Circumstellar Accretion Disks Research Articles

Time evolution of X-ray coronal activity in PMS stars; a possible relation with the evolution of accretion disks

We investigate the evolution of X-ray stellar activity from the age of the youngest known star forming regions (SFR), <1 Myr, to about 100 Myr, i.e. the zero age main sequence (ZAMS) for a ∼1 Mstar. We consider five SFR of varying age (ρ Ophiuchi, the Orion Nebula Cluster, NGC 2264, Chamaeleon I, and η Chamaeleontis) and two young clusters (the Pleiades and NGC 2516). Optical and X-ray data for these regions are retrieved both from archival observations and recent literature, and reanalyzed here in a consistent manner so as to minimize systematic differences in the results. We study trends of LX and LX/Lbol as a function of stellar mass and association age. For low mass stars (M < 1 M� ) we observe an increase in LX/Lbol in the first 3-4 Myr and a subsequent leveling off at the saturation level (LX/Lbol ∼− 3). Slowly evolving very low mass stars then retain saturated levels down to the oldest ages here considered, while for higher mass stars activity begins to decline at some age after ∼10 7 years. We find our data consistent with the following tentative picture: low mass PMS stars with no circumstellar accretion disk have saturated activity, consistently with the activity-Rossby number relation derived for MS stars. Accretion and/or the presence of disks somehow lowers the observed activity levels; disk dissipation and/or the decrease of mass accretion rate in the first few Myrs of PMS evolution is therefore responsible for the observed increase of LX/Lbol with time.

Magnetic star-disk interaction in classical T Tauri systems

We carry out 2.5D MHD simulations to study the interaction between a dipolar magnetic field of a T Tauri Star, a circumstellar accretion disk, and the halo above the disk. The initial disk is the result of 1D radiation hydrodynamics computations with opacities appropriate for low temperatures. The gas is assumed resistive, and inside the disk accretion is driven by a Shakura–Sunyaev-type eddy viscosity. Magnetocentrifugal forces due to the rotational shear between the star and the Keplerian disk cause the magnetic field to be stretched outwards and part of the field lines are opened. For a solar-mass central star and an accretion rate of 10−8 solar masses per year a field strength of 100 G (measured on the surface of the star) launches a substantial outflow from the inner parts of the disk. For a field strength of 1 kG the inner parts of disk is disrupted. The truncation of the disk turns out to be temporary, but the magnetic field structure remains changed after the disk is rebuilt.

Evolution of the Fu Orionis Object BBW 76

We have carried out a long-term photometric and spectroscopic monitoring program of the southern FU Orionis–type object BBW 76 spanning the period from 1982 to 1997. BBW 76 has the same radial velocity as the small cloud toward which it is projected, and for which a kinematic distance of about 1.8 kpc has been derived. We have determined a large reddening of E(B-V) ~ 0.7 for BBW 76. Optical and infrared spectra show the change toward later spectral type with increasing wavelength characteristic of FU Orionis stars and indicative of a hot luminous disk. High-resolution echelle spectra of BBW 76 show P Cygni profiles with extended blueshifted absorption troughs at the Hα and sodium lines from a neutral, supersonic wind. Comparison of such spectra obtained at six different epochs between 1985 and 1997 reveals major changes in these Hα and sodium line profiles. For a period of 10 years from 1985, the massive absorption troughs diminished in extent and depth, until by 1994 they had all but disappeared, while at the same time the blueshifted emission peak in the Hα line increased markedly in strength. However, when observed in 1997, the absorption had increased again and the emission had diminished. We interpret this in terms of an extended period during which accretion through a circumstellar disk decreased, with a resulting decrease in wind production. But the increased activity by 1997 shows that this is not a constant decay and that the star was not about to revert to its presumably original T Tauri stage. We monitored the star with optical photometry from 1983 to 1994, during which period it decreased almost monotonically in brightness by 0.2 mag in V. Infrared J, H, and K photometry from 1983 to 1991 shows a period of monotonic fading between 1984 and 1988, followed by more irregular behavior. In a search of the Harvard plate archives we have found a plate from the year 1900 on which BBW 76 is seen at approximately its present brightness, certainly not 2 mag brighter as expected if the optical decline between 1983 and 1994 had persisted during the whole century. Also, a plate taken for the Franklin-Adams charts in 1927 again shows BBW 76 at approximately the same brightness. This historical light curve makes BBW 76 the FU Orionis star with the longest-documented period in a high state. Overall, the observations suggest that BBW 76 is virtually identical to the prototype of its class, FU Orionis itself, in all respects except that BBW 76 has not shown the regular fading that FU Orionis has displayed after its eruption in 1936. This may be due to continued replenishment of the circumstellar accretion disk.

Spectroscopic Detection of a Stellar-like Photosphere in an Accreting Protostar

We present high-resolution (R is approximately equal to 18,000), high signal-to-noise 2 micron spectra of two luminous, X-ray flaring Class I protostars in the rho Ophiuchi cloud acquired with the NIRSPEC (near infrared spectrograph) of the Keck II telescope. We present the first spectrum of a highly veiled, strongly accreting protostar which shows photospheric absorption features and demonstrates the stellar nature of its central core. We find the spectrum of the luminous (L (sub bol) = 10 solar luminosity) protostellar source, YLW 15, to be stellar-like with numerous atomic and molecular absorption features, indicative of a K5 IV/V spectral type and a continuum veiling r(sub k) = 3.0. Its derived stellar luminosity (3 stellar luminosity) and stellar radius (3.1 solar radius) are consistent with those of a 0.5 solar mass pre-main-sequence star. However, 70% of its bolometric luminosity is due to mass accretion, whose rate we estimate to be 1.7 x 10(exp -6) solar masses yr(exp -1). We determine that excess infrared emission produced by the circumstellar accretion disk, the inner infalling envelope, and accretion shocks at the surface of the stellar core of YLW 15 all contribute significantly to its near-IR (infrared) continuum veiling. Its rotational velocity v sin i = 50 km s(exp -1) is comparable to those of flat-spectrum protostars but considerably higher than those of classical T Tauri stars in the rho Oph cloud. The protostar may be magnetically coupled to its circumstellar disk at a radius of 2 - 3 R(sub *). It is also plausible that this protostar can shed over half its angular momentum and evolve into a more slowly rotating classical T Tauri star by remaining coupled to its circumstellar disk (at increasing radius) as its accretion rate drops by an order of magnitude during the rapid transition between the Class I and Class II phases of evolution. The spectrum of WL 6 does not show any photospheric absorption features, and we estimate that its continuum veiling is r(sub k) is greater than or equal to 4.6. Its low luminosity (2 solar masses) and high veiling dictate that its central protostar is very low mass, M is approx. 0.1 solar masses. We also evaluate multi-epoch X ray data along with these spectra and conclude that the X ray variabilities of these sources are not directly related to their protostellar rotation velocities.

Simulation‐based Investigation of a Model for the Interaction between Stellar Magnetospheres and Circumstellar Accretion Disks

We examine, parametrically, the interaction between the magnetosphere of a rotating, young stellar object (YSO) and a circumstellar accretion disk using 2.5-D (cylindrically symmetric) numerical magnetoydrodynamic simulations. The interaction drives a collimated outflow, and we find that the jet formation mechanism is robust. For variations in initial disk density of a factor of 16, variations of stellar dipole strength of a factor of 4, and for various initial conditions with respect to the disk truncation radius and the existence of a disk field, outflows with similar morphologies were consistently produced. Secondly, the system is self-regulating, where the outflow properties depend relatively weakly on the parameters above. The large scale magnetic field structure rapidly evolves to a configuration that removes angular momentum from the disk at a rate that depends most strongly on the field and weakly on the rotation rate of the foot-points of the field in the disk and the mass outflow rate. Third, the simulated jets are episodic, with the timescale of jet outbursts identical to the timescale of magnetically induced oscillations of the inner edge of the disk. To better understand the physics controlling these disk oscillations, we present a semi-analytical model and confirm that the oscillation period is set by the spin down rate of the disk inner edge. Finally, our simulations offer strong evidence that it is indeed the interaction of the stellar magnetosphere with the disk, rather than some primordial field in the disk itself, that is responsible for the formation of jets from these systems.

A submillimetre survey for protostellar accretion discs using the JCMT-CSO interferometer

We present a submillimetre continuum survey for accretion discs around seven embedded protostars in the Perseus and Serpens molecular clouds. Observations were made at frequencies between 339 and 357 GHz using the James Clerk Maxwell Telescope–Caltech Submillimeter Observatory single-baseline interferometer on Mauna Kea, Hawaii. All the objects in our survey show compact dust emission on scales ≲1 arcsec, assumed to arise in a circumstellar accretion disc. We compare the properties of this compact component with evolutionary indicators, such as the ratio of compact to extended emission, and bolometric temperature. We find that discs of mass ∼0.01 M⊙ have formed by the Class 0 stage, and that similar mass discs are observed in Class I and Class II sources. A trend is observed whereby the ratio of compact to extended emission in our sources increases from Class 0 to Class II sources. For three of the objects in the survey, NGC 1333 IRAS2:CR1 and SVS13 in Perseus, and FIRS1 in Serpens, the signal-to-noise ratio is sufficient to allow us to model the brightness distributions with elliptical Gaussian and power-law disc models. The Gaussian fits give semimajor half-power radii of approximately 90 to 140 au, at the assumed distance of 350 pc to the Perseus and Serpens clouds.

The relationship between masers and massive star formation: what can be learned from the infrared?

The infrared represents an alternative wavelength regime in which to study the environments of maser emission, while at the same time complementing the information obtained through radio techniques. The near infrared (1–2 μm) yields information on outflows, shocks, and reflected dust emission, while the thermal infrared (3–30 μm) yields information on the thermal dust distribution around stars. Thus, the infrared regime yields important clues in determining whether masers exist in shocks, outflows, circumstellar accretion disks, or in the dense medium close to protostars.

Substantial reservoirs of molecular hydrogen in the debris disks around young stars.

Circumstellar accretion disks transfer matter from molecular clouds to young stars and to the sites of planet formation. The disks observed around pre-main-sequence stars have properties consistent with those expected for the pre-solar nebula from which our own Solar System formed 4.5 Gyr ago. But the 'debris' disks that encircle more than 15% of nearby main-sequence stars appear to have very small amounts of gas, based on observations of the tracer molecule carbon monoxide: these observations have yielded gas/dust ratios much less than 0.1, whereas the interstellar value is about 100 (ref. 9). Here we report observations of the lowest rotational transitions of molecular hydrogen (H2) that reveal large quantities of gas in the debris disks around the stars beta Pictoris, 49 Ceti and HD135344. The gas masses calculated from the data are several hundreds to a thousand times greater than those estimated from the CO observations, and yield gas/dust ratios of the same order as the interstellar value.

Circumstellar Disk Candidates Identified from Ultraviolet Excesses in the Orion Nebula Cluster Flanking Fields

We present an optical study of four 45' × 45' fields centered ~05 east, west, north, and south of the Orion Nebula Cluster center. We have measured V and IC photometry for ~5000 stars, and U photometry for ~1600 of these. We have obtained spectral classifications for ~300 of the stars with UVIC photometry plus an additional ~200 stars located outside the area of our photometric survey. Dereddened photometry allows us to investigate the evidence for circumstellar accretion disks from excess emission at ultraviolet wavelengths, as well as the mass and age distributions of our sample. We find ~230 active accretion disk candidates and estimate the accretion disk fraction at 40%. We use the magnitude of the ultraviolet excess to infer disk accretion rates. The mass accretion rate () decreases slowly with age over the limited age range spanned by our data, 1–3 Myr, and increases slowly with mass over the limited mass range 0.2–1.2 M⊙. Although complicated selection effects are likely to affect the overall trends with mass and age, we are sensitive to, but do not see, high -values associated with the older ages and lower masses in our sample. The mean value of is ~10-8 M⊙ yr-1, with a range of more than 1 order of magnitude at all ages and masses. We find an age and age spread for our sample of 106.0±0.4 yr with no variation within or between our fields. Meaningful constraint of the mass distribution is precluded because of the completeness limits of our survey.

Protoplanetary Disk and/or Bipolar Outflow Traced by H \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $_{2}$ \end{document} O Masers in IC 1396N

The H2O maser connected with the young stellar object in the globule IC 1396N has been mapped with the VLBA during its highest state of activity in 1996 June. The spectrum of the H2O maser consisted of a dense group of strong low-velocity features near the LSR velocity of the globule, and two high-velocity features: one redshifted to 9.3 km s-1 and the other blueshifted to -14.1 km s-1. The map of low-velocity features displays a remarkable chain of at least eight maser spots located very close to a straight line about 15 AU in extent, with LSR velocities varying linearly along the line. The two high-velocity features are offset from the low-velocity group by 410 and 10,000 AU for the blue and red features, respectively. We discuss three models that can describe the observed distribution of maser spots: a Keplerian disk, a shock front, and a molecular outflow. The final model that we propose incorporates all three of these models: the low-velocity features arise in the Keplerian disk with maser emission excited by shock waves traveling in the disk, while the high-velocity features arise at the root of the molecular outflow originating from the central 4 M☉ young star or a protostar. The mass of the disk and its angular momentum are similar to those of the solar system planets. It is suggested that it is a circumstellar accretion disk accumulating the excess angular momentum of the collapsing molecular core, which may give rise to the formation of a planetary system. This model can be tested by measurements of the proper motion and radial velocity variations of the maser spots.

X-ray emission and low-mass stars in the young open cluster NGC 2547

We present the results of deep ROSAT High Resolution Imager X-ray observations and broad-band BVIC CCD photometry of the young open cluster NGC 2547. We have been able to find cluster counterparts for the majority of the 102 significant X-ray sources detected, confirming the power of high spatial resolution X-ray observations to find low-mass, magnetically active members of distant open clusters. The age of the cluster has been determined by fitting the main-sequence turn-off and the positions of low-mass stars on their pre-main-sequence tracks. The two methods yield 55 ± 25 and 14 ± 4 Myr respectively, but we consider the lower age to be more reliable because of the comparatively large number of low-mass stars that constrain the fit. We deduce that any age spread among the low-mass stars is 20 km s−1. If this is confirmed then, even with no angular momentum loss, the fastest rotating NGC 2547 stars could not evolve into the fastest rotating stars in the α Per cluster. These results cast doubt on the assumption that rotation rates and magnetic activity seen in one cluster are representative of similar stars at the same age. We hypothesize that the solar-type stars of NGC 2547 either still possess, or have recently lost, circumstellar accretion discs which regulate their angular momentum. This would require longer disc lifetimes than commonly measured or assumed for other young stars, and we speculate on possible reasons for this.

Three‐dimensional Waves Generated at Lindblad Resonances in Thermally Stratified Disks

We analyze the linear, three-dimensional response to tidal forcing of a disk that is thin and thermally stratified in the direction normal to the disk plane. We model the vertical disk structure locally as a polytrope that represents a disk of high optical depth. We solve the three-dimensional gasdynamics equations semianalytically in the neighborhood of a Lindblad resonance. These solutions match asymptotically onto those valid away from resonances (previously obtained by Korycansky & Pringle) and provide solutions valid at all radii r. We obtain the following results: (1) A variety of waves are launched at the resonance, including r-modes and g-modes. However, the f-mode carries more than 95% of the torque exerted at the resonance. (2) These three-dimensional waves collectively transport exactly the amount of angular momentum predicted by the standard two-dimensional resonant torque formula. (3) Near resonance, the f-mode behaves compressibly and occupies the full vertical extent of the disk. Away from resonance, the f-mode behaves incompressibly, becomes confined near the surface of the disk, and, in the absence of other dissipation mechanisms, damps via shocks. In general, the radial length scale for this process is roughly rL/m (for resonant radius rL and azimuthal tidal forcing wavenumber m), independent of the disk thickness H. This wave-channeling process is due to the variations of physical quantities in r and is not due to wave refraction. (4) However, the inwardly propagating f-mode launched from an m = 2 inner Lindblad resonance experiences relatively minor channeling (accompanied by about a factor of 5 increase in nonlinearity), all the way to the radial center of the disk. We conclude that for binary stars, tidally generated waves at Lindblad resonances in highly optically thick circumbinary disks are subject to strong nonlinear damping by the channeling mechanism, while those in circumstellar accretion disks are subject to weaker nonlinear effects. We also apply our results to waves excited by young planets for which m ≈ r/H and conclude that the waves are likely damped on the scale of a few H.

GENERATION OF MAGNETIC FIELDS IN PARTIALLY IONIZED DUSTY PLASMAS CAUSED BY SHEARED FLOWS

Within a multi-fluid description appropriate to study the macroscopic dynamics of partially ionized dusty plasmas the generalized Ohm's law includes a magnetic field generation term due to relative dust-neutral fluid velocities. It is shown that this magnetic field generation mechanism can work efficiently in circumstellar accretion disks around T Tauri stars, which consequently should carry their own significant magnetic fields.

Time‐dependent Accretion by Magnetic Young Stellar Objects as a Launching Mechanism for Stellar Jets

A time-dependent jet launching and collimating mechanism is presented. Initial results of numerical simulations of the interaction between an aligned dipole rotator and a conducting circumstellar accretion disk that is initially threaded by the dipole field show that differential rotation between the disk and the star leads to the rapid expansion of the magnetic loops that connect the star to the disk. The expansion of these magnetic loops above and below the disk produces a two-component outflow. A hot, well-collimated outflow is generated by the convergent flow of attached plasma toward the rotation axis, while a cool, slower outflow is produced on the disk side of the expanding loop. The expanding loop, which later forms a plasmoid, defines the boundary between the jetlike flow and the disk wind. Episodic magnetic reconnection above and below the disk releases the jet plasma from the system and allows the process to repeat, reinforcing the hot, well-collimated outflow.

An X-ray superflare are from an infrared protostar

Class I protostars1 are very young, low-mass stellar objects that are, according to current models, composite: they include a central star (still in the process of formation) surrounded by an accretion disk ~10-10OAu in radius and embedded in an extended, infalling envelope of gas and dust up to ~104 AU in size2. X-ray emission from such protostars has recently been reported3,4, suggesting that X-ray ionization of gas and heating of dust could profoundly influence the physical and chemical properties of young stellar systems. But these observations did not have the resolution necessary to rule out a non-protostar origin for the emissions. Here we report X-ray observations of one of the nearest star-forming regions—the ρ Ophiuchi cloud—that clearly show an intense X-ray flare associated with a deeply embedded protostar. The peak X-ray luminosity, after correcting for extinction, is ≥10-100 times the Sun's bolometric luminosity. The behaviour and intensity of the flare can be modelled as arising from a magnetically confined, low-density plasma bubble ~0.050-0.3 Au in diameter (much larger than the star itself), and the X-ray luminosity equals or exceeds the bolometric luminosity of the forming star. Taken together, the evidence suggests that the X-rays are not created by the type of magnetic activity seen on the Sun or on other young low-mass stars, but rather are associated with processes in the circumstellar accretion disk or within the envelope.

Are Both Stars in a Classic T Tauri Binary Classic T Tauri Stars?

Large near-IR color excesses and emission-line spectra indicate the presence of optically thick circumstellar accretion disks and active boundary layers in young stars. We investigate whether this classic T Tauri (TT) behavior is found in one or both members of a binary system previously identified as a TT on the basis of the unresolved light. We consider the angularly resolved Brγ and, in one case, Na I, spectra of four close (13-26) TT systems. We also take into account the angularly resolved near-IR (K-L) colors of eight additional young binaries with separations between 03 and 25. We find that for all 12 systems, both components show, or have shown in the history of their observation, evidence for TT behavior. We demonstrate that this cannot be the result of random pairing of a population of single TT and weak-lined T Tauri (WT) stars. We speculate that the result that inner, AU-sized disks tend to survive for a similar length of time in both components of a close (03-26) binary suggests that a circumbinary envelope effectively regulates the common evolution of the inner disks.

The Evolution of Rotation and Activity in Young Open Clusters: IC 2391

view Abstract Citations (168) References (99) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Evolution of Rotation and Activity in Young Open Clusters: IC 2391 Patten, Brian M. ; Simon, Theodore Abstract We have undertaken a program of ground- and space-based observations to measure photometric rotation periods and X-ray luminosities for late-type stars in the young open cluster IC 2391. At an age of ∼30 Myr, IC 2391 is an ideal cluster in which to observe conditions at the zero-age main sequence (ZAMS) since the solar-type stars within this cluster have not been on the main sequence long enough to undergo significant magnetic braking. For late-type stars, the ZAMS represents the endpoint of pre-main-sequence evolution, wherein gravitational contraction and interactions with circumstellar accretion disks dominate the angular momentum evolution of the star. The ZAMS also represents the beginning of the main-sequence life of the star, wherein its subsequent evolution is dominated by magnetic braking. Knowledge of ZAMS stellar parameters, such as rotation rates and activity levels, is therefore crucial for accurate theoretical model predictions. Our survey of IC 2391 X-ray sources has revealed a number of new candidates for cluster membership, most of these being late-type stars. Among the solar-type stars in IC 2391, we find a factor of ∼20 spread in the distribution of rotation periods and also in the distribution of X-ray luminosities. We therefore conclude that stars arrive on the ZAMS with a wide range of rotation rates and activity levels. When we compare IC 2391 with older clusters, such as the Pleiades and the Hyades, we find there is an overall decline in the median rotation rate and X-ray luminosity with age, but the dispersion in X-ray luminosity, as a fraction of the median value, actually grows larger. This is because the young, rapidly rotating stars lie along a plateau of magnetic saturation, where LX has a weak dependence on rotation period, while for the older, more slowly rotating stars, the weaker LX has a strong dependence on rotation period. We find an abrupt turnover in the Rossby diagram near NR = 0.3, which suggests there is a fundamental change in the nature of dynamo activity for rapidly rotating stars. Publication: The Astrophysical Journal Supplement Series Pub Date: October 1996 DOI: 10.1086/192346 Bibcode: 1996ApJS..106..489P Keywords: GALAXY: OPEN CLUSTERS AND ASSOCIATIONS: INDIVIDUAL ALPHANUMERIC: IC 2391; STARS: CORONAE; STARS: EVOLUTION; STARS: LATE-TYPE; STARS: ROTATION; X-RAYS: STARS full text sources ADS | data products SIMBAD (117)

Rotation of T Tauri stars: accretion discs and stellar dynamos

Observed rotation periods of T Tauri stars (TTS) lie in the range |$1\lesssim P$| (days) |$\lesssim 10$|⁠, with a ‘fast rotator’ group centred at |$P\sim 3$| d, and a ‘slow rotator’ group centred at |$P\sim 8$| d. I show here that such periods are a natural consequence of the tendency of spin-down accretion torques to regulate the stellar rotation rates to values near the equilibrium rates determined by the interaction between circumstellar accretion discs and dynamo-generated magnetic fields of these stars. According to the theory sketched here, in a plot of stellar rotation periods against a parameter g, which depends on stellar properties and the accretion rate, TTS are expected to lie at or below a ‘spin-down line’. In a more approximate version of the theory, which utilizes an empirical correlation between the masses and accretion rates of TTS, the controlling parameter, gcor, depends on stellar properties alone. Predictions from both theories are in agreement with observations. I discuss the role of both classical and weak-lined TTS in this view of pre-main-sequence angular momentum regulation.

The evolutionary status of the stellar population in the rho Ophiuchi cloud core

This contribution reports the results of an infrared imaging survey aimed at characterizing the stellar populations associated with the three densest star-forming cores in the Ophiuchus molecular cloud complex. The survey has sufficient sensitivity at J, H, and K (at 5 sigma limits of 16.5, 15.4, and 14.2) to provide a complete census of embedded young stellar objects (YSOs) with masses greater than the hydrogen-burning limit, provided that their ages are less than 3 Myr and that they are obscured by no more than approximately 18 mag of visual extinction. Our data suggest (1) a large fraction (greater than 70%) of the sources located within the cores are still surrounded by circumstellar disks and/or envelopes; and (2) the shape of the initial mass function for masses, M less than 1 solar mass, appears to be consistent with that derived from the solar neighborhood. We also report the results of a deeper imaging survey of centimeter continuum sources (14 sources) in these star-forming cores and in the larger Ophiuchus complex (eight sources). A large fraction (11/14) of the radio sources associated with the cores appear to have infrared excesses diagnostic of circumstellar accretion disks and/or infalling circumstellar envelopes. In these cases, the centimeter continuum radiation most likely diagnoses the ionized component of energetic winds or jets which characterizes YSOs during the disk accretion phase. By contrast, of the eight radio sources located outside dense cores, only two show infrared excesses.

A multiwavelength study of star formation in the L1495E cloud in Taurus

view Abstract Citations (150) References (71) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Multiwavelength Study of Star Formation in the L1495E Cloud in Taurus Strom, Karen M. ; Strom, Stephen E. Abstract We have carried out a deep (t=30000s) x-ray search of the eastern portion of the L1495 cloud centered on the well known weak line T Tauri star (WTTS) V410 Tau using the ROSAT PSPC. This deep exposure enabled a search for candidate pre-main sequence (PMS) objects in this cloud to a limit 20 times more sensitive than that typical of the fields examined with the Einstein searches. Despite assertions that the PMS population in Taurus-Auriga is nearly completely known, this x-ray survey revealed 8 new PMS objects in a region 50' in diameter, as compared to a previously known stellar population of 12 objects, including deeply embedded IRAS sources. Spectroscopic and photometric observations enable us to place these objects in the HR Diagram. The newly discovered objects are predominantly stars of spectral type M0 and later, and a large fraction (6/8) appear to be surrounded by circumstellar accretion disks as judged by their infrared excess and H alpha emission. We combined the data for these x-ray discovered objects with extant and new data for the previously identified PMS stars in this region to examine the history of star formation and the frequency distribution of stellar masses in this cloud. If the ``post ROSAT'' population is either complete or representative, we conclude (1) that star formation in L1495 East took place 1 x 10^6 yrs ago and that the spread in ages is small; (2) the frequency distribution of masses, Publication: The Astrophysical Journal Pub Date: March 1994 DOI: 10.1086/173886 arXiv: arXiv:astro-ph/9309037 Bibcode: 1994ApJ...424..237S Keywords: Interstellar Matter; Molecular Clouds; Pre-Main Sequence Stars; Star Formation; Stellar Envelopes; X Ray Astronomy; Accretion Disks; Hertzsprung-Russell Diagram; Stellar Spectrophotometry; Astrophysics; STARS: CIRCUMSTELLAR MATTER; ISM: INDIVIDUAL ALPHANUMERIC: L1495; STARS: FORMATION; STARS: PRE--MAIN-SEQUENCE; X-RAYS: STARS; Astrophysics E-Print: Latex, AASTeX macros, version 3 full text sources arXiv | ADS | data products SIMBAD (63)