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Non-axisymmetric accretion on the classical TTS RW Aur A

Petrov, P. P.; Gahm, G. F.; Gameiro, J. F.; Duemmler, R.; Ilyin, I. V.; Laakkonen, T.; Lago, M. T. V. T.; Tuominen, I.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 08/02/2001 Português
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(Abridged) High-resolution spectroscopic monitoring of RW Aur A was carried out in 1996, 1998 and 1999 with simultaneous B, V photometry. A multicomponent spectrum is revealed with a veiled photospheric spectrum, broad emissions, narrow emission lines of helium, and accretion, wind and shell features. Periodic modulations in many spectral features were found. The photospheric absorption lines show sinusoidal variations in radial velocity with an amplitude of +-6 km/s and a period of about 2.77 days. The radial velocities of the narrow emission lines of He vary with the same period but in anti-phase to the photospheric lines. The equivalent widths of the narrow emissions vary with a phase-shift with respect to the velocity curve. The strength of the red-shifted accretion components of Na D and other lines is also modulated with the same period. The broad emission lines of metals vary mostly with the double period of about 5.5 days. One unexpected result is that no correlation was found between the veiling and the brightness, although both varied in wide ranges. This is partly due to a contribution of the shell absorption to the photospheric line profiles, which make them vary in width and depth thus simulating lower veiling. Most of the observed features can be interpreted in the framework of non-axisymmetric magnetospheric accretion. We consider two possible models. In the first the asymmetry is induced by orbital motion of an invisible...

The Circumstellar Environment of the TTS SU Aurigae

Oliveira, J. M.; Foing, B. H.; van Loon, J. Th.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 27/04/2000 Português
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In this contribution we investigate how a disk determines the physical and geometrical properties of the circumstellar environment of the T Tauri SU Aurigae. Our model of the spectral energy distribution of this star includes the central young star, a flat black disk and a diffuse envelope. We also describe the inner interaction region between the disk and the star, the magnetosphere, by analysing the accretion and wind signatures in several spectral lines observed during the MUSICOS 96 campaign.; Comment: to be published by ASP Conference Series ``Disks, Planetesimals and Planets'', eds. F. Garzon, Carlos Eiroa, Dolf de Winter and T. J. Mahoney

Stellar contents and star formation in the young star cluster Be 59

Pandey, A. K.; Sharma, Saurabh; Ogura, K.; Ojha, D. K.; Chen, W. P.; Bhatt, B. C.; Ghosh, S. K.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 29/10/2007 Português
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We present $UBVI_C$ CCD photometry of the young open cluster Be 59 with the aim to study the star formation scenario in the cluster. The radial extent of the cluster is found to be $\sim$ 10 arcmin (2.9 pc). The interstellar extinction in the cluster region varies between $E(B-V) \simeq$ 1.4 to 1.8 mag. The ratio of total-to-selective extinction in the cluster region is estimated as $3.7\pm0.3$. The distance of the cluster is found to be $1.00\pm0.05$ kpc. Using near-infrared colours and slitless spectroscopy, we have identified young stellar objects (YSOs) in the open cluster Be 59 region. The ages of these YSOs range between $<1$ Myr to $\sim$ 2 Myr, whereas the mean age of the massive stars in the cluster region is found to be $\sim$ 2 Myr. There is evidence for second generation star formation outside the boundary of the cluster, which may be triggered by massive stars in the cluster. The slope of the initial mass function, $\Gamma$, in the mass range $2.5 < M/M_\odot \le 28$ is found to be $-1.01\pm0.11$ which is shallower than the Salpeter value (-1.35), whereas in the mass range $1.5 < M/M_\odot \le 2.5$ the slope is almost flat. The slope of the K-band luminosity function is estimated as $0.27\pm0.02$, which is smaller than the average value ($\sim$0.4) reported for young embedded clusters. Approximately 32% of H$\alpha$ emission stars of Be 59 exhibit NIR excess indicating that inner disks of the T-Tauri star (TTS) population have not dissipated. The MSX and IRAS-HIRES images around the cluster region are also used to study the emission from unidentified infrared bands and to estimate the spatial distribution of optical depth of warm and cold interstellar dust.; Comment: Accepted for the publication in MNRAS...

Learning, Social Intelligence and the Turing Test - why an "out-of-the-box" Turing Machine will not pass the Turing Test

Edmonds, Bruce; Gershenson, Carlos
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 15/03/2012 Português
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The Turing Test (TT) checks for human intelligence, rather than any putative general intelligence. It involves repeated interaction requiring learning in the form of adaption to the human conversation partner. It is a macro-level post-hoc test in contrast to the definition of a Turing Machine (TM), which is a prior micro-level definition. This raises the question of whether learning is just another computational process, i.e. can be implemented as a TM. Here we argue that learning or adaption is fundamentally different from computation, though it does involve processes that can be seen as computations. To illustrate this difference we compare (a) designing a TM and (b) learning a TM, defining them for the purpose of the argument. We show that there is a well-defined sequence of problems which are not effectively designable but are learnable, in the form of the bounded halting problem. Some characteristics of human intelligence are reviewed including it's: interactive nature, learning abilities, imitative tendencies, linguistic ability and context-dependency. A story that explains some of these is the Social Intelligence Hypothesis. If this is broadly correct, this points to the necessity of a considerable period of acculturation (social learning in context) if an artificial intelligence is to pass the TT. Whilst it is always possible to 'compile' the results of learning into a TM...

Wealth distribution across communities of adaptive financial agents

DeLellis, Pietro; Garofalo, Franco; Iudice, Francesco Lo; Napoletano, Elena
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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This paper studies the trading volumes and wealth distribution of a novel agent-based model of an artificial financial market. In this model, heterogeneous agents, behaving according to the Von Neumann and Morgenstern utility theory, may mutually interact. A Tobin-like tax (TT) on successful investments and a flat tax are compared to assess the effects on the agents' wealth distribution. We carry out extensive numerical simulations in two alternative scenarios: i) a reference scenario, where the agents keep their utility function fixed, and ii) a focal scenario, where the agents are adaptive and self-organize in communities, emulating their neighbours by updating their own utility function. Specifically, the interactions among the agents are modelled through a directed scale-free network to account for the presence of community leaders, and the herding-like effect is tested against the reference scenario. We observe that our model is capable of replicating the benefits and drawbacks of the two taxation systems and that the interactions among the agents strongly affect the wealth distribution across the communities. Remarkably, the communities benefit from the presence of leaders with successful trading strategies, and are more likely to increase their average wealth. Moreover...

A Flexible Rule Compiler for Speech Synthesis

Skut, Wojciech; Ulrich, Stefan; Hammervold, Kathrine
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 23/03/2004 Português
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We present a flexible rule compiler developed for a text-to-speech (TTS) system. The compiler converts a set of rules into a finite-state transducer (FST). The input and output of the FST are subject to parameterization, so that the system can be applied to strings and sequences of feature-structures. The resulting transducer is guaranteed to realize a function (as opposed to a relation), and therefore can be implemented as a deterministic device (either a deterministic FST or a bimachine).; Comment: 10 pages, 6 figures

The disk-bearing young star IM Lup

Guenther, H. M.; Matt, S. P.; Schmitt, J. H. M. M.; Guedel, M.; Li, Z. -Y.; Burton, D. M.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 24/05/2010 Português
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Classical T Tauri stars (CTTS) differ in their X-ray signatures from older pre-main sequence stars, e.g. weak-lined TTS (WTTS). CTTS show a soft excess and deviations from the low-density coronal limit in the He-like triplets. We test whether these features correlate with accretion or the presence of a disk by observing IM Lup, a disk-bearing object apparently in transition between CTTS and WTTS without obvious accretion. We analyse a Chandra grating spectrum and additional XMM-Newton data of IM Lup and accompanying optical spectra, some of them taken simultaneously to the X-ray observations. We fit the X-ray emission lines and decompose the Ha emission line in different components. In X-rays IM Lup has a bright and hot active corona, where elements of low first-ionisation potential are depleted. The He-like Ne IX triplet is in the low-density state, but due to the small number of counts a high-density scenario cannot be excluded on the 90% confidence level. In all X-ray properties IM Lup resembles a main-sequence star, but it is also compatible with CTTS signatures on the 90% confidence level, thus we cannot decide if the soft excess and deviations from the low-density coronal limit in the He-like triplets in CTTS require accretion or only the presence of a disk. IM Lup is chromospherically active...

The X-ray soft excess in classical T Tauri stars

Guedel, M.; Telleschi, A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 06/09/2007 Português
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We study an anomaly in the X-ray flux (or luminosity) ratio between the OVII 21.6-22.1A triplet and the OVIII Lya line seen in classical T Tauri stars (CTTS). This ratio is unusually high when compared with ratios for main-sequence and non-accreting T Tauri stars (Telleschi et al. 2007). We compare these samples to identify the source of the excess. A sample of recently discovered X-ray stars with a soft component attributed to jet emission is also considered. We discuss data obtained from the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST) project, complemented by data from the published literature. We also present data from the CTTS RU Lup. All CTTS in the sample show an anomalous OVII/OVIII flux ratio when compared with WTTS or MS stars. The anomaly is due to an excess of cool, OVII emitting material rather than a deficiency of hotter plasma. The excess plasma must therefore have temperatures of <~2 MK. This soft excess does not correlate with UV excesses of CTTS, but seems to be related with the stellar X-ray luminosity. The spectra of the jet-driving TTS do not fit into this picture. The soft excess depends both on the presence of accretion streams in CTTS and on magnetic activity. The gas may be shock-heated near the surface...

The large-scale disk fraction of brown dwarfs in the Taurus cloud as measured with Spitzer

Monin, J. -L.; Guieu, S.; Pinte, C.; Rebull, L.; Goldsmith, P.; Fukagawa, M.; Ménard, F.; Padgett, D.; Stappelfeld, K.; McCabe, C.; Carey, S.; Noriega-Crespo, A.; Brooke, T.; Huard, T.; Terebey, S.; Hillenbrand, L.; Guedel, M.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 14/04/2010 Português
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Aims. The brown dwarf (BD) formation process has not yet been completely understood. To shed more light on the differences and similarities between star and BD formation processes, we study and compare the disk fraction among both kinds of objects over a large angular region in the Taurus cloud. In addition, we examine the spatial distribution of stars and BD relative to the underlying molecular gas Methods. In this paper, we present new and updated photometry data from the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope on 43 BDs in the Taurus cloud, and recalculate of the BD disk fraction in this region. We also useed recently available CO mm data to study the spatial distribution of stars and BDs relative to the cloud's molecular gas. Results. We find that the disk fraction among BDs in the Taurus cloud is 41 \pm 12%, a value statistically consistent with the one among TTS (58 \pm 9%). We find that BDs in transition from a state where they have a disk to a diskless state are rare, and we study one isolated example of a transitional disk with an inner radius of \approx 0.1 AU (CFHT BD Tau 12, found via its relatively small mid-IR excess compared to most members of Taurus that have disks. We find that BDs are statistically found in regions of similar molecular gas surface density to those associated with stars. Furthermore...

A Bengali HMM Based Speech Synthesis System

Mukherjee, Sankar; Mandal, Shyamal Kumar Das
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 16/06/2014 Português
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The paper presents the capability of an HMM-based TTS system to produce Bengali speech. In this synthesis method, trajectories of speech parameters are generated from the trained Hidden Markov Models. A final speech waveform is synthesized from those speech parameters. In our experiments, spectral properties were represented by Mel Cepstrum Coefficients. Both the training and synthesis issues are investigated in this paper using annotated Bengali speech database. Experimental evaluation depicts that the developed text-to-speech system is capable of producing adequately natural speech in terms of intelligibility and intonation for Bengali.

Physical Conditions of Accreting Gas in T Tauri Star Systems

Bary, Jeffrey S.; Matt, Sean P.; Skrutskie, Michael F.; Wilson, John C.; Peterson, Dawn E.; Nelson, Matthew J.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 10/07/2008 Português
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We present results from a low resolution (R~300) near-infrared spectroscopic variability survey of actively accreting T Tauri stars (TTS) in the Taurus-Auriga star forming region. Paschen and Brackett series H I recombination lines were detected in 73 spectra of 15 classical T Tauri systems. The values of the Pan/PaB, Brn/BrG, and BrG/Pan H I line ratios for all observations exhibit a scatter of < 20% about the weighted mean, not only from source to source, but also for epoch-to-epoch variations in the same source. A representative or `global' value was determined for each ratio in both the Paschen and Brackett series as well as the BrG/Pan line ratios. A comparison of observed line ratio values was made to those predicted by the temperature and electron density dependent models of Case B hydrogen recombination line theory. The measured line ratios are statistically well-fit by a tightly constrained range of temperatures (T < 2000 K) and electron densities 1e9 < n_e < 1e10 cm^-3. A comparison of the observed line ratio values to the values predicted by the optically thick and thin local thermodynamic equilibrium cases rules out these conditions for the emitting H I gas. Therefore, the emission is consistent with having an origin in a non-LTE recombining gas. While the range of electron densities is consistent with the gas densities predicted by existing magnetospheric accretion models...

High-Resolution Spectroscopy of FUors

Herbig, G. H.; Petrov, P. P.; Duemmler, R.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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High-resolution spectroscopy was obtained of the FUors FU Ori and V1057 Cyg between 1995 and 2002 with SOFIN at NOT and with HIRES at Keck I. During those years FU Ori remained about 1 mag. (in B) below its 1938-39 maximum brightness, but V1057 Cyg (B ~ 10.5 at peak in 1970-71) faded from about 13.5 to 14.9 and then recovered slightly. Their photospheric spectra resemble a rotating G0 Ib supergiant, with v_eq sin i = 70 km/s for FU Ori and 55 km/s for V1057 Cyg. As V1057 Cyg faded, P Cyg structure in Halpha and the IR CaII lines strengthened and a complex shortward-displaced shell spectrum increased in strength, disappeared in 1999, and reappeared in 2001. Night-to-night changes in the wind structure of FU Ori show evidence of sporadic infall. The strength of P Cyg absorption varied cyclically with a period of 14.8 days, with phase stability maintained over 3 seasons, and is believed to be the rotation period. The structure of the photospheric lines also varies cyclically, but with a period of 3.54 days. A similar variation may be present in V1057 Cyg. As V1057 Cyg has faded, the emission lines of a pre-existing low-excitation chromosphere have emerged, so we believe the `line doubling' in V1057 Cyg is produced by these central emission cores in the absorption lines...

Kinematics of T Tauri stars in Chamaeleon

Frink, Sabine; Roeser, Siegfried; Alcala, Juan M.; Covino, Elvira; Brandner, Wolfgang
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 02/07/1998 Português
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We study the kinematics of T Tauri stars (TTS) located in the cores of the Chamaeleon clouds as well as far off these clouds. Our sample comprises 2 early type stars known to be related to Cha I, 6 classical (CTTS) and 6 weak-line T Tauri stars (WTTS) known before the ROSAT mission, and 8 bona-fide pre-main sequence (PMS) stars as well as 23 presumably older stars discovered with ROSAT (Alcala et al. 1995; Covino et al. 1997). Altogether we present proper motions for 45 stars, taken from the Hipparcos, ACT and STARNET catalogues. For 12 stars of our sample parallaxes measured by Hipparcos are available, and we use them to derive constraints on the distance distribution of the other stars in our sample. Our analysis of the proper motions allows us to divide the sample into several subgroups. We analyse the motions of the stars in connection with different star formation scenarios and find them consistent with both the high velocity cloud (HVC) impact model (Lepine & Duvert 1994) and the cloudlet model (Feigelson 1996), whereas the data seem to be inconsistent with any kind of a dynamical ejection model.; Comment: 11 pages, 4 figures, accepted for publication in Astronomy & Astrophysics

Testing Disk-Locking in NGC 2264

Cauley, P. Wilson; Johns-Krull, Christopher M.; Hamilton, Catrina M.; Lockhart, Kelly
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 13/07/2012 Português
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We test analytic predictions from different models of magnetospheric accretion, which invoke disk-locking, using stellar and accretion parameters derived from models of low resolution optical spectra of 36 T Tauri stars (TTSs) in NGC 2264 (age~3 Myrs). Little evidence is found for models that assume purely dipolar field geometries; however, strong support is found in the data for a modified version of the X-wind model (Shu et al. 1994) which allows for non-dipolar field geometries. The trapped flux concept in the X-wind model is key to making the analytic predictions which appear supported in the data. By extension, our analysis provides support for the outflows predicted by the X-wind as these also originate in the trapped flux region. In addition, we find no support in the data for accretion powered stellar winds from young stars. By comparing the analysis presented here of NGC 2264 with a similar analysis of stars in Taurus (age~1-2 Myr), we find evidence that the equilibrium interaction between the magnetic field and accretion disk in TTS systems evolves as the stars grow older, perhaps as the result of evolution of the stellar magnetic field geometry. We compare the accretion rates we derive with accretion rates based on U-band excess...

Methodological considerations for the evaluation of TTS AD's acceptance in the Catalan context

Fernández Torné, Anna; Matamala, Anna
Fonte: Universidade Autônoma de Barcelona Publicador: Universidade Autônoma de Barcelona
Tipo: Conferência ou Objeto de Conferência Formato: application/pdf
Publicado em //2013 Português
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NSC82063; TTS 5; 1,1'-Spirobi[indane-5,6-diol], 3,3,3',3'-tetramethyl-; 1, 1'-Spirobi[indan]-5,5',6,6'-tetrol, 3,3,3',3'-tetramethyl- (8CI); 1,1'-Spirobi[1H-indene]-5,5',6,6'-tetrol, 2,2',3,3'-tetrahydro-3, 3,3',3'-tetramethyl- (9CI)

US National Cancer Institute
Fonte: Unilever Center for Molecular Informatics, Cambridge University Publicador: Unilever Center for Molecular Informatics, Cambridge University
Tipo: Outros Formato: 7478 bytes; 6738 bytes; chemical/x-cml; chemical/x-cml
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NSC25953; anti-Ethyl; Abstensil; Abstinil; Abstinyl; Alcophobin; Alk-Aubs; Antabus; Antabuse; Antadix; Antaenyl; Antaethan; Antaethyl; Antaetil; Antalcol; Antetan; Antethyl; Antetil; Anteyl; Anthethyl; Antiaethan; Anticol; Antietanol; Antietil; Antikol; Antivitium; Aversan; Averzan; Bis(diethylthiocarbamoyl) disulfide; Bis(N, N-diethylthiocarbamoyl) disulfide; Bis[(diethylamino)thioxomethyl] disulfide; Bonibal; Contralin; Contrapot; Cronetal; Disetil; Disulfan; Disulfide, bis(diethylthiocarbamoyl) (8CI); Disulfiram; Disulfuram; Ekagom TEDS; Ekagom TETDS; Ephorran; Espenal; Esperal; Etabus; Ethyl tuads; Ethyl Thiram; Ethyl Thiudad; Ethyl Thiurad; Ethyl Tuex; Ethyldithiourame; Ethyldithiurame; Exhorran; Formamide, 1, 1'-dithiobis(N,N-diethylthio)-; Hoca; Hocakrotenalnci-C02959; Krotenal; N,N,N',N'-Tetraethylthiuram disulfide; Nocbin; Noxal (VAN); Refusal; Stopaethyl; Stopethyl; Stopetyl; Tenurid; Tenutex; Tetidis; Tetradin; Tetradine; Tetraethylthioperoxydicarbonic diamide; Tetraethylthiram disulfide; Tetraethylthiuram disulfide; Tetraethylthiuran disulfide; Tetraetil; Teturam; Teturamin; Thiocid; Thioperoxydicarbonic diamide ([(H2N)C(S)]2S2), tetraethyl- (9CI); Thiosan; Thioscabin; Thireranide; Thiuram disulfide, tetraethyl-; Thiuram E; Thiuranide; Tillram; Tiuram; Tuads, ethyl; TATD; TETD; TTD; TTS; Usaf B-33; WLN: 2N2 & YUS & S 2; 1,1'-Dithiobis[N,N-diethylthioformamide]

US National Cancer Institute
Fonte: Unilever Center for Molecular Informatics, Cambridge University Publicador: Unilever Center for Molecular Informatics, Cambridge University
Tipo: Outros Formato: 7976 bytes; 8007 bytes; chemical/x-cml; chemical/x-cml
Português
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Precise High-cadence Time Series Observations of Five Variable Young Stars in Auriga with MOST

Cody, Ann Marie; Tayar, Jamie; Hillenbrand, Lynne A.; Matthews, Jaymie M.; Kallinger, Thomas
Fonte: American Astronomical Society Publicador: American Astronomical Society
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em /03/2013 Português
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To explore young star variability on a large range of timescales, we have used the MOST satellite to obtain 24 days of continuous, sub-minute cadence, high-precision optical photometry on a field of classical and weak-lined T Tauri stars (TTSs) in the Taurus-Auriga star formation complex. Observations of AB Aurigae, SU Aurigae, V396 Aurigae, V397 Aurigae, and HD 31305 reveal brightness fluctuations at the 1%-10% level on timescales of hours to weeks. We have further assessed the variability properties with Fourier, wavelet, and autocorrelation techniques, identifying one significant period per star. We present spot models in an attempt to fit the periodicities, but find that we cannot fully account for the observed variability. Rather, all stars exhibit a mixture of periodic and aperiodic behavior, with the latter dominating stochastically on timescales less than several days. After removal of the main periodicity, periodograms for each light curve display power-law trends consistent with those seen for other young accreting stars. Several of our targets exhibited unusual variability patterns not anticipated by prior studies, and we propose that this behavior originates with the circumstellar disks. The MOST observations underscore the need for investigation of TTS light variations on a wide range of timescales in order to elucidate the physical processes responsible; we provide guidelines for future time series observations.

The large-scale disk fraction of brown dwarfs in the Taurus cloud as measured with Spitzer

Monin, J.-L.; Guieu, S.; Pinte, C.; Rebull, L.; Goldsmith, P.; Fukagawa, M.; Ménard, F.; Padgett, D.; Stappelfeld, K.; McCabe, C.; Carey, S.; Noriega-Crespo, A.; Brooke, T.; Huard, T.; Terebey, S.; Hillenbrand, L.; Guedel, M.
Fonte: EDP Sciences Publicador: EDP Sciences
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em /06/2010 Português
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Aims. The brown dwarf (BD) formation process has not yet been completely understood. To shed more light on the differences and similarities between star and BD formation processes, we study and compare the disk fraction among both kinds of objects over a large angular region in the Taurus cloud. In addition, we examine the spatial distribution of stars and BD relative to the underlying molecular gas. Methods. In this paper, we present new and updated photometry data from the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope on 43 BDs in the Taurus cloud, and recalculate of the BD disk fraction in this region. We also useed recently available CO mm data to study the spatial distribution of stars and BDs relative to the cloud's molecular gas. Results. We find that the disk fraction among BDs in the Taurus cloud is 41 ± 12%, a value statistically consistent with the one among TTS (58 ± 9%). We find that BDs in transition from a state where they have a disk to a diskless state are rare, and we study one isolated example of a transitional disk with an inner radius of ≈0.1 AU (CFHT BD Tau 12, found via its relatively small mid-IR excess compared to most members of Taurus that have disks. We find that BDs are statistically found in regions of similar molecular gas surface density to those associated with stars. Furthermore...

An Optical Spectroscopic Study of T Tauri Stars. I. Photospheric Properties

Herczeg, Gregory J.; Hillenbrand, Lynne A.
Fonte: American Astronomical Society Publicador: American Astronomical Society
Tipo: Article; PeerReviewed Formato: application/pdf; application/pdf
Publicado em 10/05/2014 Português
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Estimates of the mass and age of young stars from their location in the H-R diagram are limited by not only the typical observational uncertainties that apply to field stars, but also by large systematic uncertainties related to circumstellar phenomena. In this paper, we analyze flux-calibrated optical spectra to measure accurate spectral types and extinctions of 281 nearby T Tauri stars (TTSs). The primary advances in this paper are (1) the incorporation of a simplistic accretion continuum in optical spectral type and extinction measurements calculated over the full optical wavelength range and (2) the uniform analysis of a large sample of stars, many of which are well known and can serve as benchmarks. Comparisons between the non-accreting TTS photospheric templates and stellar photosphere models are used to derive conversions from spectral type to temperature. Differences between spectral types can be subtle and difficult to discern, especially when accounting for accretion and extinction. The spectral types measured here are mostly consistent with spectral types measured over the past decade. However, our new spectral types are one to two subclasses later than literature spectral types for the original members of the TW Hya Association (TWA) and are discrepant with literature values for some well-known members of the Taurus Molecular Cloud. Our extinction measurements are consistent with other optical extinction measurements but are typically 1 mag lower than near-IR measurements...