| 1. |
- Soffel, M., et al.
(författare)
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The IAU 2000 Resolutions for Astrometry, Celestial Mechanics, and Metrology in the Relativistic Framework: Explanatory Supplement
- 2003
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X. ; 126:6, s. 2687-2706
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Tidskriftsartikel (refereegranskat)abstract
- We discuss the IAU resolutions B1.3, B1.4, B1.5, and B1.9 that wereadopted during the 24th General Assembly in Manchester, 2000, andprovides details on and explanations for these resolutions. It isexplained why they present significant progress over the correspondingIAU 1991 resolutions and why they are necessary in the light of presentaccuracies in astrometry, celestial mechanics, and metrology. In fact,most of these resolutions are consistent with astronomical models andsoftware already in use. The metric tensors and gravitational potentialsof both the Barycentric Celestial Reference System and the GeocentricCelestial Reference System are defined and discussed. The necessity andrelevance of the two celestial reference systems are explained. Thetransformations of coordinates and gravitational potentials arediscussed. Potential coefficients parameterizing the post-Newtoniangravitational potentials are expounded. Simplified versions of the timetransformations suitable for modern clock accuracies are elucidated.Various approximations used in the resolutions are explicated andjustified. Some models (e.g., for higher spin moments) that serve thepurpose of estimating orders of magnitude have actually never beenpublished before.
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| 2. |
- Elmegreen, B.G., et al.
(författare)
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Hubble Space Telescope Observations of the Interacting Galaxies NGC 2207 and IC 2163
- 2000
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Ingår i: Astronomical Journal. - : American Astronomical Society. - 1538-3881 .- 0004-6256. ; 120:2, s. 630-644
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Tidskriftsartikel (refereegranskat)abstract
- Hubble Space Telescope (HST) images of the galaxies NGC 2207 and IC 2163 show star formation and dust structures in a system that has experienced a recent grazing encounter. Tidal forces from NGC 2207 compressed and elongated the disk of IC 2163, forming an oval ridge of star formation along a caustic where the perturbed gas rebounded after its inward excursion. Gas flowing away from this ridge has a peculiar structure characterized by thin parallel dust filaments transverse to the direction of motion. The filaments become thicker and longer as the gas approaches the tidal arm. Star formation that occurs in the filaments consistently lags behind, as if the exponential disk pressure gradient pushes outward on the gas but not on the young stars. Numerical models suggest that the filaments come from flocculent spiral arms that were present before the interaction. The arms stretch out into parallel filaments as the tidal tail forms. A dust lane at the outer edge of the tidal tail is a shock front where the flow abruptly changes direction. Gas at small-to-intermediate radii along this edge flows back toward the galaxy, while elsewhere in the tidal arm, the gas flows outward.A spiral arm of NGC 2207 that is backlit by IC 2163 is seen with HST to contain several parallel, knotty filaments spanning the full width of the arm. These filaments are probably shock fronts in a density wave. The parallel structure suggests that the shocks occur in several places throughout the arm, or that the interarm gas is composed of spiral-like wisps that merge together in the arms. Blue clusters of star formation inside the clumps of these dust lanes show density-wave triggering in unprecedented detail. The star formation process seems to be one of local gravitational collapse, rather than cloud collisions. Spiral arms inside the oval of IC 2163 have a familiar geometry reminiscent of a bar, although there is no obvious stellar bar. The shape and orientation of these arms suggest they could be the result of inner Lindblad resonance–related orbits in the cos 2θ tidal potential that formed the oval. Their presence suggests that tidal forces alone may initiate a temporary nuclear gas flow and eventual starburst without first forming a stellar bar. Several emission structures resembling jets or conical flows that are 100–1000 pc long appear in these galaxies. In the western arm of NGC 2207, there is a dense dark cloud with a conical shape 400 pc long and a bright compact cluster at the tip, and there is a conical emission nebula of the same length that points away from the cluster in the other direction. This region also coincides with a nonthermal radio continuum source that is ~1000 times the luminosity of Cas A at λ = 20 cm. Surrounding clusters in arclike patterns may have been triggered by enormous explosions.
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| 3. |
- Elmegreen, D.M., et al.
(författare)
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Hubble Space Telescope Observations of Dust and Star-forming Regions in the Ocular Galaxy IC 2163 and Its Spiral Companion NGC 2207
- 2001
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Ingår i: Astronomical Journal. - : American Astronomical Society. - 1538-3881 .- 0004-6256. ; 121:1, s. 182-197
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Tidskriftsartikel (refereegranskat)abstract
- Hubble Space Telescope observations in U, B, V, and I passbands of the interacting spiral galaxies IC 2163 and NGC 2207 are used to measure extinctions in the cloud and intercloud regions and ages and luminosities of the star-forming regions. The extinction in the part of NGC 2207 seen in projection against IC 2163 was determined by using the method of White & Keel. The extinctions there and elsewhere were also determined from radiative transfer models of the magnitude differences between clouds and their surroundings. The intercloud extinction in V band ranges from 0.5 to 1 mag on the line of sight, and the cloud extinction ranges from 1 to 2 mag. The measured star-forming regions in these galaxies have a power-law relation between size and luminosity and a power-law luminosity distribution function. These power laws are consistent with a fractal dimension for the star formation that is the same as that for interstellar gas, D ~ 2.2, extending over scales ranging from 20 to 1000 pc. Fifteen compact massive star clusters that are analogous to super–star clusters found in starburst regions are in the spiral arms of NGC 2207. Nothing is peculiar about these regions except for a high H I velocity dispersion (~50 km s-1). Two more super–star clusters are in the tidally compressed oval of IC 2163. These clusters have masses ranging from ~10^4 to 2 × 10^5 M⊙ and ages of a few times 10^6 yr.
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| 4. |
- Garavini, G, et al.
(författare)
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Spectroscopic Observations and Analysis of the Peculiar SN 1999aa
- 2004
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Ingår i: Astronomical Journal. - : The American Astronomical Society.. - 0004-6256 .- 1538-3881. ; 128, s. 387-404
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Tidskriftsartikel (refereegranskat)abstract
- We present an extensive new time series of spectroscopic data of the peculiar SN 1999aa in NGC 2595. Ourdata set includes 25 optical spectra between 11 and +58 days with respect to B-band maximum light, providingan unusually complete time history. The early spectra resemble those of an SN 1991T–like object but with arelatively strong Ca H and K absorption feature. The first clear sign of Si ii k6355, characteristic of Type Iasupernovae, is found at day 7, and its velocity remains constant up to at least the first month after B-bandmaximum light. The transition to normal-looking spectra is found to occur earlier than in SN 1991T, suggestingSN 1999aa as a possible link between SN 1991T–like and Branch-normal supernovae. Comparing the observationswith synthetic spectra, doubly ionized Fe, Si, and Ni are identified at early epochs. These are characteristicof SN 1991T–like objects. Furthermore, in the day 11 spectrum, evidence is found for an absorptionfeature that could be identified as high velocity C ii k6580 or H. At the same epoch C iii k4648.8 at photosphericvelocity is probably responsible for the absorption feature at 4500 8. High-velocity Ca is found aroundmaximum light together with Si ii and Fe ii confined in a narrow velocity window. Implied constraints onsupernovae progenitor systems and explosion hydrodynamic models are briefly discussed.
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| 5. |
- Kaufman, M., et al.
(författare)
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CO observations of the interacting galaxy pair NGC 5394/95
- 2002
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Ingår i: Astronomical Journal. - : American Astronomical Society. - 1538-3881 .- 0004-6256. ; 123:2 1754, s. 702-719
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Tidskriftsartikel (refereegranskat)abstract
- BIMA 12CO J = 1 → 0 observations are presented of the spiral galaxies NGC 5394 and NGC 5395 that have undergone a recent, grazing encounter. In NGC 5394 approximately 80% of the CO emission detected by BIMA is concentrated in the central 800 pc (FWHM) starburst region, and the rest is from a portion of the inner disk south and west of the central starburst. In an encounter simulation that reproduces some of the main features of this galaxy pair, a considerable amount of gas in NGC 5394 falls into the central region early in the collision. The observed total gas distribution in the disk of NGC 5394 is lopsided, with more H I, CO, and Hα emission coming from the western or southwestern side. The innermost western arm of NGC 5394 is seen in CO and Hα emission, but the eastern inner-disk arm, which is very bright in the optical continuum, is not detected in CO or Hα emission. The NGC 5394 starburst region is similar in radio continuum luminosity and size to the M82 starburst and has a CO luminosity ∼4 times greater. A CO position-velocity diagram of the NGC 5394 nucleus reveals two separate velocity features very close to the center. This may indicate a nuclear ring or the "twin peaks" of an ILR or some depletion of 12CO J = 1 → 0 at the nucleus. From a comparison of the radio continuum, Hα, 60 μm, and CO luminosities, we estimate that the average extinction A v of the starburst nucleus is 3-4 mag, the star formation rate is ∼6 M⊙ yr-1, and the conversion factor N(H 2)/ICO in the starburst is a factor of 3-4 below the standard value. Comparison of NGC 5394 with two other systems previously studied suggests that in prograde grazing encounters a central starburst may not develop until near the end of the ocular phase. Very little of the CO emission from NGC 5395 found in previous single-dish observations is detected in the BIMA data; thus molecular gas in NGC 5395 does not appear to be strongly concentrated in compressed ridges.
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| 6. |
- Reipurth, Bo, et al.
(författare)
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Hα Emission-Line Stars in Molecular Clouds. I. The NGC 2264 Region
- 2004
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Ingår i: Astronomical Journal. - : American Astronomical Society. - 0004-6256 .- 1538-3881. ; 127:2, s. 1117-1130
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Tidskriftsartikel (refereegranskat)abstract
- We present a deep survey of Hα emission-line stars in the NGC 2264 region using wide-field objective prism films. We find 357 Hα emission-line stars, of which 113 were previously detected, within an area of 3°×3° centered on the Cone Nebula, with a majority of stars being concentrated in a dense cluster at the center of the region. We present a large-scale CO map of NGC 2264 and find a strong correlation between the cluster of Hα emission-line stars and the most massive core in the cloud complex. A more extended halo of stars poorly correlated with the gas may represent stars that have drifted away from their birth place, suggesting that star formation has taken place in NGC 2264 for several million years.
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| 7. |
- Stoica, Petre, et al.
(författare)
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Adaptive Filter-bank Approach to Restoration and Spectral Analysis of Gapped Data
- 2000
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Ingår i: Astronomical Journal. - : American Astronomical Society. - 0004-6256 .- 1538-3881. ; 120:4, s. 2163-2173
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Tidskriftsartikel (refereegranskat)abstract
- The main topic of this paper is the nonparametric estimation of complex (both amplitude and phase) spectra from gapped data, as well as the restoration of such data. The focus is on the extension of the APES (amplitude and phase estimation) approach to data sequences with gaps. APES, which is one of the most successful existing nonparametric approaches to the spectral analysis of full data sequences, uses a bank of narrowband adaptive (both frequency and data dependent) filters to estimate the spectrum. A recent interpretation of this approach showed that the filterbank used by APES and the resulting spectrum minimize a least-squares (LS) fitting criterion between the filtered sequence and its spectral decomposition. The extended approach, which is called GAPES for somewhat obvious reasons, capitalizes on the aforementioned interpretation: it minimizes the APES-LS fitting criterion with respect to the missing data as well. This should be a sensible thing to do whenever the full data sequence is stationary, and hence the missing data have the same spectral content as the available data. We use both simulated and real data examples to show that GAPES estimated spectra and interpolated data sequences have excellent accuracy. We also show the performance gain achieved by GAPES over two of the most commonly used approaches for gapped-data spectral analysis, viz., the periodogram and the parametric CLEAN method.
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| 8. |
- Zethson, Torgil, et al.
(författare)
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Sr II and [Sr II] Emission in the Ejecta of eta Carinae
- 2001
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Ingår i: The Astronomical Journal. - : American Astronomical Society. - 1538-3881 .- 0004-6256. ; 122:1, s. 322-326
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Tidskriftsartikel (refereegranskat)abstract
- We have discovered four extremely surprising emission lines of strontiumin ejecta near eta Carinae. Hubble Space Telescope (HST) SpaceTelescope Imaging Spectrograph (STIS) observations made in 1999 show twonarrow features whose wavelengths correspond to the forbiddentransitions of Sr II, and we have found no other plausibleidentification for these lines. The identifications are confirmed by newHST/STIS observations of the same stellar position, in which the Sr IIresonance lines are observed. Moreover, [Ti II, [Ni II, [Mn II, and[Co II lines are unusually strong relative to [Fe II at the sameposition. Based on observations made with the NASA/ESA Hubble SpaceTelescope and supported by STIS IDT GTO 8036 and grant GO 8327 from theSpace Telescope Science Institute. The STScI is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
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| 9. |
- Reipurth, B, et al.
(författare)
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Spectroscopic pre-main-sequence binaries. II. Haro 1-14c and Parenago 2494
- 2002
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Ingår i: The Astronomical Journal. - 1538-3881. ; 124:5, s. 2813-2820
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Tidskriftsartikel (refereegranskat)abstract
- In the course of a long-term radial velocity survey of about 100 southern pre-main-sequence (PMS) stars, we have discovered two spectroscopic PMS binaries. One is the weak-lined T Tauri star Haro 1-14c in the Ophiuchus clouds, which has a spectral type of K3 and is a member of the visual Haro 1-14 binary with a separation of 12."9. Haro 1-14c is a single-lined spectroscopic binary with a very long period of 591 days. The other is Parenago 2494, a weak-lined T Tauri star with a spectral type of K0 located in the Orion cluster. It is a double-lined spectroscopic binary with a period of 19.5 days. Our photometric monitoring shows that P2494 is a low-amplitude variable with a period of 5.77 days. We have determined accurate orbital elements for both binaries. We further discuss five low-amplitude velocity variable stars, which are possibly additional spectroscopic PMS binaries.
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| 10. |
- Stoica, P, et al.
(författare)
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Adaptive filter-bank approach to restoration and spectral analysis of gapped data
- 2000
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Ingår i: ASTRONOMICAL JOURNAL. - : UNIV CHICAGO PRESS. - 0004-6256. ; 120:4, s. 2163-2173
-
Tidskriftsartikel (refereegranskat)abstract
- The main topic of this paper is the nonparametric estimation of complex (both amplitude and phase) spectra from gapped data, as well as the restoration of such data. The focus is on the extension of the APES (amplitude and phase estimation) approach to da
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