Low-luminosity Objects Research Articles

New Distant Companions to Known Nearby Stars. II. Faint Companions ofHipparcosStars and the Frequency of Wide Binary Systems

We perform a search for faint, common proper motion companions of Hipparcos stars using the recently published LSPM-north catalog of stars with proper motion mu>0.15 arcsec/yr. Our survey uncovers a total of 521 systems with angular separations 3 arcsec 15.0. Our census is statistically complete for secondaries with angular separations 20 arcsec 1,000 AU. We observe that the distribution in orbital separations is consistent with Opik's law f(s) ds ~ s^{-1} ds only up to separation s \approx 3,000 AU, beyond which it follows a more steeply decreasing power law f(s) ds ~ s^{-l} ds with l=1.6+/-0.1. We also find that the luminosity function of the secondaries is significantly different from that of the single stars field population, showing a relative deficiency in low-luminosity (8<M_V<14) objects. The observed trends suggest either a formation mechanism biased against low-mass companions, or a disruption over time of systems with low gravitational binding energy.

Spectral types of planetary host star candidates from OGLE III

Context. The Optical Gravitational Lensing Experiment project has recently provided the OGLE III list of low-luminosity object transits from campaigns #3 and #4, reporting 40 new objects exhibiting the low-amplitude photometric eclipses expected for exoplanets. Compared to previous OGLE targets, these OGLE III candidates have been more restrictively selected and may contain low-mass planets. Aims. We have secured follow-up low-resolution spectroscopy for 28 candidates out of this list (and one from the OGLE Carina fields) to obtain an independent characterization of the pri mary stars by spectral classification and thus better constr ain the parameters of their companions. Methods. We fed the constraints from these results back into an improved light curve solution. Together with the radius ratios fro m the transit measurements, we derived the radii of the low-luminosity companions. This allows us to examine the possible sub-stellar nature of these objects. Results. Sixteen of the companions can be clearly identified as low-ma ss stars orbiting a main sequence primary, while 10 more objects are likely to have red giant primaries and therefore also host a stellar companion; 3 possibly have a sub-stellar nature (R� 0.15 R⊙). Conclusions. The planetary nature of these objects should therefore be confirmed by dynamical mass determinations.

TheSpitzerc2d Survey of Nearby Dense Cores. I. First Direct Detection of the Embedded Source in IRAM 04191+1522

We report the first detections of the Class 0 protostellar source IRAM 04191+1522 at wavelengths shortward of 60 microns with the Spitzer Space Telescope. We see extended emission in the Spitzer images that suggests the presence of an outflow cavity in the circumstellar envelope. We combine the Spitzer observations with existing data to form a complete dataset ranging from 3.6 to 1300 microns and use these data to construct radiative transfer models of the source. We conclude that the internal luminosity of IRAM 04191+1522, defined to be the sum of the luminosity from the internal sources (a star and a disk), is L_int = 0.08 +/- 0.04 L_sun, placing it among the lowest luminosity protostars known. Though it was discovered before the launch of the Spitzer Space Telescope, IRAM 04191+1522 falls within a new class of Very Low Luminosity Objects being discovered by Spitzer. Unlike the two other well-studied objects in this class, which are associated either with weak, compact outflows or no outflows at all, IRAM 04191+1522 has a well-defined molecular outflow with properties consistent with those expected based on relations derived from higher luminosity (L_int > 1 L_sun) protostars. We discuss the difficulties in understanding IRAM 04191+1522 in the context of the standard model of star formation, and suggest a possible explanation for the very low luminosity of this source.

The Spitzer c2d Survey of Nearby Dense Cores. II. Discovery of a Low-Luminosity Object in the "Evolved Starless Core" L1521F

We present Spitzer Space Telescope observations of the "evolved starless core" L1521F which reveal the presence of a very low luminosity object (L < 0.07 Lsun). The object, L1521F-IRS, is directly detected at mid-infrared wavelengths (>5 micron) but only in scattered light at shorter infrared wavelengths, showing a bipolar nebula oriented east-west which is probably tracing an outflow cavity. The nebula strongly suggests that L1521F-IRS is embedded in the L1521F core. Thus L1521F-IRS is similar to the recently discovered L1014-IRS and the previously known IRAM 04191 in its substellar luminosity and dense core environment. However these objects differ significantly in their core density, core chemistry, and outflow properties, and some may be destined to be brown dwarfs rather than stars.

Star formation in the vicinity of the IC 348 cluster

Aims. We present molecular line observations of the southwestern part of the IC 348 young cluster, and we use them together with NIR and mm continuum data to determine the distribution of dense gas, search for molecular outflows, and analyze the ongoing star formation activity in the region. Methods. Our molecular line data consists of C 18 O(1–0) and N 2 H + (1–0) maps obtained with the FCRAO telescope at a resolution of about $50''$ and CO(2–1) data obtained with the IRAM 30 m telescope at a resolution of $11''$. Results. The dense gas southwest of IC 348 is concentrated in two groups of dense cores, each of them with a few solar masses of material and indications of CO depletion at high density. One of the core groups is actively forming stars, while the other seems starless. There is evidence for at least three bipolar molecular outflows in the region, two of them powered by previously identified Class 0 sources while the other is powered by a still not well characterized low-luminosity object. The ongoing star formation activity is producing a small stellar subgroup in the cluster. Using the observed core characteristics and the star formation rate in the cluster, we propose that similar episodes of stellar birth may have produced the subclustering seen in the halo of IC 348.

Evolution of First Cores in Rotating Molecular Cores

We investigate the effect of rotation on the star formation process quantitatively using axisymmetric numerical calculations. An adiabatic hydrostatic object (the so-called first core) forms in a contracting cloud core, after the central region becomes optically thick and continues to contract, driven by mass accretion onto it. The structure of a rotating first core is characterized by its total angular momentum Jcore and mass Mcore, both of which increase by accretion with time. We find that the first core evolves with a constant Jcore/M. Evolutionary paths of first cores can be classified into two types. In a slowly rotating core with Jcore/M 0.015G/(ciso) stops its contraction before the central density reaches the H2 dissociation density and does not begin the second collapse. These rapidly rotating first cores suffer from nonaxisymmetric instabilities, such as formation of massive spiral arms, deformation into a bar, or fragmentation. Although the rotating first cores have small average luminosities of Lcore = 0.003-0.03 L☉, assuming a constant mass accretion rate core. Their lifetimes last several thousand years or more, which is much longer than those expected for nonrotating clouds (~1000 yr). We expect that at least several percent of prestellar cores contain first cores as very low luminosity objects. Furthermore, we find a core with 0.012G/(ciso) < Jcore/M < 0.015G/(ciso) may form close binary systems with initial separation of 0.02-0.1 AU after the second collapse phase.

Planets in Stellar Clusters Extensive Search. IV. A Detection of a Possible Transiting Planet Candidate in the Open Cluster NGC 2158

We have undertaken a long-term project, Planets in Stellar Clusters Extensive Search (PISCES), to search for transiting planets in open clusters. In this paper we present the results for NGC 2158, an intermediate age, populous cluster. We have monitored the cluster for over 260 hours, spread over 59 nights. We have detected one candidate transiting low luminosity object, with eclipse depth of 3.7% in the R-band. If the host star is a member of the cluster, the eclipse depth is consistent with a 1.7 R_J object. Cluster membership of the host is supported by its location on the cluster main sequence (MS) and its close proximity to the cluster center (2'). We have discovered two other stars exhibiting low-amplitude (4-5%) transits, V64 and V70, but they are most likely blends or field stars. Given the photometric precision and temporal coverage of our observations, and current best estimates for the frequency and radii of short-period planets, the expected number of detectable transiting planets in our sample is 0.13. We have observed four outbursts for the candidate cataclysmic variable V57. We have discovered 40 new variable stars in the cluster, bringing the total number of identified variables to 97, and present for them high precision light curves, spanning 13 months.

Spitzer discovery of very low luminosity objects

AbstractThe Spitzer Space Telescope allows for the .rst time to search systematically for very low luminosity (≲0.1 L⊙) objects (VeLLOs) associated with dense molecular cores. They may be the .rst candidate Class 0 sources with sub‐stellar masses. We describe such a source in the dense molecular core L1148. VeLLO natal cores show properties that are unusual for star‐forming cores. The low luminosity and in some cases the lack of prominent out.ow could be the result of the small gas supply near the VeLLO. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

AChandraX‐Ray Survey of Ultraluminous Infrared Galaxies

We present results from Chandra observations of 14 ultraluminous infrared galaxies [ULIRGs; log(L(IR)/L.) >= 12] with redshifts between 0.04 and 0.16. The goals of the observations were to investigate any correlation between infrared color or luminosity and the properties of the X-ray emission and to attempt to determine whether these objects are powered by starbursts or active galactic nuclei (AGNs). The sample contains approximately the same number of high- and low-luminosity objects and warm'' and cool'' ULIRGs. All 14 galaxies were detected by Chandra. Our analysis shows that the X-ray emission of the two Seyfert 1 galaxies in our sample is dominated by an AGN. The remaining 12 sources are too faint for conventional spectral fitting to be applicable. Hardness ratios were used to estimate the spectral properties of these faint sources. The photon indices, Gamma-values, for our sample plus the Chandra-observed 2003 sample from Ptak et al. peak in the range 1.0-1.5, consistent with expectations for X-ray binaries in a starburst, an absorbed AGN, or hot bremsstrahlung from a starburst or AGN. The values of Gamma for the objects in our sample classified as Seyferts (type 1 or 2) are larger than 2, while those classified as H II regions or LINERs tend to be less than 2. The hard X-ray to far-infrared ratios for the 12 weak sources are similar to those of starbursts, but we cannot rule out the possibility of absorbed, possibly Compton-thick, AGNs in some of these objects. Two of these faint sources were found to have X-ray counterparts to their double optical and infrared nuclei.

Bimodal Color Distribution in Hierarchical Galaxy Formation

We show how the observed bimodality in the color distribution of galaxies can be explained in the framework of the hierarchical clustering picture in terms of the interplay between the properties of the merging histories and the feedback/star formation processes in the progenitors of local galaxies. Using a semianalytic model of hierarchical galaxy formation, we compute the color distributions of galaxies with different luminosities and compare them with the observations. Our fiducial model matches the fundamental properties of the observed distributions, namely: (1) the distribution of objects brighter than Mr -18 is clearly bimodal, with a fraction of red objects increasing with luminosity; (2) for objects brighter than Mr -21, the color distribution is dominated by red objects with color u - r ≈ 2.2-2.4; (3) the spread on the distribution of the red population is smaller than that of the blue population; (4) the fraction of red galaxies is larger in denser environments, even for low-luminosity objects; and (5) the bimodality in the distribution persists up to z ≈ 1.5. We discuss the role of the different physical processes included in the model in producing the above results.

Preliminary Photometric Results for the 2003 Eclipse of Ee Cep

We report multicolour photometric observations of the 2003 eclipse of the long-period (5.6 yr) eclipsing binary EE Cep. Measurements were obtained with ten telescopes at eight observatories in four countries. In most cases, UBV(RI)C broad band filters have been used. The light curve shape shows that the obscuring body is an almost dark disk around a low-luminosity central object. However, variations of the colour indices during the eclipse indicate that the obscuring body emits a considerable amount of radiation in the near infrared.

Probing the Dust Obscuration in Seyfert Galaxies using Infrared Spectroscopy. II. Implication for the Unification of Seyfert Galaxies

We report near-IR spectroscopic observations of 11 Seyfert galaxies (six Seyfert 1s, one Seyfert 1.9, and four Seyfert 2s) and additional galaxies (four LINERs, two H II, and one type 2 transition) for comparison, obtained using the Gemini twin-channel near-IR camera on the Shane 3 m telescope at Lick Observatory. With the unique design of the Gemini camera, full J and K spectra were taken simultaneously through the same slit. This produced accurate line ratios of hydrogen recombination lines over a large wavelength baseline. For the Seyfert 1s (≤1.5), the line ratios of Paβ/Brγ are not only comparable in both broad- and narrow-line regions but also consistent with case B recombination, indicating little or no reddening in both narrow- and broad-line regions. Seyfert 2 (>1.5) galaxies, however, show substantial reddening toward the narrow-line regions. We compare optical reddening data from the literature and find significant support for the dichotomy between Seyfert 1s and Seyfert 2s, at least in low-luminosity objects. Two different scenarios are explored to explain the observed difference in reddening: a difference in reddening due to an extended dusty torus under active galactic nucleus unification, and a difference due to a different grain size distribution between the two Seyfert types. We also discuss a similar potential difference found in the strength of the 9.7 μm silicate line, along with a possible correlation between the narrow-line reddening and the strength of the silicate absorption line. We also analyzed CO band head absorption features longward of 2.3 μm to look for nonstellar contamination and evidence of recent star formation activity. The CO band head in Seyfert 1s shows heavy contamination from nonstellar radiation, which is correlated with an H - K nuclear color excess. We confirm that the CO spectroscopic indices in both Seyfert types do not show evidence of recent star formation. Taking the nonstellar contamination into account, there is little evidence from the CO index for a difference in star formation rates in the nuclei of Seyfert 1s and Seyfert 2s in our low-luminosity sample.

Black Hole Mass and Accretion Rate of Active Galactic Nuclei with Double‐peaked Broad Emission Lines

Using an empirical relation between the broad-line region size and optical continuum luminosity, we estimated the black hole mass and accretion rate for 135 active galactic nuclei (AGNs) with double-peaked broad emission lines in two samples, one from the Sloan Digital Sky Survey (SDSS) and the other from a survey of radio-loud broad emission line AGNs. With black hole masses ranging from 3 × 107 M☉ to 5 × 109 M☉, these AGNs have dimensionless accretion rates (Eddington ratios) between 0.001 and 0.1 and bolometric luminosity between 1043 and 1046 ergs s-1, both values being significantly larger than those of several previously known low-luminosity (Lbol < 1043 ergs s-1) double-peaked AGNs. The optical-X-ray spectra indices, αOX, of these high-luminosity double-peaked AGNs are between 1 and 1.9, systematically larger than those of low-luminosity objects, which are around 1. Modest correlations (with a Spearman rank correlation coefficient of 0.60) of the αOX value with the Eddington ratio and bolometric luminosity have been found, indicating that double-peaked AGNs with higher Eddington ratios or higher luminosity tend to have larger αOX values. Based on these results, we suggested that the accretion process in the central region of some high-luminosity double-peaked emission line AGNs (especially those with Eddington ratios larger than 0.01) is probably different from that of low-luminosity objects, in which a well-known ADAF-like accretion flow was thought to exist. It is likely that the accretion physics in some high-luminosity double-peaked AGNs is similar to that in normal type 1 AGNs, which is also supported by the presence of possible big blue bumps in the spectra of some double-peaked AGNs with higher Eddington ratios. We note that the prototype double-peaked emission line AGN, Arp 102B, which has a black hole mass of 108 M☉ and a dimensionless accretion rate of 0.001, may be an intermediate object between the high- and low-luminosity double-peaked AGNs. In addition, we found an apparent strong anticorrelation (with a Spearman rank correlation coefficient of -0.79) between the peak separation of double-peaked profiles and Eddington ratios. However, such an anticorrelation is probably induced by a strong correlation between the peak separation and emission-line widths and needs to be confirmed by future work. If it is real, it may provide us another clue to understanding why double-peaked broad emission lines were hardly found in luminous AGNs with Eddington ratios larger than 0.1.

Luminosity limits on white dwarfs in a Galactic shroud

We place observational constraints on a recently proposed Galactic population, dubbed the shroud (Gyuk & Gates 1999, Gates & Gyuk 2001). The shroud would be a very thick Galactic disk of low luminosity objects, most likely old white dwarfs, proposed to explain the optical depth seen in microlensing surveys towards the Magellanic clouds. The shroud is a simple alternative to the lenses being distributed in a classical, near-spherical dark halo; the advantage of the shroud is that it would compose only a fraction of a dark halo’s total mass. In this paper, we argue that stars of the Galactic shroud would be detectable in the recent proper motion survey of Oppenheimer et al. (2001) if their absolute luminosities were brighter than MR59F = 19.4 or approximately MV = 18.6. We adopt a range of simple models of the shroud’s kinematics and morphology, and the colours and luminosities of its white dwarfs; via Monte-Carlo simulations, we predict the numbers expected in the Oppenheimer et al. survey, which would be clearly separated from the numbers produced by white dwarfs of the disk, thick disk and halo. The number of white dwarf detections in the proper motion survey (98) is found to be well explained by the disk, thick disk and halo. With the most conservative kinematic and density parameters for the shroud, and an absolute luminosity of the white dwarfs of MR59F = 17.6, we find that the proper motion survey would detect over 100 WDs, just from the shroud. For a MR59F = 19.4 shroud, the survey would find 5 ± 2 peculiar objects, whereas only two white dwarfs with such characteristics are found in the original data. MR59F = 19.4 corresponds to MV = 18.6 for WDs with (V I) = 1.030.

Radio Continuum Emission at 1.4 GHz from KISS Emission-Line Galaxies

We have searched the Faint Images of the Radio Sky at Twenty centimeters (FIRST) and the NRAO VLA Sky Survey (NVSS) 1.4 GHz radio surveys for sources that are coincident with emission-line galaxy (ELG) candidates from the KPNO International Spectroscopic Survey (KISS). A total of 207 of the 2157 KISS ELGs (∼10%) in the first two Hα-selected survey lists were found to possess radio detections in FIRST, NVSS, or both. Follow-up spectra exist for all of the radio detections, allowing us to determine the activity type (star-forming vs. active nucleus) for the entire sample. We explore the properties of the radio-detected KISS galaxies in order to gain a better insight into the nature of radio-emitting galaxies in the local universe (z < 0.1). No dwarf galaxies were detected, despite the large numbers of low-luminosity galaxies present in KISS, suggesting that lower mass, lower luminosity objects do not possess strong galaxian-scale magnetic fields. Because of the selection technique used for KISS, our radio ELGs represent a quasi–volume-limited sample, which allows us to develop a clearer picture of the radio galaxy population at low redshift. Nearly two-thirds of the KISS radio galaxies are starburst or star-forming galaxies, which is in stark contrast to the results of flux-limited radio surveys, which are dominated by active galactic nuclei (AGNs) and elliptical galaxies (i.e., classical radio galaxies). While there are many AGNs among the KISS radio galaxies, there are no objects with large radio powers in our local volume. We derive a radio luminosity function (RLF) for the KISS ELGs that agrees very well with previous RLFs that adequately sample the lower luminosity radio population.

Photometric Transits from the MACHO Project Database

We have searched the MACHO project photometry database for light curves consistent with the transits of low-luminosity objects. By analyzing the 180,000 brightest stars in 14 0.5 deg2 fields toward the Galactic bulge, we have found a few hundred low-amplitude eclipsing binary systems. We have used the shapes and colors of these objects to separate the transits of objects with relatively small radii from the grazing eclipses of stars with larger radii. We find nine objects with short-period eclipses consistent with either giant extrasolar planets or late-type dwarf stars.

Observational Constraints on Radiatively Inefficient Accretion Flows in Nearby AGNs

Results of recent X-ray observations of low-luminosity AGNs are summarized. The lack of the signature of X-ray reflection features and big blue bump is in good agreement with the prediction that an optically thick accretion disk is truncated. Comparison of radio and X-ray luminosities indicates that low-luminosity objects tend to be radio loud compared to more luminous AGNs, suggesting efficient jet production in low-luminosity accretion flows.

OGLE-TR-3: A possible new transiting planet

Recently, 59 low-luminosity object transits were reported from the Optical Gravitational Lensing Experiment (OGLE). Our follow-up low-resolution spectroscopy of 16 candidates provided two objects, OGLE-TR-3 and OGLE-TR-10, which have companions with radii compatible with those of gas-giant planets. Further high-resolution spectroscopy revealed a very low velocity variation (< 500 m s-1) of the host star OGLE-TR-3 which may be caused by its unseen companion. An analysis of the radial velocity and light curve results in , , and an orbital separation of about 5 , which makes it the planet with the shortest period known. This allows to identify the low-luminosity companion of OGLE-TR-3 as a possible new gas-giant planet. If confirmed, this makes OGLE-TR-3 together with OGLE-TR-56 the first extrasolar planets detected via their transit light curves.

Spectral types of planetary host star candidates: Two new transiting planets?

Recently, 46 low-luminosity object transits were reported from the Optical Gravitational Lensing Experiment. Our follow-up spectroscopy of the 16 most promising candidates provides a spectral classification of the primary. Together with the radius ratio from the transit measurements, we derived the radii of the low-luminosity companions. This allows to examine the possible sub-stellar nature of these objects. Fourteen of them can be clearly identified as low-mass stars. Two objects, OGLE- TR-03 and OGLE-TR-10 have companions with radii of 0.15R which is very similar to the radius of the transiting planet HD 209458 B. The planetary nature of these two objects should therefore be confirmed by dynamical mass determinations.

The Evolution of the Luminosity Function in Deep Fields: A Comparison with Cold Dark Matter Models

The galaxy luminosity function (LF) has been estimated in the rest-frame B luminosity at 0 < z < 1.25 and at 1700 Å for 2.5 < z < 4.5 from deep multicolor surveys in the Hubble Deep Field-North, the Hubble Deep Field-South, and the New Technology Telescope Deep Field. The results have been compared with a recent version of galaxy formation models in the framework of hierarchical clustering in a flat cold dark matter universe with cosmological constant. The results show a general agreement for z ≲ 1, although the model LF has a steeper average slope at the faint end; at z ~ 3 such a feature results in an overprediction of the number of faint (IAB ~ 27) galaxies, while the agreement at the bright end becomes critically sensitive to the details of dust absorption at such redshifts. The discrepancies at the faint end show that a refined treatment of the physical processes involving smaller galaxies is to be pursued in the models, in terms of aggregation processes and/or stellar feedback heavily affecting the luminosity of the low-luminosity objects. The implications of our results on the evolution of the cosmological star formation rate are discussed.