calibration lines, by possible pressure shifts and by illumination effects. The absolute wavenumbers have uncertainties of typically +/- 0.001 -+/- 0.002 cm(-1) (Delta lambda approximate to 0.06-0.1 m angstrom at 2500 angstrom), while the relative wavenumbers for strong, symmetric lines in the same spectral recording have uncertainties of +/- 0.0005 cm(-1) (Delta lambda approximate to 0.03 m angstrom at 2500 angstrom) or better, depending mostly on uncertainties in the line-fitting procedure. This high relative precision greatly reduces the potential for systematic effects in the MM method, while the new Ti II measurements now allow these transitions to be used in MM analyses.
|
|
| 3. |
|
|
| 4. |
- Bautista, M A, et al.
(författare)
-
[Ti II] and [Ni II] emission from the strontium filament of eta Carinae
- 2006
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 370:4, s. 1991-2003
-
Tidskriftsartikel (refereegranskat)abstract
- We study the nature of the [Ti II] and [Ni II] emission from the so-called strontium filament found in the ejecta of eta Carinae. To this purpose, we employ multilevel models of the Ti II and Ni II systems, which are used to investigate the physical condition of the filament and the excitation mechanisms of the observed lines. For the Ti II ion, for which no atomic data were previously available, we carry out ab initio calculations of radiative transition rates and electron impact excitation rate coefficients. It is found that the observed spectrum is consistent with the lines being excited in a mostly neutral region with an electron density of the order of 10(7) cm(-3) and a temperature around 6000 K. In analysing three observations with different slit orientations recorded between 2000 March and 2001 November, we find line ratios that change among various observations, in a way consistent with changes of up to an order of magnitude in the strength of the continuum radiation field. These changes result from different samplings of the extended filament due to the different slit orientations used for each observation, and yield clues on the spatial extent and optical depth of the filament. The observed emission indicates a large Ti/Ni abundance ratio relative to solar abundances. It is suggested that the observed high Ti/Ni ratio in gas is caused by dust-gas fractionation processes and does not reflect the absolute Ti/Ni ratio in the ejecta of eta Carinae. We study the condensation chemistry of Ti, Ni and Fe within the filament and suggest that the observed gas phase overabundance of Ti is likely the result of selective photoevaporation of Ti-bearing grains. Some mechanisms for such a scenario are proposed.
|
|
| 5. |
- Bensby, Thomas, et al.
(författare)
-
The origin and chemical evolution of carbon in the Galactic thin and thick discs
- 2006
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 367:3, s. 1181-1193
-
Tidskriftsartikel (refereegranskat)abstract
- In order to trace the origin and evolution of carbon in the Galactic disc, we have determined carbon abundances in 51 nearby F and G dwarf stars. The sample is divided into two kinematically distinct subsamples with 35 and 16 stars that are representative of the Galactic thin and thick discs, respectively. The analysis is based on spectral synthesis of the forbidden [C I] line at 872.7 nm using spectra of very high resolution (R approximate to 220 000) and high signal-to-noise ratio (S/N greater than or similar to 300) that were obtained with the Coude Echelle Spectrograph (CES) spectrograph by the European Southern Observatory (ESO) 3.6-m telescope at La Silla in Chile. We find that [C/Fe] versus [Fe/H] trends for the thin and thick discs are totally merged and flat for subsolar metallicities. The thin disc that extends to higher metallicities than the thick disc shows a shallow decline in [C/Fe] from [Fe/H] approximate to 0 and up to [Fe/H]approximate to+0.4. The [C/O] versus [O/H] trends are well separated between the two discs (due to differences in the oxygen abundances) and bear a great resemblance to the [Fe/O] versus [O/H] trends. Our interpretation of our abundance trends is that the sources that are responsible for the carbon enrichment in the Galactic thin and thick discs have operated on a time-scale very similar to those that are responsible for the Fe and Y enrichment [i.e. SN Ia and asymptotic giant branch (AGB) stars, respectively]. We further note that there exist other observational data in the literature that favour massive stars as the main sources for carbon. In order to match our carbon trends, we believe that the carbon yields from massive stars then must be very dependent on metallicity for the C, Fe and Y trends to be so finely tuned in the two disc populations. Such metallicity-dependent yields are no longer supported by the new stellar models in the recent literature. For the Galaxy, we hence conclude that the carbon enrichment at metallicities typical of the disc is mainly due to low- and intermediate-mass stars, while massive stars are still the main carbon contributor at low metallicities (halo and metal-poor thick disc).
|
|
| 6. |
- Blackwell-Whitehead, RJ, et al.
(författare)
-
Hyperfine structure of the ground state in singly ionized manganese
- 2005
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 364:2, s. 705-711
-
Tidskriftsartikel (refereegranskat)abstract
- We report the first experimental measurements and theoretical calculations for the hyperfine splitting of the 3d(5) 4s ground state of Mn II. The hyperfine structure constants were determined using Fourier transform spectroscopy with a hollow cathode discharge lamp. The uncertainties in the magnetic dipole constants, A, are between 3 x 10(-4) and 5 x 10(-4) cm(-1). Extensive configuration interaction calculations of the hyperfine splitting of the a(7)S(3) ground state have been carried out to compare with the experimental measurements. In addition, we give wavelengths for three resonant transitions to the ground state of Mn II, accurate to a few parts in 10(8). These wavelengths are of interest in astrophysical applications such as studies of the time variation of the fine structure constant.
|
|
| 7. |
- Blackwell-Whitehead, R J, et al.
(författare)
-
Experimental oscillator strengths for the spectrum of neutral manganese
- 2005
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 361:4, s. 1281-1286
-
Tidskriftsartikel (refereegranskat)abstract
- We report laboratory measurements of oscillator strengths for 44 transitions in the Mn T spectrum covering the wavelength range 209-2780 nm. Nine energy level lifetimes have been measured using time resolved laser-induced fluorescence (LIF). The lifetimes have been combined with branching fractions measured by Fourier transform spectroscopy to obtain absolute oscillator strengths. In total, 24 of these oscillator strengths are measured for the first time, including transitions in the previously unobserved infrared region above 2.0 mu m.
|
|
| 8. |
- Blackwell-Whitehead, R. J., et al.
(författare)
-
Experimental TiI oscillator strengths and their application to cool star analysis
- 2006
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 373:4, s. 1603-1609
-
Tidskriftsartikel (refereegranskat)abstract
- We report experimental oscillator strengths for 88 Ti I transitions covering the wavelength range 465-3892 nm, 67 of which had no previous experimental values. Radiative lifetimes for 13 energy levels, including the low energy levels 3d(2)(F-3) 4s4p ((3)p) z(5)D(j)degrees have been measured using time-resolved laser-induced fluorescence. Intensity-calibrated Ti I spectra have been measured using Fourier transform spectroscopy to determine branching fractions for the decay channels of these levels. The branching fractions are combined with the radiative lifetimes to yield absolute transition probabilities and oscillator strengths. Our measurements include 50 transitions in the previously unobserved infrared region lambda > 1.0 mu m, a region of particular interest to the analysis of cool stars and brown dwarfs.
|
|
| 9. |
- Casassus, S., et al.
(författare)
-
A centimetre-wave excess over free-free emission in planetary nebulae
- 2007
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 382:4, s. 1607-1622
-
Tidskriftsartikel (refereegranskat)abstract
- We report a centimetre-wave (cm-wave, 5-31 GHz) excess over free-free emission in planetary nebulae (PNe). Accurate 31- and 250-GHz measurements show that the 31-GHz flux densities in our sample are systematically higher than the level of optically thin free-free continuum extrapolated from 250-GHz. The 31-GHz excess is observed, within one standard deviation, in all 18 PNe with reliable 31- and 250-GHz data, and is significant in nine PNe. The only exception is the peculiar object M2-9, whose radio spectrum is that of an optically thick stellar wind. On average the fraction of non-free-free emission represents 51 per cent of the total flux density at 31 GHz, with a scatter of 11 per cent. The average 31-250 GHz spectral index of our sample is = -0.43 +/- 0.03 (in flux density, with a scatter of 0.14). The 31-250 GHz drop is reminiscent of the anomalous foreground observed in the diffuse interstellar medium (ISM) by cosmic microwave background (CMB) anisotropy experiments. The 5-31 GHz spectral indices are consistent with both flat spectra and spinning dust emissivities, given the 10 per cent calibration uncertainty of the comparison 5-GHz data. However, a detailed study of the objects with the largest cm-excess, including the low-frequency data available in the literature, shows that present spinning dust models alone cannot explain the cm-wave excess in PNe. Although we have no definitive interpretation of our data, the least implausible explanation involves a synchrotron component absorbed by a cold nebular screen. We give flux densities for 37 objects at 31 GHz, and for 26 objects at 250 GHz.
|
|
| 10. |
- Casu, S., et al.
(författare)
-
The diffuse clouds towards Cyg OB2 No. 5 and No. 12
- 2005
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 359:1, s. 73-78
-
Tidskriftsartikel (refereegranskat)abstract
- We have investigated, in the 13CO(1-0) line, the region between the stars Cyg OB2 No. 5 and No, 12. The integrated intensity contour map reveals the existence of a condensation at 12 km s-1 (core C), north-west of Cyg OB2 No. 5, with a core-halo structure. The morphology and the derived physical parameters make it quite similar to core A, previously discovered towards Cyg OB2 No. 12. A second condensation at 7 km s -1, near Cyg OB2 No. 12, has been mapped in the same line (1-0) of 13CO. A summary of different observations, in all spectral regions, towards Cyg OB2 No. 5 and No. 12 is presented to provide the material for a further comprehensive discussion of this intricate and still intriguing region.
|
|
