Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Infrared Nebulae around Young Stellar Objects
We present a K-band atlas of 106 reflection nebulae, 41 of which are newdiscoveries. We observed these nebulae with the University of Hawaii 2.2m telescope in the course of an imaging survey of 197 objects that wereselected as nearby young Class I sources. K-band images andflux-calibrated surface brightness contour plots of each nebula arepresented. We found that the near-IR luminosities and physical sizes ofthe nebulae increase with the bolometric luminosity of the illuminatingsources. Only 22 nebulae, about 10% of these candidate Class I sources,have indications of shocked H2 emission. The great variety ofnebulae that we observed prevented us from classifying them based onmorphology. However, we note that as the spectral index decreases, thecentral star is more frequently visible at K band, and the flux from thecentral star tends to be dominant over the flux from the nebula. Forobjects that have a higher spectral index, most of the K-band flux isfrom the reflection nebula, and the central star is less frequentlyvisible. The nebula around IRAS 05450+0019 has a unique morphology, andwe speculate that it may be an example of a disk shadow being projectedinto the surrounding cloud. We present J-, H-, and K-band images of thisobject with surface brightness contours, as well as its spectral energydistribution from 1.2 to 100 μm.
| X-ray emission from MP Muscae: an old classical T Tauri star
Aims.We study the properties of X-ray emitting plasma of MPMus, an old classical T Tauri star. We check whether anaccretion process could produce the observed X-ray emission and wederive the accretion parameters and the characteristics of theshock-heated plasma. We compare the properties of MPMus with those of younger classical T Tauri stars to testwhether age is related to the properties of the X-ray emittingplasma. Methods: .XMM-Newton X-ray spectra allow us to measureplasma temperatures, abundances, and electron density. The density ofcool plasma probes whether X-ray emission is produced by plasma heatedin the accretion process. Results: .X-ray emission fromMP Mus originates from high density cool plasma but ahot flaring component is also present, suggesting that both coronalmagnetic activity and accretion contribute to the observed X-rayemission. We find a Ne/O ratio similar to that observed in the muchyounger classical T Tauri star BP Tau. From the softpart of the X-ray emission, mostly produced by plasma heated in theaccretion shock, we derive a mass accretion rate of5×10-11 M_ȯ yr-1.
| Unraveling the Origins of Nearby Young Stars
A systematic search for close conjunctions and clusterings in the pastof nearby stars younger than the Pleiades is undertaken, which mayreveal the time, location, and mechanism of formation of these oftenisolated, disconnected from clusters and star-forming regions, objects.The sample under investigation includes 101 T Tauri, post-TT, andmain-sequence stars and stellar systems with signs of youth, culled fromthe literature. Their Galactic orbits are traced back in time and nearapproaches are evaluated in time, distance, and relative velocity.Numerous clustering events are detected, providing clues to the originof very young, isolated stars. Each star's orbit is also matched withthose of nearby young open clusters, OB and TT associations andstar-forming molecular clouds, including the Ophiuchus, Lupus, CoronaAustralis, and Chamaeleon regions. Ejection of young stars from openclusters is ruled out for nearly all investigated objects, but thenearest OB associations in Scorpius-Centaurus, and especially, the denseclouds in Ophiuchus and Corona Australis have likely played a major rolein the generation of the local streams (TWA, Beta Pic, andTucana-Horologium) that happen to be close to the Sun today. The core ofthe Tucana-Horologium association probably originated from the vicinityof the Upper Scorpius association 28 Myr ago. A few proposed members ofthe AB Dor moving group were in conjunction with the coeval Cepheus OB6association 38 Myr ago.
| Relation between the Luminosity of Young Stellar Objects and Their Circumstellar Environment
We present a new model-independent method of comparison of NIRvisibility data of YSOs. The method is based on scaling the measuredbaseline with the YSO's distance and luminosity, which removes thedependence of visibility on these two variables. We use this method tocompare all available NIR visibility data and demonstrate that itdistinguishes YSOs of luminosity L*<~103Lsolar (low L) from YSOs of L*>~103Lsolar (high L). This confirms earlier suggestions, based onfits of image models to the visibility data, for the difference betweenthe NIR sizes of these two luminosity groups. When plotted against the``scaled'' baseline, the visibility creates the following data clusters:low-L Herbig Ae/Be stars, T Tauri stars, and high-L Herbig Be stars. Wemodel the shape and size of clusters with different image models andfind that low-L Herbig stars are best explained by the uniformbrightness ring and the halo model, T Tauri stars with the halo model,and high-L Herbig stars with the accretion disk model. However, theplausibility of each model is not well established. Therefore, we try tobuild a descriptive model of the circumstellar environment consistentwith various observed properties of YSOs. We argue that low-L YSOs haveoptically thick disks with the optically thin inner dust sublimationcavity and an optically thin dusty outflow above the inner disk regions.High-L YSOs have optically thick accretion disks with high accretionrates enabling gas to dominate the NIR emission over dust. Althoughobservations would favor such a description of YSOs, the required dustdistribution is not supported by our current understanding of dustdynamics.
| On the Nature of the Extended Radio Emission Surrounding T Tauri South
At centimeter wavelengths, the young stellar system T Tauri is known tobe composed of two sources, the northern one associated with the opticalstar T Tau itself, and the southern one related to the infraredcompanion T Tau S. Here we reexamine the origin of the radio emissionfrom these two components using archival 2 cm, 3.6 cm, and 6 cm VLAobservations. The emission from the northern member is confirmed to belargely dominated by free-free radiation from an ionized wind, while thesouthern radio source is confirmed to consist of a compact component ofmagnetic origin, surrounded by an extended halo. Only moderatelyvariable, the extended structure associated with the southern source ismost likely the result of free-free radiation related to stellar winds.However, its flat spectral energy distribution, its extent, and the lackof variation of its size with the frequency of observation areincompatible with the classical picture of a fully ionized wind withconstant velocity and mass-loss rate leading to an electron densitydistribution of ne(r)~r-2. Instead, we propose amodel in which the ionization results from the impact of a supersonicwind driven by T Tau Sb onto dense surrounding material, possiblyassociated with the circumbinary disk recently identified around the TTau Sa/T Tau Sb pair. The timescales for cooling and recombination insuch a situation are in good agreement with the observed morphologicalchanges undergone by the extended structure as its driving source movesthrough the environment.
| Modeling T Tauri Winds from He I λ10830 Profiles
The high opacity of He I λ10830 makes it an exceptionallysensitive probe of the inner wind geometry of accreting T Tauri stars.In this line, blueshifted absorption below the continuum results fromsimple scattering of stellar photons, a situation that is readilymodeled without definite knowledge of the physical conditions andrecourse to multilevel radiative transfer. We present theoretical lineprofiles for scattering in two possible wind geometries, a disk wind anda wind emerging radially from the star, and compare them to observed He I λ10830 profiles from a survey of classical T Tauri stars. Thecomparison indicates that subcontinuum blueshifted absorption ischaracteristic of disk winds in ~30% of the stars and of stellar windsin ~40%. We further conclude that for many stars the emission profile ofhelium likely arises in stellar winds, increasing the fraction ofaccreting stars inferred to have accretion-powered stellar winds to~60%. Stars with the highest disk accretion rates are more likely tohave stellar wind than disk wind signatures and less likely to haveredshifted absorption from magnetospheric funnel flows. This suggeststhe possibility that when accretion rates are high, disks can extendcloser to the star, magnetospheric accretion zones can be reduced insize, and conditions can arise that favor radially outflowing stellarwinds.
| High Spectral Resolution Near-IR Observations of ESO-Hα 279A and 279B
We present high spectral resolution near-IR observations of the starsESO-Hα 279A and 279B located in the Serpens star formationcomplex. ESO-Hα 279A is a known young T Tauri star driving aHerbig-Haro flow, while ESO-Hα 279B is a nearby (8", 2500 AU)extremely red companion. Previous work has suggested that this is not aphysically bound system but a chance alignment of a young star and abackground giant. Below, we further investigate the nature ofESO-Hα 279B and probe the emission characteristics of ESO-Hα279A. We find that ESO-Hα 279B shows many narrow absorptionfeatures typical of late-type giant stars and unlike those seen in FUOrionis objects and Herbig-Haro energy sources, the only young starsthat exhibit such deep CO absorption. This result confirms thatESO-Hα 279B is a background M-type giant viewed through themolecular cloud hosting ESO-Hα 279A, and therefore, theassociation of ESO-Hα 279A and 279B is fortuitous. ForESO-Hα 279A we find that the Na doublet lines are broadened andself-absorbed and that the v=2-0 CO overtone band head is similar inshape to that observed in the young pre-main-sequence object SVS 13 inNGC 1333, and not characteristic of a rotating-disk velocity dispersion.We consider the origin of the CO emission in relation to current modelsand suggest that it perhaps is more indicative of creation in a wind orfunnel flow rather than in the inner hot regions of a circumstellardisk.
| Evidence of Stellar-driven Outflows from the Classical T Tauri Star RY Tau
RY Tau is a rapidly rotating classical T Tauri star observed close toedge-on. The combination of new HST/STIS observations obtained in 2001with HST/GHRS archive data from 1993 has allowed us to get, for thefirst time, information on the thermal structure and velocity of thewind. The repeated observations of the Si III] and C III] lines show alack of changes with time on the blue side of the profile (dominated bythe wind contribution). Very high temperature plasma(logTe=4.8) is detected at densities of9.5<=logne(cm-3)<=10.2 associated with thewind. The emitting volumes are ~(0.35 Rsolar)3,suggesting a stellar origin. The wind kinematics derived from theprofiles (Si III], C III], and [O II]) does not satisfy the theoreticalpredictions of MHD centrifugally driven disk winds. The profiles'asymmetry, large velocity dispersions, and small variability as well asthe small emitting volumes are best explained if the wind is produced bythe contributions of several outflows from atmospheric open-fieldstructures like those observed in the Sun.
| Spectropolarimetry of the Classical T Tauri Star TW Hydrae
We present high-resolution (R~60,000) circular spectropolarimetry of theclassical T Tauri star TW Hydrae. We analyze 12 photospheric absorptionlines and measure the net longitudinal magnetic field for sixconsecutive nights. While no net polarization is detected the first fivenights, a significant photospheric field of Bz=149+/-33 G isfound on the sixth night. To rule out spurious instrumentalpolarization, we apply the same analysis technique to severalnonmagnetic telluric lines, detecting no significant polarization. Wefurther demonstrate the reality of this field detection by showing thatthe splitting between right and left polarized components in these 12photospheric lines shows a linear trend with Landé g-factor timeswavelength squared, as predicted by the Zeeman effect. However, thislongitudinal field detection is still much lower than that which wouldresult if a pure dipole magnetic geometry is responsible for the meanmagnetic field strength of 2.6 kG previously reported for TW Hya. Wealso detect strong circular polarization in the He I λ5876 and CaII λ8498 emission lines, indicating a strong field in the lineformation region of these features. The polarization of the Ca II lineis substantially weaker than that of the He I line, which we interpretas being due to a larger contribution to the Ca II line fromchromospheric emission in which the polarization signals cancel.However, the presence of polarization in the Ca II line indicates thataccretion shocks on classical T Tauri stars do produce narrow emissionfeatures in the infrared triplet lines of calcium.
| A deep XMM-Newton X-ray observation of the Chamaeleon I dark cloud
Context: .Low-mass stars are known to exhibit strong X-ray emissionduring the early stages of evolution. Nearby star forming regions areideal targets to study the X-ray properties of pre-main sequencestars. Aims: .A deep XMM-Newton exposure is used to investigateX-ray properties of the pre-main sequence population of the Chamaeleon Istar forming region. Methods: .The northern-eastern fringe of theChameleon I dark cloud was observed with XMM-Newton, revisiting a regionobserved with ROSAT 15 years ago. Centered on the extended X-ray sourceCHXR 49 we are able to resolve it into three major contributingcomponents and to analyse their spectral properties. Furthermore, thedeep exposure allows not only the detection of numerous, previouslyunknown X-ray sources, but also the investigation of variability and thestudy of the X-ray properties for the brighter targets in the field. Weuse EPIC spectra, to determine X-ray brightness, coronal temperaturesand emission measures for these sources, compare the properties ofclassical and weak-line T Tauri stars and make a comparison with resultsfrom the ROSAT observation. Results: .X-ray properties of T Tauristars in Cha I are presented. The XMM-Newton images resolve somepreviously blended X-ray sources, confirm several possible ones anddetect many new X-ray targets, resulting in the most comprehensive listwith 71 X-ray sources in the northern Cha I dark cloud. The analysis ofmedium resolution spectra shows an overlapping distribution of spectralproperties for classical and weak-line T Tauri stars, with the X-raybrighter stars having hotter coronae and a higher L_X/L_bol ratio. X-rayluminosity correlates with bolometric luminosity, whereas the L_X/L_bolratio is slightly lower for the classical T Tauri stars. Large flares aswell as a low iron and a high neon abundance are found in both types ofT Tauri stars. Abundance pattern, plasma temperatures and emissionmeasure distributions during quiescent phases are attributed to a highlevel of magnetic activity as the dominant source of their X-rayemission.
| Results of the ROTOR-program. I. The long-term photometric variability of classical T Tauri stars
Context: .T Tauri stars exhibit variability on all timescales, whoseorigin is still debated. Aims: .We investigate the long termvariability of CTTs over up to 20 years, characterize it from a set ofstatistical parameters and discuss its origin. Methods: .Wepresent a unique, homogeneous database of photometric measurements forClassical T Tauri stars extending up to 20 years. The database containsmore than 21 000 UBVR observations of 72 CTTs. All the data werecollected within the framework of the ROTOR-program at Mount MaidanakObservatory (Uzbekistan) and together they constitute the longesthomogeneous, accurate record of TTS variability ever assembled. Wecharacterize the long term photometric variations of 49 CTTs withsufficient data to allow a robust statistical analysis and propose anempirical classification scheme. Results: .Several patterns oflong term photometric variability are identified. The most commonpattern, exhibited by a group of 15 stars which includes T Tau itself,consists of low level variability (Δ V≤0.4 mag) with nosignificant changes occurring from season to season over many years. Arelated subgroup of 22 stars exhibits a similar stable long termvariability pattern, though with larger amplitudes (up to ΔV≃1.6 mag). Besides these representative groups, we identify threesmaller groups of 3-5 stars each which have distinctive photometricproperties. Conclusions: .The long term variability of most CTTsis fairly stable and merely reflects shorter term variability due tocold and hot surface spots. Only a small fraction of CTTs undergosignificant brightness changes on the long term (months, years), whichprobably arise from slowly varying circumstellar extinction.
| Dynamical Mass Estimates for Incomplete Orbits: Young Multiple Stars in Taurus and Ophiuchus
We present recent measurements of the orbital motion in the binaries DFTau and ZZ Tau and in the triples Elias 12, T Tau, and V853 Oph. Weobserved these systems with the Fine Guidance Sensors on the HubbleSpace Telescope and with adaptive optics imaging at the W. M. Keck andGemini North Observatories. Based on our measurements and thosepresented in the literature we perform preliminary orbital analyses forDF Tau, ZZ Tau, Elias 12 Na-Nb, and T Tau Sa-Sb. Because the orbitalcoverage in most of these systems does not yet span a sufficient portionof the orbit, we are not able to find definitive orbit solutions. Byusing a Monte Carlo search technique we explored the orbital parameterspace allowed by the current set of data available for each binary. Weconstructed weighted distributions for the total mass of the binariesderived from a large sample of possible orbits that fit the data. Thesemass distributions show that the total mass is already well defined. Wecompute total mass estimates of 0.78+0.25-0.15,0.66+0.15-0.11,1.13+0.36-0.09, and4.13+1.58-0.97 Msolar for DF Tau, ZZTau, Elias 12 Na-Nb, and T Tau Sa-Sb, respectively, using a distance of140 pc. For Elias 12 Na-Nb, where the orbital coverage spans ~164°,we compute a preliminary orbit solution with a period of ~9-12 yr. Byincluding an earlier lunar occultation measurement, we also find alikely orbit solution for ZZ Tau with a period of ~32 yr. Withadditional measurements to continue mapping the orbits the deriveddynamical masses will be useful in constraining the theoretical tracksof pre-main-sequence evolution.
| Mid-Infrared Observations of T Tauri Stars: Probing the Star-Disk Connection in Rotational Evolution
We present mid-IR N-band (λeff=10.2 μm) photometryof a carefully selected sample of T Tauri stars thought to be singlefrom the Taurus-Auriga molecular cloud. Infrared excesses in these starsare generally attributed to circumstellar dust disks. Combiningobservations at 2.16 (Ks band) and 10.2 μm (N band) weprobe a region in the circumstellar dust disk from a few stellar radiithrough the terrestrial planet zone (0.02-1.0 AU). By analyzing thedistribution of the (Ks-N) color index with respect topreviously measured photometric rotation periods we investigate whatrole circumstellar disks play in the rotational evolution of the centralstar. The resulting positive correlation between these two variables isconsistent with the notion that a star-disk interaction facilitates theregulation of angular momentum during the T Tauri stage. We alsodemonstrate how including nonsingle stars in such an analysis weakensany correlation in the relation between (Ks-N) color andperiod. To further understand disk properties we also present spectralenergy distributions for a few objects with new ground-based M-band(λeff=4.8 μm) and Q-band (λeff=20μm) data and compare them to a geometrically thin, optically thickdisk model.
| X-ray accretion signatures in the close CTTS binary V4046 Sagittarii
We present Chandra HETGS observations of the classical T Tauri star(CTTS) V4046 Sgr. The He-like triplets of O VII, NiIX, and Si XIII are clearly detected. Similar to the CTTS TW Hya and BPTau, the forbidden lines of O VII and Ne IX are weak compared to theintercombination line, indicating high plasma densities in the X-rayemitting regions. The Si XIII triplet, however, is within thelow-density limit, in agreement with the predictions of the accretionfunnel infall model with an additional stellar corona. V4046 Sgr is thefirst close binary exhibiting these features. Together with previoushigh-resolution X-ray data on TW Hya and BPTau, and in contrast to T Tau, now threeout of four CTTS show evidence of accretion funnels.
| Self-Correlation Analysis of the Photometric Variability of T Tauri Stars
T Tauri stars are variable stars that are in an early phase ofevolution, in which accretion and contraction to the main sequence arestill taking place. Their photometric variability is complex; it takesplace on a variety of timescales, due to a variety of physicalprocesses. Periodic variability occurs due to rotation and the presenceof cool or hot spots on the star. It may also occur due to periodicobscuration of the star by inhomogeneities in the still presentaccretion disk. But the periodicity may be masked by other forms ofvariability, or by time variation in the cool or hot spots, or theobscuring inhomogeneities. For other types of variable stars,self-correlation has proven to be a useful adjunct to Fourier analysisfor studying semiregular variability; it determines the cycle-to-cyclebehavior of the star, averaged over all the data. We have therefore usedit to investigate the photometric variability of about 30 T Tauri starsusing existing data. It has provided useful information about periodsand their coherence, about the amplitude of the periodic variation, orits upper limit, and about the ``profile'' of the amplitude-timescalebehavior. In most cases, it has confirmed periods previously determinedby Fourier analysis, but in some cases it has suggested that thepreviously determined period is spurious.
| A Unified Model for Bipolar Outflows from Young Stars
We develop a unified model for molecular outflows in star formation. Themodel incorporates essential features expected of the primary wind,which is thought to be driven magnetocentrifugally from close to thecentral stellar object, and the ambient core material shaped byanisotropic magnetic support. The primary wind is modeled as atoroidally magnetized fast outflow moving radially away from the origin,with an angle-dependent density distribution: a dense axial jetsurrounded by a more tenuous wide-angle wind, as expected in the X-windmodel. If dynamically significant magnetic fields are present, thestar-forming core will settle faster along the field lines than across,forming a toroid-like structure. We approximate the structure with asingular isothermal toroid whose density distribution can be obtainedanalytically. The interaction of the laterally stratified wind and theambient toroid is followed using the Zeus2D magnetohydrodynamics (MHD)code. We find that the lobes produced by the interaction resemble manysystematics observed in molecular outflows from very young stars,ranging from Class 0 to I sources. In particular, both the dense axialjet and the wide-angle wind participate in the wind-ambient interaction.In our model, the jet- and wind-driven pictures of molecular outflowsare unified. We discuss the observational implications of the unifiedpicture, including the possibility of detecting the primary jet/winddirectly.
| Accurate stellar masses in the multiple system T Tauri
Aims.We obtain accurate estimates for the individual masses of thecomponents of the tight binary system T Tau S to settle the ongoingdebate on the nature of T Tau Sa, a so-called infrared companion.Methods: .We take advantage of the fact that T Tau S belongs to a triplesystem composed of two hierarchical orbits to simultaneously analyze themotion of T Tau Sb in the rest frames of T Tau Sa and T Tau N. With thismethod, it is possible to pinpoint the location of the center of mass ofT Tau S and, thereby, to determine individual masses for T Tau Sa and TTau Sb with no prior assumption about the mass/flux ratio of the system.This improvement over previous studies of the system results in muchbetter constraints on orbital parameters. Results: .We findindividual masses of 2.73±0.31 M_ȯ for T Tau Sa and of0.61±0.17 M_ȯ for T Tau Sb (in agreement with its early-Mspectral type), including the uncertainty on the distance to the system.These are among the most precise estimates of the mass of any Pre-MainSequence star, a remarkable result since this is the first system inwhich individual masses of T Tauri stars can be determined fromastrometry only. This model-independent analysis confirms that T Tau Sais an intermediate-mass star, presumably a very young Herbig Ae star,that may possess an almost edge-on disk.
| A Survey and Analysis of Spitzer Infrared Spectrograph Spectra of T Tauri Stars in Taurus
We present mid-infrared spectra of T Tauri stars in the Taurusstar-forming region obtained with the Spitzer Infrared Spectrograph(IRS). For the first time, the 5-36 μm spectra of a large sample of TTauri stars belonging to the same star-forming region is studied,revealing details of the mid-infrared excess due to dust incircumstellar disks. We analyze common features and differences in themid-IR spectra based on disk structure, dust grain properties, and thepresence of companions. Our analysis encompasses spectral energydistributions from the optical to the far-infrared, a morphologicalsequence based on the IRS spectra, and spectral indices in IRS wavebands representative of continuum emission. By comparing the observedspectra to a grid of accretion disk models, we infer some basic diskproperties for our sample of T Tauri stars and find additional evidencefor dust settling.
| The Evolution of Outflow-Envelope Interactions in Low-Mass Protostars
We present multiline and continuum observations of the circumstellarenvironment within 104 AU of a sample of protostars toinvestigate how the effects of outflows on their immediate environmentchange over time. 12CO (1-0) emission probes thehigh-velocity molecular outflows near the protostars and demonstratesthat the outflow opening angle widens as the nascent star evolves. Mapsof the 13CO (1-0) and HCO+ (1-0) outflow emissionshow that protostellar winds erode the circumstellar envelope throughthe entrainment of the outer envelope gas. The spatial and velocitydistribution of the dense circumstellar envelope, as well as its mass,is traced by the C18O (1-0) emission and also displaysevolutionary changes. We show that outflows are largely responsible forthese changes and propose an empirical model for the evolution ofoutflow-envelope interactions. In addition, some of the outflows in oursample appear to affect the chemical composition of the surroundingenvironment, enhancing the HCO+ abundance. Overall, ourresults confirm that outflows play a major role in the star formationprocess through their strong physical and chemical impacts on theenvironments of the young protostars.
| Astrophysics in 2005
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
| The Origins of Fluorescent H2 Emission From T Tauri Stars
We survey fluorescent H2 emission in HST STIS spectra of theclassical T Tauri stars (CTTSs) TW Hya, DF Tau, RU Lupi, T Tau, and DGTau, and the weak-lined T Tauri star (WTTS) V836 Tau. From each of thosesources we detect between 41 and 209 narrow H2 emissionlines, most of which are pumped by strong Lyα emission.H2 emission is not detected from the WTTS V410 Tau. Thefluorescent H2 emission appears to be common to circumstellarenvironments around all CTTSs, but high spectral and spatial resolutionSTIS observations reveal diverse phenomenon. Blueshifted H2emission detected from RU Lupi, T Tau, and DG Tau is consistent with anorigin in an outflow. The H2 emission from TW Hya, DF Tau,and V836 Tau is centered at the radial velocity of the star and isconsistent with an origin in a warm disk surface. The H 2lines from RU Lupi, DF Tau, and T Tau also have excess blueshiftedH2 emission that extends to as much as -100 kms-1. The strength of this blueshifted component from DF Tauand T Tau depends on the upper level of the transition. In all cases,the small aperture and attenuation of H2 emission by stellarwinds restricts the H2 emission to be formed close to thestar. In the observation of RU Lupi, the Lyα emission and theH2 emission that is blueshifted by 15 km s-1 areextended to the SW by ~0.07", although the faster H2 gas thatextends to ~100 km s-1 is not spatially extended. We alsofind a small reservoir of H 2 emission from TW Hya and DF Tauconsistent with an excitation temperature of ~2.5×104K.
| Probing T Tauri Accretion and Outflow with 1 Micron Spectroscopy
In a high-dispersion 1 μm survey of 39 classical T Tauri stars(CTTSs) veiling is detected in 80% of the stars, and He I λ10830and Pγ line emission in 97%. On average, the 1 μm veilingexceeds the level expected from previously identified sources of excessemission, suggesting the presence of an additional contributor toaccretion luminosity in the star-disk interface region. Strengths ofboth lines correlate with veiling, and at Pγ there is a systematicprogression in profile morphology with veiling. He I λ10830 hasan unprecedented sensitivity to inner winds, showing blueshiftedabsorption below the continuum in 71% of the CTTSs, compared to 0% atPγ. This line is also sensitive to magnetospheric accretion flows,with redshifted absorption below the continuum found in 47% of theCTTSs, compared to 24% at Pγ. The blueshifted absorption at He Iλ10830 shows considerable diversity in its breadth andpenetration depth into the continuum, indicating that a range of innerwind conditions exist in accreting stars. We interpret the broadest anddeepest blue absorptions as formed from scattering of the 1 μmcontinuum by outflowing gas whose full acceleration region envelopes thestar, suggesting radial outflow from the star. In contrast, narrow blueabsorption with a range of radial velocities more likely arises viascattering of the 1 μm continuum by a wind emerging from the innerdisk. Both stellar and disk winds are accretion powered, since neitheris seen in nonaccreting WTTSs and among the CTTSs helium strengthcorrelates with veiling.
| The Spatial Distribution of Brown Dwarfs in Taurus
By combining photometry from the 2MASS Point Source Catalog and theUSNO-B1.0 catalog with optical and infrared spectroscopy, I haveperformed a search for young brown dwarfs in an area of 225deg2 encompassing all of the Taurus star-forming region(τ~1 Myr). From this work, I have discovered 22 new members ofTaurus, 5 of which were independently found by Guieu and coworkers.Sixteen of these new members have spectral types later than M6 and thusare likely to be brown dwarfs according to the theoretical evolutionarymodels of Chabrier and Baraffe. After adding these new members to thepreviously known members of Taurus, I have compared the spatialdistributions of stars and brown dwarfs across the entire region. I findno statistically significant difference between these two distributions.Taurus does not contain the large, extended population of brown dwarfsthat has been predicted by some embryo ejection models for the formationof brown dwarfs. However, these results are consistent with otherejection models, as well as models in which stars and brown dwarfs sharea common formation mechanism.
| CCS and NH3 Emission Associated with Low-Mass Young Stellar Objects
In this work we present a sensitive and systematic single-dish survey ofCCS emission (complemented with ammonia observations) at 1 cm, toward asample of low- and intermediate-mass young star-forming regions known toharbor water maser emission, made with NASA's 70 m antenna at Robledo deChavela, Spain. Out of the 40 star-forming regions surveyed in the CCS(21-10) line, only six low-mass sources show CCSemission: one transitional object between the prestellar andprotostellar Class 0 phase (GF9-2), three Class 0 protostars(L1448-IRS3, L1448C, and B1-IRS), a Class I source (L1251A), and a youngT Tauri star (NGC 2071 North). Since CCS is considered an ``early-time''(<~105 yr) molecule, we explain these results by eitherproposing a revision of the classification of the age of NGC 2071 Northand L1251A, or suggesting the possibility that the particular physicalconditions and processes of each source affect thedestruction/production of the CCS. No statistically significantrelationship was found between the presence of CCS and parameters of themolecular outflows and their driving sources. Nevertheless, we found asignificant relationship between the detectability of CCS and theammonia peak intensity (higher in regions with CCS), but not with itsintegrated intensity. This tendency may suggest that the narrowerammonia line widths in the less turbulent medium associated with youngercores may compensate for the differences in ammonia peak intensity,rendering differences in integrated intensity negligible. From the CCSdetection rate we derive a lifetime of this molecule of~=(0.7-3)×104 yr in low-mass star-forming regions.
| Subaru Near Infrared Coronagraphic Images of T Tauri
High angular resolution near-infrared (JHK) adaptive optics images of TTau were obtained with the infrared camera Coronagraphic Imager withAdaptive Optics (CIAO) mounted on the 8.2m Subaru Telescope in 2002 and2004. The images resolve a complex circumstellar structure around amultiple system. We resolved T Tau Sa and Sb as well as T Tau N and S.The estimated orbit of T Tau Sb indicates that it is probably bound to TTau Sa. The K band flux of T Tau S decreased by 1.7 Jy in 2002November compared with that in 2001 mainly because T Tau Sa becamefainter. The arc-like ridge detected in our near-infrared images isconsistent with what is seen at visible wavelengths, supporting theinterpretation in previous studies that the arc is part of the cavitywall seen relatively pole-on. Halo emission is detected out to2''from T Tau N. This may be light scattered off the commonenvelope surrounding the T Tauri multiple system.
| The infrared Hourglass cluster in M8*†
A detailed study of the Hourglass nebula in the M8 star-forming regionis presented. The study is mainly based on recent subarcsec-resolutionJHKs images taken at Las Campanas Observatory andcomplemented with archival Hubble Space Telescope (HST) images andlong-slit spectroscopy retrieved from the European Southern ObservatoryArchive Facility. Using the new numerical code CHORIZOS, we estimate thedistance to the earliest stars in the region to be 1.25 kpc. Infraredphotometry of all the sources detected in the field is given. Fromanalysis of the JHKs colour-colour diagrams, we find that animportant fraction of these sources exhibit significant infrared excess.These objects are candidates to be low- and intermediate-masspre-main-sequence stars. Based on HST observations, the spatialdistribution of gas, dust and stars in the region is analysed. Amorphological analysis of these images also reveals a rich variety ofstructures related to star formation (proplyds, jets, bow shocks),similar to those observed in M16 and M42, along with the detection ofthe first four Herbig-Haro objects in the region. Furthermore, along-slit spectrum obtained with the New Technology Telescope confirmsthe identification of one of them (HH 870) in the core of the Hourglassnebula, providing the first direct evidence of active star formation byaccretion in M8.
| On the interstellar extinction law toward young stars
We have determined the atomic hydrogen column density N HI toward all ofthe young stars from the Taurus-Auriga-Perseus star-forming complex forwhich the corresponding spectra are available in the Hubble SpaceTelescope archive (nine stars) by analyzing the Lyα line profile.We show that the stars studied, except DR Tau, lie not far from the edgeof the gaseous cloud of the star-forming region closest to us or, moreprecisely, inside the outer H I shell of the cloud. This shell with acolumn density of N HI ≃ 6 × 1020 cm-2 surrounds themolecular gas of the cloud composed of a diffuse component (theso-called diffuse screen) in which dense, compact TMC-1 cores areembedded. The properties of the dust grains toward the stars that lie atthe front edge of the cloud most likely differ only slightly from thoseof the interstellar dust outside star-forming regions. This casts doubton the validity of the hypothesis that the extinction curve toward youngstars has an anomalously low amplitude of the 2175 Åbump—such an extinction curve is observed for the field stars HD29647 and HD 283809 toward which the line of sight passes through theTMC-1 core.
| Three Years of ChaMPlane Northern Field WIYN Spectroscopy
We present initial results of WIYN spectroscopic observations ofselected objects detected in the Chandra Multiwavelength Plane(ChaMPlane) Survey in fields toward the Galactic anticenter. ChaMPlaneis designed to identify low-luminosity X-ray sources, bothaccretion-powered and stellar coronal, in the Galaxy. It also includes awide-field optical imaging Survey conducted with the NOAO Mosaic camerasto identify optical counterparts, as well as Hα-selected objectsin the ~5 times larger field. We report spectroscopic classificationsfor 1069 objects in Galactic anticenter (i.e., northern) fields,resulting in 612 type determinations. These include 5 new cataclysmicvariables, 4 Be stars, 14 lithium-absorption stars, 182 stellar coronalsources (primarily dMe stars), and 30 new quasars. Bright opticalcounterparts of Chandra sources in this sample are most frequently dMestars, whereas a majority of the faintest (R>20.5 mag)spectroscopically classified Chandra source counterparts are quasars.The bulk of Hα-selected sources appears to be roughly evenlydivided between dMe stars and M stars at all magnitudes.
| Investigating Disk Evolution: A High Spatial Resolution Mid-Infrared Survey of T Tauri Stars
We present a high spatial resolution, 10-20 μm survey of 65 T Tauribinary stars in Taurus, Ophiuchus, and Corona Australis using the Keck10 m telescopes. Designed to probe the inner ~1 AU region of thecircumstellar disks around the individual stellar components in thesebinary systems, this study increases the number of binaries withspatially resolved measurements at 10 μm by a factor of ~5. Combinedwith resolved near-infrared photometry and spectroscopic accretiondiagnostics, we find that ~10% of stars with a mid-infrared excess donot appear to be accreting. In contrast to an actively accreting disksystem, these passive disks have significantly lower near-infraredcolors that are, in most cases, consistent with photospheric emission,suggesting the presence of an inner disk hole. In addition, thereappears to be a spectral type/mass dependence associated with thepresence of a passive disk, with all passive disks occurring aroundM-type stars. The presence of a passive disk does not appear to berelated to the fact that these objects are in visual binary systems; thepassive disk systems span the entire range of binary separations presentin the sample, and a similar fraction of passive disks is observed in asample of single stars. The possibility that the passive disks arecaused by the presence of an as yet undetected companion at a smallseparation (0.3-3 AU) is possible for any individual system; however, itcannot account for the spectral type dependence of the passive disksample as a whole. We propose that these passive disks represent asubset of T Tauri stars that are undergoing significant disk evolution.The fraction of observed passive disks and the observed spectral typedependence can both be explained by models of disk evolution thatinclude disk photoevaporation from the central star.
| Spectropolarimetry of the Classical T Tauri Star T Tauri
High-resolution (R~60,000) circular spectropolarimetry of the classicalT Tauri star T Tau is presented. The star was observed on 1997 November21 and 22. Analyzing 12 photospheric absorption lines, the meanlongitudinal magnetic field is found to be Bz=12+/-35 G. The3 σ upper limit of |Bz|<=105 G. Previously, T Tauwas reported to have a mean longitudinal field of 160+/-40 G. A strongmean magnetic field (~2.4 kG) has been reported on the surface of T Taubased on Zeeman broadening measurements in unpolarized light. Thepresent observations indicate that it is very unlikely that this fieldis dipolar in nature. In order to verify the observing techniques andanalysis methods used on T Tau, spectra of the Sun obtained by observingthe asteroid Vesta are analyzed in the same fashion. Here the meanlongitudinal field is Bz=-4+/-3 G, which is well within thelimits of previous observations. As a further check on our results, wealso present data for the magnetic Ap star 53 Cam, which gives a meanlongitudinal magnetic field that agrees well with the published fieldvariations for this star.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Taurus |
| Ascensió Recta: | 04h21m59.44s |
| Declinació: | +19°32'06.4" |
| Magnitud Aparent: | 9.884 |
| Moviment propi RA: | 15.1 |
| Moviment propi Dec: | -13.7 |
| B-T magnitude: | 11.334 |
| V-T magnitude: | 10.004 |
Catàlegs i designacions:
|
INHABITED SKY
