Bolometric Temperature Research Articles

Characterizing the nature of embedded young stellar objects through silicate, ice and millimeter observations

(Abridged) Classification schemes for YSOs are based on evaluating the degree of dissipation of the surrounding envelope, whose main effects are the extinction of the optical radiation from the central YSO and re-emission in the far-infrared. Since extinction is a property of column density along the line of sight, the presence of a protoplanetary disk may lead to a misclassification when the system is viewed edge-on. We performed radiative transfer calculations, using the axysimmetric 3D radiative transfer codes RADMC and RADICAL, to show the effects of different geometries on the main indicators of YSO evolutionary stage, like the slope of the flux between 2 and 24mum, the bolometric temperature and the optical depth of silicates and ices. We show that for systems viewed at intermediate angles the 'classical' indicators of evolution accurately trace the envelope column density, and they all agree with each other. On the other hand, edge-on system are misclassified for inclinations larger than ~65deg. In particular, silicate emission, typical of pre-main sequence stars with disks, turns into absorption when the disk column density reaches 1e22cm-2, corresponding e.g. to a 5e-3 Msun flaring disk viewed at 64deg. A similar effect is noticed in all the other classification indicators studied alpha, Tbol, and the H2O and CO2 ices absorption strengths. This misclassification has a big impact on the nature of the flat-spectrum sources (alpha ~0), whose number can be explained by simple geometrical arguments without invoking evolution. A reliable classification scheme using a minimal number of observations is constituted by observations of the mm-flux with both a single dish and an interferometer.

The effect of activity on stellar temperatures and radii

Recent analyses of low-mass eclipsing binary stars have unveiled a significant disagreement between the observations and the predictions of stellar structure models. Results show that theoretical models underestimate the radii and overestimate the effective temperatures of low-mass stars but yield luminosities that accord with observations. A hypothesis based upon the effects of stellar activity was put forward to explain the discrepancies. In this paper we study the existence of the same trend in single active stars and provide a consistent scenario to explain systematic differences between active and inactive stars in the H-R diagram reported earlier. The analysis is done using single field stars of spectral types late-K and M and computing their bolometric magnitudes and temperatures through infrared colours and spectral indices. The properties of the stars in samples of active and inactive stars are compared statistically to reveal systematic differences. After accounting for a number of possible bias effects, active stars are shown to be cooler than inactive stars of similar luminosity therefore implying a larger radius as well, in proportions that are in excellent agreement with those found from eclipsing binaries. The present results generalise the existence of strong radius and temperature dependences on stellar activity to the entire population of low-mass stars, regardless of their membership in close binary systems.

Mass-luminosity relation of intermediate-mass stars

The mass–luminosity relation (MLR) for intermediate-mass stars is based on data on detached double-lined eclipsing binaries. However, there is a notable difference between the parameters of B0V–G0V components of eclipsing binaries and those of single stars. Single early-type stars are rapid rotators, whereas tidal forces produce synchronous rotation in close binaries and all such pairs are synchronized, so components of close binaries rotate more slowly. As is well known, stellar rotation changes stellar evolution and the global parameters of a star. In this work we collect data on fundamental parameters of stars with masses m > 1.5 m⊙. They are components of binaries with P > 15 d and consequently are not synchronized with the orbital periods and presumably are rapid rotators. These stars are believed to evolve similarly with single stars. Modern data on masses, absolute and bolometric luminosities, radii and temperatures of detached main-sequence double-lined eclipsing binary components (i.e. presumably slow rotators) are also collected. Mass–luminosity, mass–temperature and mass–radius relations of close and wide binaries are presented, as well as their Hertzsprung–Russell diagram. For the mass range 4.5 < m/m⊙ < 5.5 (late B stars) it was found that rapid rotators exhibit slightly higher luminosities and larger radii than predicted by the standard relations, and their main sequence is shifted to the right-hand side with respect to that of the close binary components. The resulting relations for rapidly and slowly rotating A–F and early B stars are not statistically different. As our estimations show, for the given mass range the effect on the initial mass function (IMF) is marginal, but there is no way to estimate the degree to which the effect may be important for higher masses. Available observational data for m > 12 m⊙ are too poor to make definite conclusions. Knowledge of the MLR should come from dynamical mass determinations of visual binaries combined with spatially resolved precise photometry. Then the IMF should be revised for that mass range.

High Spatial Resolution Observations of Two Young Protostars in the R Corona Australis Region

We present multi-wavelength, high spatial resolution imaging of the IRS 7 region in the R Corona Australis molecular cloud. Our observations include 1.1 mm continuum and HCO^+ J = $3 \to 2$ images from the SMA, ^{12}CO J = $3 \to 2$ outflow maps from the DesertStar heterodyne array receiver on the HHT, 450 $\mu$m and 850 $\mu$m continuum images from SCUBA, and archival Spitzer IRAC and MIPS 24 \micron images. The accurate astrometry of the IRAC images allow us to identify IRS 7 with the cm source VLA 10W (IRS 7A) and the X-ray source X_W. The SMA 1.1 mm image reveals two compact continuum sources which are also distinguishable at 450 $\mu$m. SMA 1 coincides with X-ray source CXOU J190156.4-365728 and VLA cm source 10E (IRS 7B) and is seen in the IRAC and MIPS images. SMA 2 has no infrared counterpart but coincides with cm source VLA 9. Spectral energy distributions constructed from SMA, SCUBA and Spitzer data yield bolometric temperatures of 83 K for SMA 1 and $\leq$70 K for SMA 2. These temperatures along with the submillimeter to total luminosity ratios indicate that SMA 2 is a Class 0 protostar, while SMA 1 is a Class 0/Class I transitional object (L=$17\pm6$ \Lsun). The ^{12}CO J = $3 \to 2$ outflow map shows one major and possibly several smaller outflows centered on the IRS 7 region, with masses and energetics consistent with previous work. We identify the Class 0 source SMA 2/VLA 9 as the main driver of this outflow. The complex and clumpy spatial and velocity distribution of the HCO^+ J = $3 \to 2$ emission is not consistent with either bulk rotation, or any known molecular outflow activity.

Gas and Dust Condensations and a Peculiar Class 0 Object in the Lupus 3 Star‐Forming Cloud

The Lupus 3 molecular cloud has been surveyed for dense gas and dust cores and embedded objects in radio [H 13 CO + J ¼ 1Y0 line and 1.2 mm continuum] and infrared [JHKsL 0 MN1 bands and H2 v ¼1Y0 S(1) line] wavelengths. These observations unveil a filamentary cloud, three dense cores, an embedded millimeter-wave source (MMS),andanassociatedelongatedobjectintheKband.ThepropertiesofthethreedensecoresareM ¼ 3:5Y5:6 M� , R ¼ 0:04Y0:06pc,andn(H2) ¼ (1:0Y3:9) ; 10 5 cm � 3 ,propertiessimilartothoseinTaurus.Twoofthesethreeobjects are likely to be prestellar cores, while the other one exhibits ongoing star formation. The spectral energy distribution (SED) analysis of the MMS shows that it is a remarkably cold Class 0 object with molecular outflow detected in the CO(J ¼ 3Y2) line and peculiar near-IR detections. From the estimated low bolometric temperature (39.5 K), faint bolometric luminosity (0.16 L� ), and sufficiently large envelope mass (0.52 M� ), the MMS is expected to be in a very earlyphase(� 10 4 yr)ofmassaccretion.TheK-bandelongatedfeatureappearstobescatteredlightoriginatingfromthe embeddedcentral objectofthe MMS seenthrough the outflowcavityopening towardHH 78onthe nearsideasshown by the blueshifted CO wings. The MMS has also been detected by the Spitzer Space Telescope, and its near-IR images exhibit butterfly-shaped nebulosity emission as scattered light through the bipolar cavities in contrast to that in the K band. Together with the Spitzer and NTT JHK photometric data, the observed SED has a short-wavelength cutoff suggesting a low effective temperature (<1400 K) of the central object. Subject headingg dust, extinction — ISM: clouds — ISM: individual (Lupus cloud) — ISM: jets and outflows — stars: formation — stars: preYmain-sequence

The Late‐T Dwarf Companion to the Exoplanet Host Star HD 3651: A New Benchmark for Gravity and Metallicity Effects in Ultracool Spectra

(Abridged) We present near-IR photometry and spectroscopy of HD 3651B, the low-luminosity, wide-separation (480 AU) companion to the K0V exoplanet host star HD 3651A. We find a spectral type of T7.5+/-0.5, a temperature of 780-840 K, and a mass of 40-72 Mjup; its bolometric luminosity and effective temperature are among the lowest measured for any brown dwarf. We estimate an age of 3-12 Gyr for the primary star HD 3651A and find that it is ~3x older than the K4V star Gl 570A (1-5 Gyr), the host star of the T7.5 dwarf Gl 570D. HD 3651B belongs to the rare class of substellar objects that are companions to main-sequence stars and thus provides a new benchmark for studying very low-temperature objects. Given their similar temperatures (within ~30 K) and metallicities (within ~0.1 dex) but different ages, a comparison of HD 3651B and GL 570D allows us to examine gravity-sensitive diagnostics in ultracool spectra. We find that the expected signature of HD 3651B's higher surface gravity due to its older age, namely a suppressed K-band flux relative to GL 570D, is not seen. Instead, the K-band flux of HD 3651B is enhanced compared to GL 570D, indicative of a younger age. Thus, the relative ages derived from interpretation of T dwarf spectra and from stellar activity indicators appear to be in discord. One likely explanation is that the K-band fluxes are also very sensitive to metallicity differences. Metallicity variations may be as important as surface gravity variations in causing spectral differences among field late-T dwarfs.

The Emergent Flux and Effective Temperature of δ Canis Majoris

AbstractNew angular diameter determinations for the bright southern F8 supergiant δ CMa enable the bolometric emergent flux and effective temperature of the star to be determined with improved accuracy. The spectral flux distribution and bolometric flux have been determined from published photometry and spectrophotometry and combined with the angular diameter to derive the bolometric emergent flux ℱ = (6.50 ± 0.24) × 107 Wm−2 and the effective temperature Teff = 5818 ± 53 K. The new value for the effective temperature is compared with previous interferometric and infrared flux method determinations. The accuracy of the effective temperature is now limited by the uncertainty in the bolometric flux rather than by the uncertainty in the angular diameter.

Accurate fundamental parameters for lower main-sequence stars

We derive an empirical effective temperature and bolometric luminosity calibration for G and K dwarfs, by applying our own implementation of the InfraRed Flux Method to multi-band photometry. Our study is based on 104 stars for which we have excellent BVRIJHK photometry, excellent parallaxes and good metallicities. Colours computed from the most recent synthetic libraries (ATLAS9 and MARCS) are found to be in good agreement with the empirical colours in the optical bands, but some discrepancies still remain in the infrared. Synthetic and empirical bolometric corrections also show fair agreement. A careful comparison to temperatures, luminosities and angular diameters obtained with other methods in literature shows that systematic effects still exist in the calibrations at the level of a few percent. Our InfraRed Flux Method temperature scale is 100K hotter than recent analogous determinations in the literature, but is in agreement with spectroscopically calibrated temperature scales and fits well the colours of the Sun. Our angular diameters are typically 3% smaller when compared to other (indirect) determinations of angular diameter for such stars, but are consistent with the limb-darkening corrected predictions of the latest 3D model atmospheres and also with the results of asteroseismology. Very tight empirical relations are derived for bolometric luminosity, effective temperature and angular diameter from photometric indices. We find that much of the discrepancy with other temperature scales and the uncertainties in the infrared synthetic colours arise from the uncertainties in the use of Vega as the flux calibrator. Angular diameter measurements for a well chosen set of G and K dwarfs would go a long way to addressing this problem.

Cooling of the quasi-persistent neutron star X-ray transients KS 1731-260 and MXB 1659-29

We present Chandra and XMM‐Newton X-ray observations that monitor the neutron star cooling of the quasi-persistent neutron star X-ray transients KS 1731−260 and MXB 1659−29 for approximately 4 yr after these sources returned to quiescence from prolonged outbursts. In both sources the outbursts were long enough to significantly heat the neutron star crust out of thermal equilibrium with the core. We analyse the X-ray spectra by fitting absorbed neutron star atmosphere models to the observations. The results of our analysis strengthen the preliminary findings of Wijnands et al. that in both sources the neutron star crust cools down very rapidly suggesting it has a high heat conductivity and that the neutron star core requires enhanced core cooling processes. Importantly, we now detect the flattening of the cooling in both sources as the crust returns to thermal equilibrium with the core. We measure the thermal equilbrium flux and temperature in both sources by fitting a curve that decays exponentially to a constant level. The cooling curves cannot be fit with just a simple exponential decay without the constant offset. We find the constant bolometric flux and effective temperature components to be (9.2 ± 0.9) × 10 −14 erg cm −2 s −1 and 70.0 ± 1.6 eV in KS 1731−260 and (1.7 ± 0.3) × 10 −14 erg cm −2 s −1 and 51.6 ± 1.4 eV in MXB 1659−29. We note that these values are dependent on the assumed distance to the sources and the column density which was tied between the observations due to the low number of photons in the latter observations. However, importantly, the shape of the cooling curves is independent of the distance assumed. In addition, we find that the crust of KS 1731−260 cools faster than that of MXB 1659−29 by a factor of ∼2, likely due to different crustal properties. This is the first time that the cooling of a neutron star crust into thermal equilibrium with the core has been observed in such detail.

The possiblity of detection of ultracool dwarfs with the UKIRT Infrared Deep Sky Survey

We present predictions for the numbers of ultra--cool dwarfs in the Galactic disk population that could be detected by the WFCAM/UKIDSS Large Area Survey and Ultra Deep Survey. Simulated samples of objects are created with masses and ages drawn from different mass functions and birthrates. Each object is then given absolute magnitudes in different passbands based on empirically derived bolometric correction vs. effective temperature relationships (or model predictions for Y dwarfs). These are then combined with simulated space positions, velocities and photometric errors to yield observables such as apparent magnitudes and proper motions. Such observables are then passed through the survey selection mechanism to yield histograms in colour. This technique also produces predictions for the proper motion histograms for ultra--cool dwarfs and estimated numbers for the as yet undetected Y dwarfs. Finally it is shown that these techniques could be used to constrain the ultra low--mass mass function and birthrate of the Galactic disk population

ChandraObservations of SDSS J1004+4112: Constraints on the Lensing Cluster and Anomalous X‐Ray Flux Ratios of the Quadruply Imaged Quasar

We present results from Chandra observations of SDSS J1004+4112, a strongly lensed quasar system with a maximum image separation of 15''. All four bright images of the quasar, as well as resolved X-ray emission originating from the lensing cluster, are clearly detected. The emission from the lensing cluster extends out to approximately 15. We measure the bolometric X-ray luminosity and temperature of the lensing cluster to be 4.7 × 1044 ergs s-1 and 6.4 keV, consistent with the luminosity-temperature relation for distant clusters. The mass estimated from the X-ray observation shows excellent agreement with the mass derived from gravitational lensing. The X-ray flux ratios of the quasar images differ markedly from the optical flux ratios, and the combined X-ray spectrum of the images possesses an unusually strong Fe Kα emission line, both of which are indicative of microlensing.

Evolution of Class 0 protostars: models versus observations

The rates at which mass accumulates into protostellar cores can now be predicted in numerical simulations. Our purpose here is to develop methods to compare the statistical properties of the predicted protostars with the observable parameters. This requires (1) an evolutionary scheme to convert numerically derived mass accretion rates into evolutionary tracks and (2) a technique to compare the tracks to the observed statistics of protostars. Here, we use a 3D Kolmogorov–Smirnov test to quantitatively compare model evolutionary tracks and observations of Class 0 protostars. We find that the wide range of accretion functions and time-scales associated with gravoturbulent simulations naturally overcome difficulties associated with schemes that use a fixed accretion pattern. This implies that the location of a protostar on an evolutionary track does not precisely determine the present age or final accrued mass. Rather, we find that predictions of the final mass for protostars from observed Tbol–Lbol values are uncertain by a factor of 2 and that the bolometric temperature is not always a reliable measure of the evolutionary stage. Furthermore, we constrain several parameters of the evolutionary scheme and estimate a lifetime of Class 0 sources of 2–6 × 104 yr, which is related to the local free-fall time and thus to the local density at the onset of the collapse. Models with Mach numbers smaller than six are found to best explain the observational data. Generally, only a probability of 70 per cent was found that our models explain the current observations. This is caused by not well-understood selection effects in the observational sample and the simplified assumptions in the models.

Optical Spectroscopy of the Surface Population of the ρ Ophiuchi Molecular Cloud: The First Wave of Star Formation

We present the results of optical spectroscopy of 139 stars obtained with the Hydra multiobject spectrograph. The objects extend over a 1.3 deg2 area surrounding the main cloud of the ρ Oph complex. The objects were selected from narrowband images to have Hα in emission. Using the presence of strong Hα emission, lithium absorption, location in the Hertzsprung-Russell diagram, or previously reported X-ray emission, we were able to identify 88 objects as young stars associated with the cloud. Strong Hα emission was confirmed in 39 objects with line widths consistent with their origin in magnetospheric accretion columns. Two of the strongest emission-line objects are young, X-ray-emitting brown dwarf candidates with M8 spectral types. Comparisons of the bolometric luminosities and effective temperatures with theoretical models suggest a median age for this population of 2.1 Myr, which is significantly older than the ages derived for objects in the cloud core. It appears that these stars formed contemporaneously with low-mass stars in the Upper Scorpius subgroup, likely triggered by massive stars in the Upper Centaurus subgroup.

Evolutionary Signatures in the Formation of Low‐Mass Protostars

We present an evolutionary picture of a forming star. We assume a singular isothermal sphere as the initial state of the core that undergoes collapse, as described by Shu. We include the evolution of a first hydrostatic core at early times and allow a disk to grow, as predicted by Adams & Shu. We use a one-dimensional radiative transfer code to calculate the spectral energy distribution for the evolving protostar from the beginning of collapse to the point when all envelope material has accreted onto the star + disk system. Then, we calculate various observational signatures (Tbol, Lbol/Lsmm, and infrared colors) as a function of time. As defined by the bolometric temperature criterion, the Class 0 stage should be very short, while the Class I stage persists for much of the protostar's early life. We present physical distinctions among the classes of forming stars and calculate the observational signatures for these classes. Finally, we present models of infrared color-magnitude diagrams, as observed by the Spitzer Space Telescope, that should be strong discriminators in determining the stage of evolution for a protostar.

Period-colour and amplitude-colour relations in classical Cepheid variables - II. The Galactic Cepheid models

In this paper, we construct full amplitude non-linear hydrodynamical models of fundamental mode Galactic Cepheids and analyse the resulting theoretical period-colour (PC) and amplitude-colour (AC) relations at maximum, mean and minimum light. These theoretical relations match the general form of the observed relations well. This agreement is, to some extent, independent of the mass-luminosity (ML) relations used, pulsation code, numerical techniques, details of the input physics and methods to convert theoretical quantities, such as bolometric luminosity and temperature, to observational quantities, such as V-band magnitudes or (V - I) colours. We show that the PC and AC properties of fundamental mode Galactic Cepheids with periods such that log (P) > 0.8 can be explained by a simple application of the Stefan-Boltzmann law and the interaction of the photosphere with the hydrogen ionization front (HIF). We discuss the implications of our results for explaining the behaviour of Galactic Cepheid PC and period-luminosity (PL) relations at mean light.

Monitoring Chandra Observations of the Quasi-persistent Neutron Star X-Ray Transient MXB 1659-29 in Quiescence: The Cooling Curve of the Heated Neutron Star Crust

We have observed the quasi-persistent neutron-star X-ray transient and eclipsing binary MXB 1659-29 in quiescence on three occasions with Chandra. The purpose of our observations was to monitor the quiescent behavior of the source after its last prolonged (~2.5 years) outburst which ended in September 2001. The X-ray spectra of the source are consistent with thermal radiation from the neutron-star surface. We found that the bolometric flux of the source decreased by a factor of 7-9 over the time-span of 1.5 years between our first and last Chandra observations. The effective temperature also decreased, by a factor of 1.6-1.7. The decrease in time of the bolometric flux and effective temperature can be described using exponential decay functions, with e-folding times of 0.7 and ~3 years, respectively. Our results are consistent with the hypothesis that we observed a cooling neutron-star crust which was heated considerably during the prolonged accretion event and which is still out of thermal equilibrium with the neutron-star core. We could only determine upper-limits for any luminosity contribution due to the thermal state of the neutron-star core. The rapid cooling of the neutron-star crust implies that it has a large thermal conductivity. Our results also suggest that enhanced cooling processes are present in the neutron-star core.

Aeolian processes in Proctor Crater on Mars: Sedimentary history as analyzed from multiple data sets

Proctor Crater is a 150 km diameter crater in Noachis Terra, within the southern highlands of Mars. The analysis leading to the sedimentary history incorporates several data sets including imagery, elevation, composition, and thermal inertia, mostly from the Mars Global Surveyor mission. The resulting stratigraphy reveals that the sedimentary history of Proctor Crater has involved a complex interaction of accumulating and eroding sedimentation. Aeolian features spanning much of the history of the crater interior dominate its surface, including large erosional pits, stratified beds of aeolian sediment, sand dunes, erosional and depositional streaks, dust devil tracks, and small bright bed forms that are probably granule ripples. Long ago, up to 450 m of layered sediment filled the crater basin, now exposed in eroded pits on the crater floor. These sediments are probably part of an ancient deposit of aeolian volcaniclastic material. Since then, some quantity of this material has been eroded from the top layers of the strata. Small, bright dune forms lie stratigraphically beneath the large dark dune field. Relative to the large dark dunes, the bright bed forms are immobile, although in places, their orientations are clearly influenced by the presence of the larger dunes. Their prevalence in the crater and their lack of compositional and thermal distinctiveness relative to the crater floor suggests that these features were produced locally from the eroding basin fill. Dust devil tracks form during the spring and summer, following a west‐southwesterly wind. Early in the spring the dust devils are largely restricted to dark patches of sand. As the summer approaches, dust devil tracks become more plentiful and spread to the rest of the crater floor, indicating that the entire region acquires an annual deposit of dust that is revealed by seasonal dust devils. The dark dunes contain few dust devil tracks, suggesting that accumulated dust is swept away directly by saltation, rather than by the passage of dust devils. Spectral deconvolution indicates that the dark dunes have infrared spectra consistent with basalt‐like materials. The average thermal inertia calculated from Thermal Emission Spectrometer bolometric temperatures is 277 ± 17 J m−2 s−0.5 K−1, leading to an effective grain size of 740 ± 170 μm, which is consistent with coarse sand and within the range expected for Martian sand. The coarse sand that composes the large dune field may have originated from outside the crater, saltating in from the southwest. Most of the transport pathway that delivered this sand to the dune field has since been eroded away or buried. The sand was transported to the east center of the crater floor, where beneath the present‐day dunes a 50 m high mound of sand has accumulated. Dune slip faces indicate a wind regime consisting of three opposing winds. Some of these wind directions are correlated with the orientations of dust devil tracks and bright bed forms. The combination of a tall mound of sand and three opposing winds is consistent with a convergent wind regime, which produces the large reversing transverse and star dunes that dominate the dune field. The dark dunes have both active slip faces and seemingly inactive slip faces, suggesting that the dunes vary spatially in their relative activity. Nevertheless, the aeolian activity that has dominated the history of Proctor Crater still continues today.

Far-infrared photometry of deeply embedded outflow sources

We present far-infrared maps and spectroscopy for a number of deeply embedded protostellar objects (Cep E, HH 211-MM, IC 1396 W, L 1157, L 1211, and RNO 15 FIR) from data that we acquired with the Infrared Space Observatory (ISO) photopolarimeter (PHOT) and Long Wavelength Spectrometer (LWS). Several previously undetected deeply embedded sources are found in the vicinity of our targets. We determine temperatures and luminosities of seven objects and locate them on an L b o l -T b o l diagram - the equivalent of a Hertzsprung-Russell diagram for protostars. Their masses and ages, according to their location on tracks taken from our evolutionary model, are derived. L 1211 and Cep E appear to be intermediate-mass objects which will reach final masses of about 3 M O ., whereas the other sources are in or below the solar mass range. The derived ages of 15 000 to 30000 yr are consistent with their current Class 0 state. A comparison of the luminosity of the associated outflows in the 1 - 0 S(1) line of molecular hydrogen with the source properties (bolometric luminosity, bolometric temperature and envelope mass) of 16 Class 0 sources shows no statistically significant correlations. Nevertheless, the data are consistent with a scheme in which the outflow strength and protostar evolve simultaneously. We show that the relationship is partially disguised, however, by the local properties of the surrounding material, the extinction and by short-term flux variability.

Near-infrared photometry of 20 hipparcos carbon stars

Near-infrared JHK photometry was carried out for 20 carbon stars observed by the satellite Hipparcos. From the observations, bolometric corrections in the K band ( BC K ), apparent bolometric magnitudes ( m rmbo ) and effective temperatures ( T e ) of these stars were estimated. Combined with the trigonometric parallaxes measured by Hipparcos, the absolute bolometric magnitudes ( M bol) of some of the stars were obtained.

The Evolution of the Circumstellar Environment of Embedded Young Stars from Observations of Rare Species of Carbon Monoxide

Studies of the properties of the circumstellar material around young embedded sources can provide important insights into the relative importance of infall and outflow in clearing the circumstellar environment. In this paper we report one such study of 20 of the most deeply embedded young stars in the Taurus molecular cloud using the J = 1 → 0, J = 2 → 1, and J = 3 → 2 transitions of C18O and C17O. The profiles comprise a narrow component, with a line width of ~0.7 km s-1 plus in many cases a second broader component that contains a similar mass of material. The broad-component line width decreases with increasing source age as measured by the source bolometric temperature, consistent with what would be expected if the broad component traces the interaction between a stellar wind and ambient material. The momentum flux of the broad-component emission in the immediate vicinity of the forming star is comparable to that in the spatially extended outflows from these sources and also decreases with increasing source age. Comparison with models of this interaction provide a reasonable match to the behavior of the broad component with source age. The derived total column density toward the sources shows a systematic decrease with increasing bolometric temperature and implies a circumstellar mass-loss rate that varies from ~10-4 M☉ yr-1 for the youngest sources to ~10-8 M☉ yr-1 for sources in the late Class I phase. The broad components are sufficiently massive and energetic to clear the circumstellar environment on timescales of ~105 yr, and we argue that the broad component of the C18O line profiles is the dense, low-velocity inner part of the outflow from the sources and may be the material being directly swept up by the stellar wind. The widths of the broad and narrow components are correlated, indicating that the narrow-component material is already being stirred by the stellar wind, even in the youngest sources.