AN ENHANCED SPECTROSCOPIC CENSUS OF THE ORION NEBULA CLUSTER

We present U, B, V, I broadband, 6200 A TiO mediumband, and Hα narrowband photometry of the Orion Nebula Cluster (ONC) obtained with the WFI imager at the ESO/MPI 2.2 telescope at La Silla Observatory. The nearly simultaneous observations cover the entire ONC in a field of about 34 × 34 arcmin. They enable us to determine stellar colors avoiding the additional scatter in the photometry induced by stellar variability typical of pre-main-sequence stars. We identify 2612 point-like sources in the I band; 58%, 43%, and 17% of them are also detected in V, B, and U, respectively. 1040 sources are identified in the Hα band. In this paper we present the observations, the calibration techniques adopted, and the resulting catalog. We show the derived color-magnitude diagram of the population and discuss the completeness of our photometry. We define a spectrophotometric TiO index that takes into account the fluxes in the V, I, and TiO bands. Comparing it with spectral types of ONC members in the literature, we find a correlation between the index and the spectral type valid for M-type stars, which is accurate to better than 1 spectral subclass for M3-M6 types and better than 2 spectral subclasses for M0-M2 types.. This allows us to newly classify 217 stars. In a similar way, we subtract from our Hα photometry the photospheric continuum at its wavelength, deriving calibrated line excess for the full sample. This represents the largest Hα star catalog obtained to date on the ONC. This data set enables a full re-analysis of the properties of the pre-main-sequence population in the Orion Nebula Cluster to be presented in an accompanying paper.

We present a new analysis of the stellar population of the Orion Nebula Cluster (ONC) based on multi-band optical photometry and spectroscopy. We study the color-color diagrams in BVI, plus a narrow-band filter centered at 6200A, finding evidences that intrinsic color scales valid for main-sequence dwarfs are incompatible with the ONC, while a better agreement is found employing synthetic intrinsic colors obtained constraining the typical lower surface gravity of young stars. We refine these model colors even further, empirically, by comparison with a sample of ONC stars with no accretion and no extinction. We consider the stars with known spectral types from the literature, and add 65 newly classified stars from slit spectroscopy and 182 M-type from narrow-band photometry; in this way we isolate a sample of about 1000 stars with known spectral type. We introduce a new method to self-consistently derive reddening and accretion excess from the location of each star in the BVI color-color diagram. This enables us to accurately determine the extinction of the ONC members. We adopt a lower distance for the ONC than previously assumed, based on recent parallax measurements. With a careful choice also of the spectral type-temperature transformation, we produce the new H-R diagram of the ONC population, more populated than previous works. With respect to previous works, we find higher luminosity for late-type stars and a lower luminosity for early types. We determine the age distribution of the population, peaking at 2-3 Myr, a higher age than previously estimated. We study the distribution of the members in the mass-age plane, and find that taking into account selection effects due to incompleteness removes an apparent correlation between mass and age. We derive the IMF for low- and intermediate-mass members of the ONC, which turns out to be model-dependent, and shows a turn-over at ~<0.2Msun.

We derive the spectral types of the cool gi- ants in about 100 symbiotic systems. Our classication is mainly based on near IR spectra in order to avoid the contamination of the spectrum by the nebula and the hot component in the visual region. The accuracy of our spec- tral types is approximately one spectral subclass, similar to previous near IR classication work, and much better than visual spectral type estimates. Strong, intrinsic spectral type variations (> 2 spectral subtypes) are only seen in systems containing pulsating mira variables. We present a catalogue of spectral types for cool giants in symbiotic systems which also includes determinations taken from the literature. The catalogue gives spectral types for the cool giants in about 170 systems which is nearly the full set of conrmed symbiotics. Based on our classications we discuss the distribution of spectral types of the cool giants in galactic symbiotic bi- naries. We nd that the spectral types cluster strongly be- tween M3 and M6, with a peak at M5. The distribution of systems with a mira variable component peaks even later, at spectral types M6 and M7. This is a strong bias towards late spectral types when compared to red giants in the so- lar neighbourhood. Also the frequency of mira variables is much larger among symbiotic giants. This predominance of very late M-giants in symbiotic systems seems to indi- cate that large mass loss is a key ingredient for triggering symbiotic activity on a white dwarf companion. Further we nd for symbiotic systems a strong corre- lation between the spectral type of the cool giant and the orbital period. In particular we nd a tight relation for the minimum orbital period for symbiotic systems with ? Based on observations obtained with the 1.52 m and 3.6 m telescopes of the European Southern Observatory (ESO), the 1.93 m telescope of the Observatoire de Haute-Provence (OHP), the 2.3 m telescope of the Australian National University (ANU) at Siding Spring, and the William Herschel Telescope (WHT) at La Palma. This research has made use of the AFOEV database, operated at CDS, France. red giants of a given spectral type. This limiting line in the spectral type { orbital period diagram seems to be equivalent with the relation R '1=2, where R is the ra- dius of the red giant and '1 the distance from the center of the giant to the inner Lagrangian point L1. This cor- relation possibly discloses that symbiotic stars are { with probably only one exception in our sample { well detached binary systems.

We report on XMM—Newton observations of the young open cluster NGC 2547 which allow us to characterize coronal activity in solar-type stars, and stars of lower mass, at an age of 30 Myr. X-ray emission is seen from stars at all spectral types, peaking among G stars at luminosities (0.3-3 keV) of Lx≃ 1030.5 erg s−1 and declining to Lx≤ 1029.0 erg s−1 among M stars with masses ≥0.2 M⊙. Coronal spectra show evidence for multi-temperature differential emission measures and low coronal metal abundances of Z≃ 0.3. The G- and K-type stars of NGC 2547 follow the same relationship between X-ray activity and Rossby number established in older clusters and field stars, although most of the solar-type stars in NGC 2547 exhibit saturated or even supersaturated X-ray activity levels. The median levels of Lx and Lx/Lbol in the solar-type stars of NGC 2547 are very similar to those in T-Tauri stars of the Orion Nebula cluster (ONC), but an order of magnitude higher than in the older Pleiades. The spread in X-ray activity levels among solar-type stars in NGC 2547 is much smaller than in older or younger clusters. Coronal temperatures increase with Lx, Lx/Lbol and surface X-ray flux. The most active solar-type stars in NGC 2547 have coronal temperatures intermediate between those in the ONC and the most active older zero-age main-sequence (ZAMS) stars. We show that simple scaling arguments predict higher coronal temperature in coronally saturated stars with lower gravities. A number of candidate flares were identified among the low-mass members and a flaring rate [for total flare energies (0.3-3 keV) &amp;gt; 1034 erg] of one every 350+350−120 ks was found for solar-type stars, which is similar to rates found in the ONC and Pleiades. Comparison with ROSAT High Resolution Imager (HRI) data taken 7 yr earlier reveals that only 10-15 per cent of solar-type stars or stars with Lx &amp;gt; 3 × 1029 erg s−1 exhibit X-ray variability by more than a factor of 2. This is comparable with clusters of similar age but less than in both older and younger clusters. The similar median levels of X-ray activity and rate of occurrence for large flares in NGC 2547 and the ONC demonstrate that the X-ray radiation environment around young solar-type stars remains relatively constant over their first 30 Myr.

We present near-infrared long-slit and multislit spectra of low-mass brown dwarf candidates in the Orion nebula cluster. The long-slit data were observed in the H and K bands using NIRI on the Gemini-North Telescope. The multi-object spectroscopic observations were made using IRIS2 on the Anglo-Australian Telescope at H band. We develop a spectral typing scheme based on optically calibrated, near-infrared spectra of young sources in the Taurus and IC 348 star-forming regions with spectral types M3.0 to M9.5. We apply our spectral typing scheme to 52 sources, including previously published UKIRT and GNIRS spectra. 40 objects show strong water absorption with spectral types of M3 to >M9.5. The latest type objects are provisionally classified as early L types. We plot our sources on Hertzsprung-Russell diagrams overlaid with theoretical pre-main-sequence isochrones. The majority of our objects lie close to or above the 1-Myr isochrone, leading to an average cluster age that is <1 Myr. We find 38 sources lie at or below the hydrogen-burning limit (0.075 M ⊙ ). 10 sources potentially have masses below the deuterium-burning limit (0.012 M ⊙ ). We use a Monte Carlo approach to model the observed luminosity function with a variety of cluster age and mass distributions. The lowest x 2 values are produced by an age distribution centred at 1 Myr, with a mass function that declines at substellar masses according to an M α power law in the range a = 0.3-0.6. We find that truncating the mass function at 0.012 M ⊙ produces luminosity functions that are starved of the faintest magnitudes, even when using bimodal age populations that contain 10-Myr-old sources. The results of these Monte Carlo simulations therefore support the existence of a planetary mass population in the ONC.

We present a study of protoplanetary disks in spatially resolved low-mass binary stars in the Orion Nebula Cluster (ONC) to assess the impact of binarity on the properties of circumstellar disks. This is currently the largest such study in a clustered high-stellar-density star-forming environment. We particularly aim to determine the presence of magnetospheric accretion and dust disks for each binary component, and measure the overall disk frequency. We carried out spatially resolved adaptive-optics-assisted observations to acquire near-IR photometry and spectroscopy of 26 binaries in the ONC, and determine stellar parameters such as effective temperatures, spectral types, luminosities, and masses, as well as accretion properties and near-infrared excesses for the individual binary components. A fraction of 40(+10/-9)% of the binary components in the sample can be inferred to be T Tauri stars possessing an accretion disk, marginally fewer than the disk fraction of single stars. We find that disks in wide binaries of >200AU separation are consistent with random pairing, while the evolution of circumprimary and circumsecondary disks is observed to be synchronized in close binaries (separations <200AU). Circumbinary disks appear to be unsuitable to explain this difference. Furthermore, we identify several mixed pairs of accreting and non-accreting components, suggesting that these systems are common and that there is no preference for either the more or less massive component to evolve faster. The derived accretion luminosities and mass accretion rates of the ONC binary components are of similar magnitude as those for both ONC single stars and binaries in the Taurus star-forming region. The paper concludes with a discussion of the (presumably weak) connection between the presence of inner accretion disks in young binary systems and the existence of planets in stellar multiples.(abridged)

Spectral types for eleven Orion stars are presented. Key words: spectral types - Orion nebula

view Abstract Citations (73) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Infrared Stars Wing, Robert F. ; Spinrad, Hyron ; Kuhi, L. V. Abstract As the number of extremely red stars found by means of infrared sky surveys rapidly becomes larger, it becomes increasingly important to determine the spectral characteristics and the nature of the variability of at least a few such stars. We have made a detailed study of the three "infrared stars" identified by Neugebauer, Martz, and Leighton-the Taurus object, the Cygnus object, and TX Cam Our results depend primarily upon the measurements of molecular band strengths and unblanketed energy distributions made with photoelectric spectrum scanners in the wavelength range 0 8-11 j#. For the Taurus object we also have 32 A/mm coude spectrograms centered at 1 08 and a low-dispersion blue spectrogram. UB V photometry has been obtained for the Taurus object and TX Cam. That the Taurus object is a Mira-type variable is indicated by its visual and infrared light-curves, the strength of its hydrogen emission lines of the Balmer and Paschen series, its irregular Balmer decrement, and the shortward displacement of its hydrogen line P with respect to the absorption spectrum. It appears that we have not yet observed a complete cycle, and that the period is longer than 532d TX Cam was previously known to be a Mira variable, likewise with an exceptionally long period (557d) In the case of the Cygnus object, on the other hand, we have found no definite variation in magnitude, color, or molecular band strength. The radial velocity of the Taurus object is +63 l 5 km/sec; its proper motion is less than 0"02/ year. It lies far from the galactic plane (b = - 310) and probably is a disk population star like Mira itself. The great strength of its molecular hydrogen feature at Xl 1172 confirms the low temperature indicated by its energy distribution The scanner measurements have been used to define a color index, a TiO index, a VO index, and a rough index of stellar H2O; they also indicate when the molecules CN and ZrO are present. Definitions of very late spectral types, depending primarily upon the strength of the VO band complex near 1 OS are proposed. The color index is free of blanketing for all spectral types earlier than M8 and has been corrected for blanketing, in an approximate fashion, for the latest types We have observed an adequate number of giant stars and long-period variables of type M to establish that such stars normally fall upon a well-defined and nearly linear sequence (the "M sequence") in the diagram of band strength (TiO+ VO) versus color. The infrared stars in Taurus and Cygnus, like stars of type S, fall well below the M sequence These stars, which have abnormally weak TiO and VO bands for their colors, also have weak bands of 1120. In the Taurus object, the band strengths of all three molecules increase with decreasing temperature. We discuss the various mechanisms that can affect the relation between band strength and color. For the Taurus object our observational data are sufficiently complete to eliminate all but one-a low ratio of oxygen to carbon-as the cause of the observed discrepancy. This star appears to have an oxygen/ carbon ratio similar to that of the 8-type star R And, without, how ever, sharing the latter's enrichment in s-process elements Interstellar reddening may be important in the case of the Cygnus object but here the discrepancy is so great that it seems most reasonable to suppose that it is due primarily to a low oxygen/carbon ratio. It is shown that changes of only a few per cent in this ratio can produce important changes in the molecular band strengths We interpret TX Cam as a moderately reddened star of normal chemical composition. A review of the available information on the spectral types and light variations of infrared stars leads to the suggestion that the majority of such stars are Mira variables with periods longer than 1 year. Publication: The Astrophysical Journal Pub Date: January 1967 DOI: 10.1086/148985 Bibcode: 1967ApJ...147..117W full text sources ADS | data products SIMBAD (34)

view Abstract Citations (46) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Detection of a Dozen X-Ray--Emitting Main-Sequence B6--A3 Stars in Orion Caillault, Jean-Pierre ; Zoonematkermani, Saeid Abstract The detection of 12 X-ray-emitting main-sequence B6-A3 stars in the Orion Nebula is reported. The luminosity of these stars in the 0.2-3.5 keV band is considerably in excess of field stars of the same spectral type. Consideration is given to the possibilities that these stars may not actually be the source of the X-rays, but that the emission may be attributable to T Tauri or 'naked' T Tauri star companions, or that these hot stars are indeed the sources of emission and that current theories of X-ray emission from early-type stars must be amended. Publication: The Astrophysical Journal Pub Date: March 1989 DOI: 10.1086/185400 Bibcode: 1989ApJ...338L..57C Keywords: Early Stars; Main Sequence Stars; Orion Nebula; Stellar Luminosity; X Ray Stars; Companion Stars; Hot Stars; Stellar Spectra; Taurus Constellation; Astrophysics; NEBULAE: ORION NEBULA; STARS: EARLY-TYPE; STARS: X-RAYS full text sources ADS | data products SIMBAD (2)

We highlight differences in spectral types and intrinsic colors observed in pre-main sequence (pre-MS) stars. Spectral types of pre-MS stars are wavelength-dependent, with near-infrared spectra being 3-5 spectral sub-classes later than the spectral types determined from optical spectra. In addition, the intrinsic colors of young stars differ from that of main-sequence stars at a given spectral type. We caution observers to adopt optical spectral types over near-infrared types, since Hertzsprung-Russell (H-R) diagram positions derived from optical spectral types provide consistency between dynamical masses and theoretical evolutionary tracks. We also urge observers to deredden pre-MS stars with tabulations of intrinsic colors specifically constructed for young stars, since their unreddened colors differ from that of main sequence dwarfs. Otherwise, V-band extinctions as much as ~0.6 mag erroneously higher than the true extinction may result, which would introduce systematic errors in the H-R diagram positions and thus bias the inferred ages.

view Abstract Citations (52) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Spectra and Radial Velocities of Stars in the Orion Nebula Cluster. Johnson, Hugh M. Abstract Radial velocity and spectral type are known for forty-five stars within 24' of Ol Orionis, most of them from new data in this paper. The kinematics and spectral peculiarities of the Orion Nebula cluster are discussed in terms of these stars, with reference also to astrometric data. Various parameters are determined, including the distance of the cluster, mean radial velocity and significant departures from it, the dispersion of the velocities, and the distinction between atmospheric and mass components of the velocity. The data are consistent with a uniform expansion of the cluster, but do not establish it. There are notable atmospheric motions in some member stars. Publication: The Astrophysical Journal Pub Date: October 1965 DOI: 10.1086/148365 Bibcode: 1965ApJ...142..964J full text sources ADS | data products SIMBAD (45)

I present an analysis of archival spectra of 200 sources toward the Orion Nebula Cluster (ONC) that were obtained with the Near-Infrared Spectrograph (NIRSpec) onboard the James Webb Space Telescope (JWST). I have used these data to assess cluster membership and measure spectral types for the targets. Fifty-three sources are classified as likely cluster members, 24 of which have spectral types that are suggestive of brown dwarfs (&amp;gt;M6). Seven of the NIRSpec targets were previously identified as ‘Jupiter-mass binary objects’ (JuMBOs), all of which are background sources rather than brown dwarfs based on the NIRSpec data. The spectral classifications of those objects are consistent with the results of my recent study of the JWST photometry in the ONC, which found that only a few JuMBO components have the colours expected for brown dwarfs, none of which form pairs that have uniquely wide separations or low masses relative to known binary brown dwarfs.

view Abstract Citations (105) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stellar Motions in the Orion Nebula Cluster. Strand, K. Aa. Abstract This paper investigates the motions of the stars near the center of the Orion Nebula cluster. The observational material consists of plates taken with the Yerkes 40-inch refractor with an interval of fifty years between first- and second-epoch plates. A comparison of the plate constants for first- and second-epoch plates reveals that the reference frame is expanding, leading to an expansion age of the cluster equal to 3 X l0 years. Assuming the same velocity distribution along any axis within the cluster, the proper motions and the radial velocities lead to a distance of 520 parsecs for the cluster. The colormagnitude diagram of the cluster members shows the existence of a broad continuous sequence from O6 to K2 with stars as early as A0 departed from the standard main sequence, indicating that stars of later spectral types may still be in the process of gravitational contraction. Publication: The Astrophysical Journal Pub Date: July 1958 DOI: 10.1086/146511 Bibcode: 1958ApJ...128...14S full text sources ADS | data products SIMBAD (234) Related Materials (1) Erratum: 1972ApJ...174..721S

Investigation of the progenitors and outbursts of classical and recurrent novae

We report on a sequence of Chandra X-ray Observatory observations of the TW Hydrae brown dwarf (BD) 2MASSW J1139511-315921 (2M1139). In the combined 31 ks ACIS-S exposure, 2M1139 is detected at the 3-sigma confidence level. We find an X-ray luminosity of L_X = 1.4^(+2.7)_(-1.0) x 10^26 ergs s^-1 or log(L_X/L_bol) = -4.8 +/- 0.3. This object is similar to another TW Hydrae BD member, CD-33 7795B (TWA 5B): both have H-alpha emission, both show no signatures of accretion, and both have comparable ages and spectral types. TWA 5B was previously detected in X-rays with a luminosity of L_X = 4 x 10^27 ergs s^-1 or log(L_X/L_bol) = -3.4, an order of magnitude more luminous in X-rays than 2M1139. We find that the discrepancy between the X-ray luminosity of 2M1139 and TWA 5B is consistent with the spread in X-ray luminosity in the Orion Nebula Cluster (ONC) for BDs of similar spectral types. Though rotation may play a role in the X-ray activity of ultracool dwarfs like 2M1139 and TWA 5B, the discrepancy cannot be explained by rotation alone. We also examine two X-ray bright objects in the FOV of our Chandra observations and find one to be of spectral type K0IV and identify it as a possible RS Canum Venaticorum (RS CVn), and another X-ray bright object whose light-curve clearly shows the decay phase of an X-ray flare.