Orionis Stars Research Articles

Stellar Rotation: A Clue to the Origin of High-Mass Stars?

We present the results of a study aimed at assessing whether low and high mass stars form similarly. Our approach is (1) to examine the observed projected rotational velocities among a large sample of newly-formed stars spanning a range in mass between 0.2 and 50 M ; and (2) to search for evidence of a discontinuity in rotational properties that might indicate a difference in the stellar formation process at some characteristic mass. Our database includes both recently published values of v sin i for young intermediate- and low- mass stars in Orion, as well as new observations of O stars located in young clusters and OB associations. We find that the median of the quantity v_obs/v_c (observed rotational speed/equatorial breakup velocity) is typically about 0.15 and shows no evidence of a discontinuity over the full range of stellar masses, while the quantity Jsini/M (derived angular momentum per unit mass) exhibits a slow, monotonic rise with increasing mass with no evidence of a discontinuity. We suggest that these observations are most simply interpreted as indicative of a single stellar formation and angular momentum regulation mechanism: one that results in rotation rates well below breakup, and angular momenta per unit mass that differ systematically by no more than a factor of 3-4 over a mass range spanning a factor of 250.

Massive Protoplanetary Disks in the Trapezium Region

We determine the disk mass distribution around 336 stars in the young (~1 Myr) Orion Nebula cluster by imaging a 25 × 25 region in 3 mm continuum emission with the Owens Valley Millimeter Array. For this sample of 336 stars, we observe 3 mm emission above the 3 σ noise level toward 10 sources, six of which have also been detected optically in silhouette against the bright nebular background. In addition, we detect 20 objects in 3 mm continuum emission that do not correspond to known near-IR cluster members. Comparisons of our measured fluxes with longer wavelength observations enable rough separation of dust emission from thermal free-free emission, and we find substantial dust emission toward most objects. For the sample of 10 objects detected at both 3 mm and near-IR wavelengths, eight exhibit substantial dust emission. Excluding the two high-mass stars (θ1 Ori A and the BN object) and assuming a gas-to-dust ratio of 100, we estimate circumstellar masses ranging from 0.13 to 0.39 M☉. For the cluster members not detected at 3 mm, images of individual objects are stacked to constrain the mean 3 mm flux of the ensemble. The average flux is detected at the 3 σ confidence level and implies an average disk mass of 0.005 M☉, comparable to the minimum-mass solar nebula. The percentage of stars in Orion surrounded by disks more massive than ~0.1 M☉ is consistent with the disk mass distribution in Taurus, and we argue that massive disks in Orion do not appear to be truncated through close encounters with high-mass stars. Comparison of the average disk mass and number of massive dusty structures in Orion with similar surveys of the NGC 2024 and IC 348 clusters is used to constrain the evolutionary timescales of massive circumstellar disks in clustered environments.

Irradiated and Bent Jets in the Orion Nebula

We present new images and proper-motion measurements of irradiated outflows from young stars in the outskirts of the Orion Nebula obtained with the Advanced Camera for Surveys onboard the Hubble Space Telescope. The images reveal many new outflows and new details in previously detected flows. The large-scale bipolar flows such as HH 502, 505, 874, and 876 contain multiple shocks whose velocities systematically decrease with increasing distance from their sources as the flows bend away from the core of the Orion Nebula. We consider several mechanisms for bending jets, including radiation pressure, the rocket effect on a neutral jet, and the impact of a side wind. While mild bends may be explained by either radiation pressure or the rocket effect, the extreme bends of some Orion jets and LL Ori–type bow shocks are best explained by the interaction of these outflows with a large-scale flow from the nebular core. The jet Hα emission measures and geometry are used to estimate their mass-loss rates, which range from about ~ 10-9 to 10-6 M⊙ yr-1. Many of the quasi-parabolic bow shocks that wrap around young stars in Orion (the LL Ori–type objects) exhibit large gaps between the observed jets and the parabolic fronts facing the core of the nebula. These may indicate the additional action of a wide-angle outflow component such as a T Tauri stellar wind, a proplyd photoablation flow, or possibly a photoablation flow from the jet itself. The Hα surface brightness and radii of the LL Ori fronts are used to estimate that the Vw products of the wide-angle flow components have values around 10-6 M⊙ yr-1 km s-1. The side-wind density and velocity are also constrained. Outside the nebular core, the greatest concentration of bent jets and LL Ori–type parabolic fronts is located south and west of the Trapezium. The nonuniform, clumpy spatial distribution of jets, outflow sources, and proplyds in the outskirts of the Orion Nebula indicate that star formation occurred in small hierarchical clusters.

Proper motion study of the ROSAT selected WTTS candidates around Taurus-Auriga

Astrometric study of the T Tauri stars (TTS) candidates and non-TTS X-ray sources around Tau-Aur [Li, Hu 132 (1998) 173], based on the Hipparcos and the ACT Reference Catalogue, is presented in this paper. The ROSAT selected X-ray sources are found to have complicated nature. A few of them are associated with the Tau-Aur or the Orion Star Forming Regions (SFR). Some, with distances similar to that of Tau-Aur but indicating discrepant proper motions, are probable or solid Pleiades supercluster members and other late type young active stars of unresolved nature, the latter are more likely formed in rapidly moving cloudlets, or else have originated from different sites rather than Tau-Aur and dispersed to the present locations. A number of the non-TTS X-ray sources studied are possible Hyades cluster members. Some TTS candidates could be foreground pre-main sequence stars or actually young dwarfs not yet depleted their Lithium.

The Status of ROSAT X-ray Active Young Stars toward Taurus-Auriga

We present an astrometric study of the candidates of T Tauri stars (TTS) and non-TTS X-ray sources around Tau-Aur, based on the Hipparcos Catalogue and the ACT Reference Catalogue. The ROSAT selected X-ray sources are found to be a mixed population. A few of them are associated with the Tau-Aur or Orion Star Forming Regions (SFR). Some, with distances similar to that of Tau-Aur but with discrepant proper motions, are probable or sure Pleiades super-cluster members or other late type young active stars with unresolved nature, more likely to originate in rapidly moving cloudlets, or else having originated from dierent sites other than Tau-Aur and moved to the present locations. A good many of the non-TTS X-ray sources are considered as Hyades cluster members. Some TTS candidates could be foreground pre-main sequence stars or actually young dwarfs not yet depleted of their Lithium. Under the hypothesis that the sources we studied are representative of the ROSAT selected TTS candidates discovered in the outskirts of the Tau-Aur region, we conclude that only up to one third of the weak-line TTS candidates could be expected to be physically associated with the Tau-Aur association. Along with the parallax and proper motion analysis of the non-TTS X-ray sources around the Tau-Aur SFR, our result suggests that the vast majority of the young active X-ray sources within an angular diameter of about 30 of the Tau-Aur SFR, belong to four main subgroups that are spatially separate.

High Angular Resolution Mid-Infrared Imaging of Young Stars in Orion BN/KL

We present Keck Long Wavelength Spectrometer images of the Orion BN/KL star-forming region obtained in the first multiwavelength study to have 03-05 resolution from 4.7 to 22 μm. The young stellar objects designated infrared source n and radio source I are believed to dominate the BN/KL region. We have detected extended emission from a probable accretion disk around source n but infer a stellar luminosity on the order of only 2000 L☉. Although source I is believed to be more luminous, we do not detect an infrared counterpart even at the longest wavelengths. However, we resolve the close-by infrared source, IRc2, into an arc of knots ~103 AU long at all wavelengths. Although the physical relation of source I to IRc2 remains ambiguous, we suggest that these sources mark a high-density core (107-108 pc-3 over ~103 AU) within the larger BN/KL star-forming cluster. The high density may be a consequence of the core being young and heavily embedded. We suggest that the energetics of the BN/KL region may be dominated by this cluster core rather than one or two individual sources.

Star-like activity from a very young ‘isolated planet’

The discovery of isolated bodies of planetary mass has challenged the paradigm that planets form only as companions to stars. To determine whether 'isolated planets', brown dwarfs and stars can have a common origin, we have made deep submillimetre observations of part of the p Oph B star formation region. Spectroscopy of the 9-Jupiter-mass core Oph B-11 has revealed carbon monoxide line wings such as those of a protostar. Moreover, the estimated mass of outflowing gas lies on the force versus core-mass relation for protostars and protobrown dwarfs. This is evidence for a common process that can form any object between planetary and stellar masses in a molecular cloud. In a submillimetre continuum map, six compact cores in p Oph B were found to have masses presently below the deuterium-burning limit, extending the core mass function down to 0.01 M O . with the approximate form dN/dM M - 3 / 2 . If these lowest-mass cores are not transient and can collapse under gravity, then isolated planets should be very common in p Oph in the future, as is the case in the Orion star formation region. In fact, the isolated planetary objects that may form from these cores would outnumber the massive planets that have been found as companions to stars.

Further identification of ROSAT all-sky survey sources in Orion

We report on the identification of 22 ROSAT All-Sky Survey (RASS) X-ray sources distributed in the general direc- tion of the Orion star-forming region. The X-ray sample contains sources from the ROSAT bright source catalogue and from previous detections. The optical identifications are based on intermediate-resolution spectroscopy and UBV Johnson photom- etry using a 1m-class telescope. The strengths of the Hα ,N a D2 and lithium lines for the stellar counterparts are evaluated applying the spectral subtraction technique, using templates of the same spectral type. Radial velocities of the optical coun- terparts are also reported. Thirteen of the optical counterparts show the lithium absorption line in their spectra and have radial velocities consistent with the Orion star forming region. Four of these objects can be classified as new bona-fide T Tauri stars.

Oscillations in the PMS δ Scuti star V346 Ori

We report the discovery of periodicities in the light curve of the Pre-Main Sequence (PMS) Herbig Ae star V346 Ori. We interpret variations in the light curve of the star as the superposition of at least two periodic signals that result from δ Scuti-type stellar oscillations. The computation of linear non-adiabatic radial pulsation models for PMS stars reproduces these periods for a 1.55 M O . star with T e f f = 7410 K and log L/L O . = 0.74 pulsating in the fundamental and in the second overtone. If our assumption of radial oscillations is indeed correct, the star's location on the HR diagram places it at a distance significantly smaller than that of the Orion star forming region, hence raising questions regarding the evolutionary status of V346 Ori. Observations with better time sampling and spanning a longer period of time are needed if the latter question is to be addressed fully.

Gamma-rays from massive stars in Cygnus and Orion

Radioactive 26Al, ejected by massive stars through winds and supernova explosions, leads to γ-ray line emission that can serve as a probe of the interstellar environment in and near young star clusters. The ~ 1 Myr decay time of 26Al is long enough to allow transport over significant distances, which can cause substantial angular offsets between γ-ray emission and cluster stars. Details of such offsets are determined by the morphology of the ISM. We discuss observations in Cygnus and Orion, and models based on population synthesis methods.

Polarimetric Variations of Binary Stars. IV. Pre–Main-Sequence Spectroscopic Binaries Located in Taurus, Auriga, and Orion

We present polarimetric observations of 14 pre-main-sequence (PMS) binaries located in the Taurus, Auriga, and Orion star forming regions. The majority of the average observed polarizations are below 0.5%, and none are above 0.9%. After removal of estimates of the interstellar polarization, about half the binaries have an intrinsic polarization above 0.5%, even though most of them do not present other evidences for the presence of circumstellar dust. Various tests reveal that 77% of the PMS binaries have or possibly have a variable polarization. LkCa3, Par1540, and Par2494 present detectable periodic and phase-locked variations. The periodic polarimetric variations are noisier and of a lesser amplitude (~0.1%) than for other types of binaries, such as hot stars. This could be due to stochastic events that produce deviations in the average polarization, a non-favorable geometry (circumbinary envelope), or the nature of the scatterers (dust grains are less efficient polarizers than electrons). Par1540 is a Weak-line TTauri Star, but nonetheless has enough dust in its environment to produce detectable levels of polarization and variations. A fourth interesting case is W134, which displays rapid changes in polarization that could be due to eclipses. We compare the observations with some of our numerical simulations, and also show that an analysis of the periodic polarimetric variations with the Brown, McLean, & Emslie (BME) formalism to find the orbital inclination is for the moment premature: non-periodic events introduce stochastic noise that partially masks the periodic low-amplitude variations and prevents the BME formalism from finding a reasonable estimate of the orbital inclination.

New Observations of the Pulsating PMS Star V351 Ori

AbstractWe present new photoelectric observations of the Pre–Main-Sequence δ Scuti star, V351 Ori. These new data suggest that V351 Ori could be a multiperiodic variable. The comparison between observations and detailed pulsational models allows us to put independent constraints on the mass and luminosity of the star. The predicted distance is 210 pc, indicating that V351 Ori is much closer than the Orion star forming region. With an inferred mass of ∼1.8 M⊙ and an uncertain evolutionary stage, V351 Ori represents an excellent candidate for future asteroseismological studies that will assess whether it is a young PMS star (∼6 Myr) or an evolved object (∼1 Gyr) leaving the main-sequence.

Nature of OH maser and SiO thermal emission towards carbon star: IRAS 05373–0810 (V1187 Ori)

We present observational evidence that IRAS 05373-0810 is a genuine carbon star with an ISO SWS spectrum closely resembling that of RScl. Modelling of the spectral energy distribution of IRAS05373-0810 ;suggests that the star has luminosity of order of 8000 L(.) and loses mass at a rate of about 2-3 x 10(-7) M(.) yr(-1). The detected OH maser emission at 1612, 1665 and 1667 MHz arid SiO thermal emission at 86.85 GHz towards IRAS05373-0810 is not associated with this source. The available observations imply that these lines, typical for O-rich sources, come from the molecular cloud L 1641 in the Orion star forming region (OH) and. very likely, from the NGC 2149 molecular complex (SiO).

An ultra-high-resolution study of the interstellar medium towards Orion

We report ultra-high-resolution observations (R approximate to 9 x 10(5)) of Na I, Ca II, K I, CH and CH+ for interstellar sightlines towards 12 bright stars in Orion. These data enable the detection of many more absorption components than previously recognized, providing a more accurate perspective on the absorbing medium. This is especially so for the line of sight to the Orion nebula, a region not previously studied at very high resolution. Model fits have been constructed for the absorption-line profiles, providing estimates for the column density, velocity dispersion and central velocity for each constituent velocity component. A comparison between the absorption occurring in sightlines with small angular separations has been used, along with comparisons with other studies, to estimate the line-of-sight velocity structures. Comparisons with earlier studies have also revealed temporal variability in the absorption-line profile of zeta Ori, highlighting the presence of small-scale spatial structure in the interstellar medium on scales of approximate to 10 au. where absorption from both Na-0 and K-0 is observed for a particular cloud, a comparison of the velocity dispersions measured for each of these species provides rigorous limits on both the kinetic temperature and turbulent velocity prevailing in each cloud. Our results indicate the turbulent motions to be subsonic in each case. Na-0/Ca+ abundance ratios are derived for individual clouds, providing an indication of their physical state.

Near-Infrared Photometric Variability of Stars toward the Orion A Molecular Cloud

We present an analysis of J, H, and K_s time-series photometry obtained with the southern 2MASS telescope over a 0.°84 × 6° region centered near the Trapezium region of the Orion Nebula cluster. These data are used to establish the near-infrared variability properties of pre–main-sequence stars in Orion on timescales of ~1–36 days, ~2 months, and ~2 years. A total of 1235 near-infrared variable stars are identified, ~93% of which are likely associated with the Orion A molecular cloud. The variable stars exhibit a diversity of photometric behavior with time, including cyclic fluctuations with periods up to 15 days, aperiodic day-to-day fluctuations, eclipses, slow drifts in brightness over 1 month or longer, colorless variability (within the noise limits of the data), stars that become redder as they fade, and stars that become bluer as they fade. The mean peak-to-peak amplitudes of the photometric fluctuations are ~0.2 mag in each band, and 77% of the variable stars have color variations less than 0.05 mag. The more extreme stars in our sample have amplitudes as large as ~2 mag and change in color by as much as ~1 mag. The typical timescale of the photometric fluctuations is less than a few days, indicating that near-infrared variability results primarily from short-term processes. We examine rotational modulation of cool and hot starspots, variable obscuration from an inner circumstellar disk, and changes in the mass accretion rate and other physical properties in a circumstellar disk as possible physical origins of the near-infrared variability. Cool spots alone can explain the observed variability characteristics in ~56%–77% of the stars, while the properties of the photometric fluctuations are more consistent with hot spots or extinction changes in at least 23% of the stars, and with variations in the disk mass accretion rate or inner disk radius in ~1% of our sample. However, differences between the details of the observations and the details of variability predicted by hot-spot, extinction, and accretion disk models suggest either that another variability mechanism not considered here may be operative or that the observed variability represents the net results of several of these phenomena. Analysis of the star-count data indicates that the Orion Nebula cluster is part of a larger area of enhanced stellar surface density that extends over a 0.°4 × 2.°4 (3.4 pc × 20 pc) region containing ~2700 stars brighter than Ks = 14 mag.

New observations of the pulsating PMS star V351 Ori

We present new precise photoelectric observations of the Pre{Main-Sequence Scuti star, V351 Ori. These new data show that V351 Ori pulsates in a mixture of several radial modes (at least four). The comparison between observations and detailed pulsational models, allows us to provide independent constraints on the mass and luminosity of the star. The predicted distance is 210 pc, indicating that V351 Ori is much closer than the Orion star forming region. With an inferred mass of 1.8 M and an uncertain evolutionary stage, V351 Ori represents an excellent candidate for future asteroseismological studies that will assess whether it is a young PMS star (6 Myr) or an evolved object (1 Gyr) leaving the main-sequence.

A 10 Micron Search for Truncated Disks Among Pre–Main-Sequence Stars with Photometric Rotation Periods

We use mid-IR (primarily 10 μm) photometry as a diagnostic for the presence of disks with inner cavities among 32 pre–main-sequence stars in Orion and in Taurus-Auriga for which rotation periods are known and for which there is no evidence of inner disks at near-IR wavelengths. Disks with inner cavities are predicted by magnetic disk-locking models that seek to explain the regulation of angular momentum in T Tauri stars. Only three stars in our sample show evidence of excess mid-IR emission. Although these three stars may possess truncated disks consistent with magnetic disk-locking models, the remaining 29 stars in our sample do not. Apparently, stars lacking near-IR excesses in general do not possess truncated disks to which they are magnetically coupled. We discuss the implications of this result for the hypothesis of disk-regulated angular momentum. Evidently, young stars can exist as slow rotators without the aid of present disk locking, and there exist very young stars already rotating at nearly breakup velocity whose subsequent angular momentum evolution will not be regulated by disks. Moreover, we question whether disks, when present, truncate in the manner required by disk-locking scenarios. Finally, we discuss the need for rotational evolution models to take full account of the large dispersion of rotation rates present at 1 Myr; doing so may allow the models to explain the rotational evolution of low-mass pre–main-sequence stars in a way that does not depend on braking by disks.

The PMS Eclipsing Binary RXJ 0529.4+0041

We report the discovery of the first low–mass pre–main sequence eclipsing binary among a sample of double-lined spectroscopic binaries in the Orion star forming region found in a previous high-resolution spectroscopic investigation on ROSAT–discovered weak-T Tauri stars. Here we present the preliminary results from the combined analysis of the spectroscopic orbit and B and V light–curves, using data available till spring 2000. We then compare the fundamental stellar parameters derived from the orbital solution with those inferred from some widely used theoretical evolutionary models.

A comparison of the rotational properties of T Tauri stars in Orion and Taurus

We analyse the distribution of projected equatorial velocities (v sin i) for a magnitude-limited sample of stars in Taurus, in order to assess whether this sample can contain a population of fast rotators (missed in previous photometric monitoring campaigns) similar to those recently discovered in Orion by Stassun et al. We find strong evidence, in line with the results of photometric monitoring campaigns in Taurus, that there is no such population of stars in Taurus that rotate at a large fraction of breakup velocity. We thus demonstrate that the stellar rotation distributions in the two star-forming regions are intrinsically different (with a statistical significance of this discrepancy in excess of 3σ), and discuss possible origins for this difference.

A comparison of the rotational properties of T Tauri stars in Orion and Taurus

A comparison of the rotational properties of T Tauri stars in Orion and Taurus