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We present measurements of magnetic field strength and geometry on the surfaces of T Tauri stars (TTS) with and without circumstellar disks. We use these measurements to argue that magnetospheric accretion models should not assume that a fixed fraction of the stellar surface contains magnetic field lines that couple with the disk. We predict the fractional area of accretion footpoints, using magnetospheric accretion models and assuming field strength is roughly constant for all TTS. Analysis of Zeeman broadened infrared line profiles shows that individual TTS each have a distribution of surface magnetic field strengths extending up to 6 kG. Averaging over this distribution yields mean magnetic field strengths of 1-3 kG for all TTS, regardless of whether the star is surrounded by a disk. These strong magnetic fields suggest that magnetic pressure dominates gas pressure in TTS photospheres, indicating the need for new model atmospheres. The He I 5876 Å emission line in TTS can be strongly polarized, so that magnetic field lines at the footpoints of accretion have uniform polarity. The circular polarization signal appears to be rotationally modulated, implying that accretion and perhaps the magnetosphere are not axisymmetric. Time series spectropolarimetry is fitted reasonably well by a simple model with one magnetic spot on the surface of a rotating star. On the other hand, spectropolarimetry of photospheric absorption lines rules out a global dipolar field at the stellar surface for at least some TTS. 相似文献
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Matteo Correnti Francesco Paresce Rossella Aversa Giacomo Beccari Guido De Marchi Marcella Di Criscienzo Xiaoying Pang Loredana Spezzi Elena Valenti Paolo Ventura 《Astrophysics and Space Science》2012,340(2):263-279
We have used new, deep, visible and near infrared observations of the compact starburst cluster in the giant HII region NGC 3603 and its surroundings with the WFC3 on HST and HAWK-I on the VLT to study in detail the physical properties of its intermediate mass (∼1–3 M⊙) stellar population. We show that after correction for differential extinction and actively accreting stars, and the study of field star contamination, strong evidence remains for a continuous spread in the ages of pre-main sequence stars in the range ∼2 to ∼30 Myr within the temporal resolution available. Existing differences among presently available theoretical models account for the largest possible variation in shape of the measured age histograms within these limits. We also find that this isochronal age spread in the near infrared and visible Colour-Magnitude Diagrams cannot be reproduced by any other presently known source of astrophysical or instrumental scatter that could mimic the luminosity spread seen in our observations except, possibly, episodic accretion. The measured age spread and the stellar spatial distribution in the cluster are consistent with the hypothesis that star formation started at least 20–30 Myrs ago progressing slowly but continuously up to at least a few million years ago. All the stars in the considered mass range are distributed in a flattened oblate spheroidal pattern with the major axis oriented in an approximate South-East–North-West direction, and with the length of the equatorial axis decreasing with increasing age. This asymmetry is most likely due to the fact that star formation occurred along a filament of gas and dust in the natal molecular cloud oriented locally in this direction. 相似文献
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L. Origlia E. Valenti R. M. Rich 《Monthly notices of the Royal Astronomical Society》2005,356(4):1276-1282
Using the Near Infrared Spectrometer (NIRSPEC) spectrograph at Keck II, we have obtained infrared (IR) echelle spectra covering the range 1.5 –1.8 μm for the moderately reddened bulge globular clusters NGC 6342 and 6528, finding [Fe/H]=−0.60 and −0.17 dex, respectively. We measure an average α-enhancement of ≈+0.33 dex in both clusters, consistent with previous measurements on other metal-rich bulge clusters, and favouring the scenario of a rapid bulge formation and chemical enrichment. We also measure very low 12 C/13 C isotopic ratios (≈5 in NGC 6342 and ≈8 in NGC 6528), suggesting that extra-mixing mechanisms resulting from cool bottom processing are at work during evolution along the red giant branch (RGB). 相似文献
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Summary The magnetosphere depends on the astronomical orientation of the geomagnetic field with respect to the solar wind. The statistical distribution of polar auroras must therefore depend on the orientation of the geomagnetic field with respect to the ecliptic plane. We have investigated this peculiar feature of auroras that we call auroral astronomical geometry. We give here some preliminary results concerning a limited set of pre-IGY auroras. The criteria that we have chosen to prepare the auroral collection are also briefly summarized. The results conform to the hypothesis of the auroral origin from the magnetospheric neutral sheet. Auroral particles are found to impinge over the earth with low angles with respect to the ecliptic plane (40°). Only in a 4-hours interval around midnight they are found to impinge with angles up to 70°. Definite evidence of these facts requires further investigation with a larger amount of data. — We have also prepared a complex code for recording the morphological features of each aurora, namely: standard information, movement. intensity, color, sunlight illumination, period of pulsation, location in the sky among stars and planets, time evolution, duration and general features of the auroral display. It is well known in fact that the auroral morphology affects auroral heights and latitudes, etc.; presumably it should also affect its astronomical geometry, which we will investigate later.Presented at the Inter-Union Symposium on Solar-Terrestrial Physics — Belgrade 1966. 相似文献
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N. Elias-Rosa S. Benetti M. Turatto E. Cappellaro S. Valenti A. A. Arkharov J. E. Beckman A. Di Paola M. Dolci A. V. Filippenko R. J. Foley K. Krisciunas V. M. Larionov W. Li W. P. S. Meikle A. Pastorello G. Valentini W. Hillebrandt 《Monthly notices of the Royal Astronomical Society》2008,384(1):107-122
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Encarni Montoya Valenti Rull Nathan D. Stansell Broxton W. Bird Sandra Nogué Teresa Vegas‐vilarrúbia Mark B. Abbott Wilmer A. Díaz 《第四纪科学杂志》2011,26(2):207-218
The occurrence of the Younger Dryas cold reversal in northern South America midlands and lowlands remains controversial. We present a palaeoecological analysis of a Late Glacial lacustrine section from a midland lake (Lake Chonita, 4.6501 °N, 61.0157 °W, 884 m elevation) located in the Venezuelan Gran Sabana, based on physical and biological proxies. The sediments were mostly barren from ～15.3 to 12.7 k cal a BP, probably due to poor preservation. A ligneous community with no clear modern analogues was dominant from 12.7 to 11.7 k cal a BP (Younger Dryas chronozone). At present, similar shrublands are situated around 200 m elevation above the lake, suggesting a cooling‐driven downward shift in vegetation during that period. The interval from 11.7 to 10.6 k cal a BP is marked by a dramatic replacement of the shrubland by savannas and a conspicuous increase in fire incidence. The intensification of local and regional fires at this interval could have played a role in the vegetation shift. A change to wetter, and probably warmer, conditions is deduced after 11.7 k cal a BP, coinciding with the early Holocene warming. These results support the hypothesis of a mixed origin (climate and fire) of the Gran Sabana savannas, and highlight the climatic instability of the Neotropics during the Late Glacial. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
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L. Tomasella S. Benetti E. Cappellaro A. Pastorello M. Turatto R. Barbon N. Elias‐Rosa A. Harutyunyan P. Ochner L. Tartaglia S. Valenti 《Astronomische Nachrichten》2014,335(8):841-849
We present the compilation of the first 221 supernovae classified during the Asiago Classification Program (ACP). The details of transients classification and the preliminarily reduced spectra, in fits format, are immediately posted on the Padova‐Asiago SN group web site. The achieved performances for the first 2 years of the ACP are analysed, showing that half of all our classifications were made within 5 days from transient detection. The distribution of the supernova types of this sample resembles the distribution of the general list of all the supernovae listed in the Asiago SN catalog (ASNC, Barbon et al. 1999). Finally, we use our subsample of 78 core‐collapse supernovae, for which we retrieve the host‐galaxy morphology and r ‐band absolute magnitudes, to study the observed subtype distribution in dwarf compared to giant galaxies. This ongoing program will give its contribution to the classification of the large number of transients that will be soon delivered by the Gaia mission. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献