A method to determine distances to molecular clouds using near-IR photometry

The aim of the paper is to estimate distances to dense molecular cores that harbour Very Low Luminosity Objects (VeLLO) detected by Spitzer Space Telescope and to confirm their VeLLO nature. The cloud distances are estimated using near-IR photometric method. We use a technique that provides spectral classification of stars lying towards the fields containing the clouds into main sequence and giants. In this technique, the observed (J-H) and (H-Ks) colours are dereddened simultaneously using trial values of A_V and a normal interstellar extinction law. The best fit of the dereddened colours to the intrinsic colours giving a minimum value of chi^{2} then yields the corresponding spectral type and A_V for the star. The main sequence stars, thus classified, are then utilized in an A_V versus distance plot to bracket the cloud distances. The typical error in the estimation of distances to the clouds are found to be ~18%.We estimated distances to seven cloud cores, IRAM04191, L1521F, BHR111, L328, L673-7, L1014, and L1148 using the above method. These clouds contain VeLLO candidates. The estimated distances to the cores are found to be 127\pm25 pc (IRAM04191), 136\pm36 pc (L1521F), 355\pm65 pc (BHR111), 217\pm30 pc (L328), 240\pm45 pc (L673-7), 258\pm50 pc (L1014), and 301\pm55 pc (L1148). We re-evaluated the internal luminosities of the VeLLOs discovered in these seven clouds using the distances estimated from this work. Except for L1014-IRS (L_{int}=0.15 L_{\odot}), all other VeLLO candidates are found to be consistent with the definition of a VeLLO (L_{int}<=0.1L_{\odot}). In addition to the cores that harbour VeLLO candidates, we also obtained distances to the clouds L323, L675, L676, CB 188, L1122, L1152, L1155, L1157 and L1158 which are located in the directions of the above seven cores. Towards L1521F and L1148 we found evidence of the presence of multiple dust layers.

view Abstract Citations (67) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Optical and infrared properties of the newly formed stars in Canis Major R1. Herbst, W. ; Racine, R. ; Warner, J. W. Abstract UBVRIJHKL photometry and MK spectral types have been obtained for stars illuminating nebulae in the CMa RI association. Five stars, including the two classical Herbig emission stars Z CMa and HD 53367, have K - L excesses and/or emission-line spectra indicating the presence of circumstellar matter. A number of stars also have V - K indices larger than expected on the basis of their spectral types and B - V indices, suggesting that a steeper than normal extinction law applies to these stars. The color-magnitude diagram for the association is unusual; many stars over the entire spectral range of the association (B0 to Al) lie more than 1 mag above the zero-age main sequence (ZAMS) in V. The middle and early-type B stars with this property are all variable stars and most have rotationally broadened spectra and/or shell indicators. The stars with K - L excesses all lie well above the ZAMS. An age of 3 x 10 years is suggested for most of the stars, compatible with the suggestion that a supernova explosion triggered star formation in this region. A comparison is made between the observations and theoretical models of pre-main-sequence stars. Some of the latest-type stars (A0-Al) lie close to the ZAMS, suggesting that either (1) star formation in the association was not coeval, or (2) isochrones derived from Iben's pre-main-sequence models do not represent real stellar groups. A bolometric luminosity of Mbol < - 4.3 is derived for Z CMa, implying a core mass of M> 10 M0 and a spectral type of B 1, if the star is on the ZAM S. Subject headings: clusters: associations - stars: early-type - stars: emission-line Publication: The Astrophysical Journal Pub Date: July 1978 DOI: 10.1086/156282 Bibcode: 1978ApJ...223..471H Keywords: Early Stars; Infrared Astronomy; Optical Properties; Stellar Spectrophotometry; A Stars; Astronomical Catalogs; B Stars; Emission Spectra; Main Sequence Stars; Nebulae; Astronomy; Early-Type Stars:Photometry; Emission-Line Stars:Infrared Excesses; Emission-Line Stars:Stellar Associations; Pre-Main-Sequence Stars:Stellar Groups; Stellar Groups: Ages full text sources ADS | data products SIMBAD (37) GCPD (1)

view Abstract Citations (225) References (62) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Activity, Variability, and Rotation of Lower Main-Sequence Hyades Stars Radick, Richard R. ; Thompson, D. T. ; Lockwood, G. W. ; Duncan, D. K. ; Baggett, W. E. Abstract High-precision differential b, y photometric observations of 24 Hyades stars, spectral types F4 V to K8 V, were made at Lowell Observatory during late 1983 and early 1984. It was found that all 18 stars of spectral type F8 V or later in this sample were variable, confirming that low-level photometric variability is a ubiquitous and persistent characteristic of Hyades stars later than spectral type F8 V. On the other hand, the six stars earlier than spectral type F7 V were not detectably variable. Contemporaneous Ca II H + K emission flux measurements for five of the 24 stars, spectral types F8 V to G2 V, were obtained at Mount Wilson Observatory and showed that H + K emission flux varies inversely with respect to photometric brightness on both long-term and rotational time scales. Rotation periods were measured for a total of 23 Hyades stars ranging in spectral type from F8 V to K8 V. It was found that Hyades stars conform reasonably well to the 'Rossby relation', which relates chromospheric activity to the ratio of rotation period and convective turnover time scale. Publication: The Astrophysical Journal Pub Date: October 1987 DOI: 10.1086/165645 Bibcode: 1987ApJ...321..459R Keywords: Late Stars; Main Sequence Stars; Stellar Activity; Stellar Rotation; Variable Stars; Calcium; Emission Spectra; Potassium; Stellar Spectra; Stellar Spectrophotometry; Astrophysics; CA II EMISSION; CLUSTERS: OPEN; PHOTOMETRY; STARS: ROTATION; STARS: VARIABLES full text sources ADS | data products SIMBAD (31)

Four young star clusters were studied in order to characterize their anomalous extinction or variable reddening that could be due to a possible contamination by dense clouds or circumstellar effects. The extinction law (Rv) was evaluated by adopting two methods: (i) the use of theoretical expressions based on the colour-excess of stars with known spectral type, and (ii) the analysis of two-colour diagrams, where the slope of observed colours distribution is compared to the normal distribution. An algorithm to reproduce the zero age main sequence (ZAMS) reddened colours was developed in order to derive the average visual extinction (Av) that provides the best fitting of the observational data. The structure of the clouds was evaluated by means of statistical fractal analysis, aiming to compare their geometric structure with the spatial distribution of the cluster members. The cluster NGC 6530 is the only object of our sample showing anomalous extinction. In average, the other clusters are suffering normal extinction, but several of their members, mainly in NGC 2264, seem to have high Rv, probably due to circumstellar effects. The ZAMS fitting provides Av values that are in good agreement with those found in the literature. The fractal analysis shows that NGC 6530 has a centrally concentrated distribution of stars that is different of the sub-structures found in the density distribution of the cloud projected in the Av map, suggesting that the original cloud has been changed with the cluster formation. On the other hand, the fractal dimension and the statistical parameters of Berkeley 86, NGC 2244, and NGC 2264 indicate a good cloud-cluster correlation, when compared to other works based on artificial distribution of points.

Aims.The main objective of this project is to characterise chromospheric activity of FGK stars from the HARPS archive. We start, in this first paper, by presenting a catalogue of homogeneously determined chromospheric emission (CE), stellar atmospheric parameters, and ages for 1674 FGK main sequence (MS), subgiant, and giant stars. The analysis of CE level and variability is also performed.Methods.We measured CE in the Ca IIH&amp;K lines using more than 180 000 high-resolution spectra from the HARPS spectrograph, as compiled in the AMBRE project, obtained between 2003 and 2019. We converted the fluxes to bolometric and photospheric corrected chromospheric emission ratio,RHK′. Stellar atmospheric parametersTeff, logg, and [Fe/H] were retrieved from the literature or determined using a homogeneous method.M⋆,R⋆, and ages were determined from isochrone fitting.Results.We show that our sample has a distribution of CE for MS stars that is consistent with an unbiased sample of solar-neighbour MS stars. We analysed the CE distribution for the different luminosity classes and spectral types and confirmed the existence of the very inactive (VI) star and very active star populations at logRHK′ &lt; −5.1 and &gt; − 4.2 dex, respectively. We found indications that the VI population is composed mainly of subgiant and giant stars and that logRHK′ = −5.1 dex marks a transition in stellar evolution. Overall, CE variability decreases with decreasing CE level but its distribution is complex. There appears to be at least three regimes of variability for inactive, active, and very active stars, with the inactive and active regimes separated by a diagonal, extended Vaughan-Preston (VP) gap. We show that stars with low activity levels do not necessarily have low variability. In the case of K dwarfs, which show high CE variability, inactive and active stars have similar levels of activity variability. This means that activity levels alone are not enough to infer the activity variability of a star. We also explain the shape of the VP gap observed in the distribution of CE using the CE variability-level diagram. In the CE variability-level diagram, the Sun is located in the high-variability region of the inactive MS stars zone. A method to extract the probability density function of the CE variability for a given logRHK′level is discussed, and a python code to retrieve it is provided.

UBVRIJHKL photometry and MK spectral types have been obtained for stars illuminating nebulae in the CMa R1 association. Five stars, including the two classical Herbig-emission-stars Z CMa and HD 53367, have (K-L) excesses and/or emission line spectra indicating the presence of circumstellar matter. A number of stars also have (V-K) indices larger than expected on the basis of their spectral types and (B-V) indices, suggesting that a steeper than normal extinction law applies to these stars. The color-magnitude diagram for the association is unusual; many stars over the entire spectral range of the association (B0 to A1) lie more than 1 magnitude above the ZAMS in V. The middle and early type B stars with this property are all variable stars and most have rotationally broadened spectra and/or shell indicators. No stars later than B5 are on the main sequence, making CMa R1 probably the youngest stellar group yet identified. It suggests an age of ~3 × 105 years, compatible with the suggestion that a supernova explosion triggered star formation in this region. A comparison is made between the observations and theoretical models of pre-main sequence stars. A bolometric luminosity of Mbol &lt; −4m.3 is derived for Z CMa implying a core mass of M &gt; 10 M⊙, and a spectral type of ~B1, if the star is on the ZAMS.

The first eight elements of the periodic table are discussed: H, He, Li, Be, B, C, N, and O. They are referred to as key elements, given their important role in stellar evolution. It is noteworthy that all of them were initially synthesized in the Big Bang. The primordial abundances of these elements calculated using the Standard Model of the Big Bang (SMBB) are presented in this review. The good agreement between the SMBB and observations of the primordial abundances of the isotopes of hydrogen and helium, D, 3He, and 4He, is noted, but there is a difference of ~0.5 dex for lithium (the isotope 7Li) between the SMBB and observations of old stars in the galactic halo that has not yet been explained. The abundances of light elements in stellar atmospheres depends on the initial rotation velocity, so the typical rotation velocities of young Main Sequence (MS) stars are examined. Since the data on the abundances of light elements in stars are very extensive, the main emphasis in this review is on several unsolved problems. The helium abundance He/H in early B-type of the MS stars shows an increment with age; in particular, for the most massive B stars with masses M = 12−19M⦿, He/H increases by more than a factor of two toward the end of the MS. Theoretical models of stars with rotation cannot explain such a large increase in He/H. For early B- and late O-type MS stars that are components of close binary systems, He/H undergoes a sharp jump in the middle of the MS stage that is a mystery for the theory. The anomalous abundance of helium (and lithium) in the atmospheres of chemically peculiar stars (types He-s, He-w, HgMn, Ap, and Am) is explained in terms of the diffusion of atoms in surface layers of the stars, but this hypothesis cannot yet explain all the features of the chemical composition of these stars. The abundances of lithium, beryllium, and boron in FGK-dwarfs manifest a trend with decreasing effective temperature T eff as well as a dip at T eff ~ 6600 K in the Hyades and other old clusters. The two effects are among the unsolved problems. In the case of lithium, there is special interest in FGK-giants and supergiants that are rich in lithium (they have logε(Li)≥ 2). Most of them cannot be explained in terms of the standard theory of stellar evolution, so nonstandard hypotheses are invoked: the recent synthesis of lithium in a star and the engulfment by a star of a giant planet with mass equal to that of Jupiter or greater. An analysis of the abundances of carbon, nitrogen, and oxygen in early B- and late O-stars of the MS indicates that the C II, N II, and O II ions are overionized in their atmospheres. For early B-type MS stars, good agreement is found between observations of the N/O ratio and model calculations for rotating stars. A quantitative explanation of the well-known “nitrogen-oxygen” anticorrelation in FGK-giants and supergiants is found. It reflects the dependence of the anomalies in N and C on the initial rotation velocity V0. The stellar rotation models which yield successful explanations for C, N. and O cannot, however, explain the observed helium enrichment in early B-type MS stars.

In this study, the dynamic effects of radiation pressure —varying with spectral type —on main -sequence stars and their potential applications for interstellar space missions are investigated. High -resolution spectral data from the UVES Paranal Observatory Project (UVES -POP), obtained using ESO’s Very Large Telescope (VLT), were used to determine the effective temperatures, surface gravities, and luminosities of the target stars. Astrometric parameters from Gaia DR3 were utilized to derive distances and confirm the stars’ positions on the main sequence. Mass -loss rates were calculated using the method of de Jager, Nieuwenhuijzen, and van der Hucht (1988), and stellar wind parameters were derived based on these rates. The resulting radiation pressure (Prad) values were modeled as the radiative force acting on an idealized low -mass light sail positioned near the stars. Consequently, the acceleration and orbital deviations of the light sail were evaluated in a comparative manner based on spectral type. The results suggest that main -sequence stars of different spectral types could serve as potential “propulsion sources” or “navigation waypoints” in future interstellar missions. In this context, the study is considered to contribute to the optimization of trajectories for light sail -based interstellar missions.

We use the data of the Chandra Orion Ultradeep Project (COUP) to study the nearly 600 X-ray sources that can be reliably identified with optically well characterized T Tauri stars (TTS) in the Orion Nebula Cluster. We detect X-ray emission from more than 97% of the optically visible late-type (spectral types F to M) cluster stars. This proofs that there is no ``X-ray quiet'' population of late-type stars with suppressed magnetic activity. All TTS with known rotation periods lie in the saturated or super-saturated regime of the relation between activity and Rossby numbers seen for main-sequence (MS) stars, but the TTS show a much larger scatter in X-ray activity than seen for the MS stars. Strong near-linear relations between X-ray luminosities, bolometric luminosities and mass are present. We also find that the fractional X-ray luminosity rises slowly with mass over the 0.1 - 2 M_sun range. The plasma temperatures determined from the X-ray spectra of the TTS are much hotter than in MS stars, but seem to follow a general solar-stellar correlation between plasma temperature and activity level. The large scatter about the relations between X-ray activity and stellar parameters seems to be related to the influence of accretion on the X-ray emission. While the X-ray activity of the non-accreting TTS is consistent with that of rapidly rotating MS stars, the accreting stars are less X-ray active (by a factor of ~2-3 on average) and produce much less well defined correlations than the non-accretors. We discuss possible reasons for the suppression of X-ray emission by accretion and the implications of our findings on long-standing questions related to the origin of the X-ray emission from young stars.

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)

view Abstract Citations (35) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Properties of minimum-flux coronae in dwarfs and giants. Mullan, D. J. Abstract Hearn's (1975) minimum-flux corona model is applied to main-sequence and giant stars. In this model, the corona is assumed to have a strictly radial magnetic field and to adjust itself in such a way that for a given pressure at its base, the sum of radiative, conductive, and stellar-wind energy fluxes from it is a minimum. Numerical results for main-sequence stars of spectral type M6 V through O5 V are presented in terms of coronal temperatures and efficiency factors for converting thermal energy to mechanical energy. Similar results are given for the giant stars Alpha Aur, Beta Gem, Alpha Boo, and Alpha Tau. It is shown that: (1) coronae of red dwarfs must be up to three times cooler than the solar corona unless the efficiency of conversion increases by many orders of magnitude in later spectral types relative to the efficiency of the sun; (2) coronae of upper-main-sequence stars may be twice as hot as the solar corona; (3) the coronae of Beta Gem and Alpha Aur are hot enough to explain the presence of O V, O VI, Si III, and N V lines; (4) the coronae of Alpha Boo and Alpha Tau are so cool that insignificant O V emission is expected; and (5) line asymmetries due to stellar winds are most easily detectable in Alpha Boo and Alpha Aur because of the larger mass-loss rates per unit area in these stars. Publication: The Astrophysical Journal Pub Date: October 1976 DOI: 10.1086/154706 Bibcode: 1976ApJ...209..171M Keywords: Coronas; Dwarf Stars; Giant Stars; Main Sequence Stars; Stellar Atmospheres; Stellar Luminosity; Energy Conversion Efficiency; Light Curve; Radiant Flux Density; Solar Flares; Stellar Spectra; Stellar Winds; X Ray Sources; Astrophysics full text sources ADS | data products SIMBAD (6)

ζ Aurigae is an eclipsing binary star system with a property shared by only a few other systems. It consists of two stars in orbit around one another: a cool supergiant star (spectral type K2 II) and a hot main sequence (spectral type B8 v) star. The supergiant star is nearly 200 times larger than the Sun, while the main sequence star is only about seven times larger than the Sun. The Earth lies in the orbital plane of the two stars, and thus they periodically eclipse one another. When the relatively small B star goes into eclipse behind the cool supergiant star, its light passes through extended outer gaseous layers—the outer atmosphere—of the cool star. Thus the B star is an astrophysical light source which serves as a probe of the atmosphere of the K star. The eclipses, which occur every 972 days, have been well studied at visible wavelengths1,2. We report here IUE observations of the ζ Aur stellar system carried out between September 1979 and January 1980. A preliminary look at the data shows the advantage of UV observations of the system. There is strong evidence for cool, outflowing material near the primary star and hotter material farther from the primary perhaps in a shockwave. The onset of the atmospheric phase of the eclipse of the system—difficult to predict because of the irregularities of the supergiant atmosphere—began between 15 September and 1 November 1979.

We present the discovery of a gap near M G ≈ 10 in the main sequence on the Hertzsprung–Russell Diagram (HRD) based on measurements presented in Gaia Data Release 2 (DR2). Using an observational form of the HRD with M G representing luminosity and G BP − G RP representing temperature, the gap presents a diagonal feature that dips toward lower luminosities at redder colors. The gap is seen in samples extracted from DR2 with various distances, and is not unique to the Gaia photometry—it also appears when using near-IR (NIR) photometry (J − K s versus ). The gap is very narrow (∼0.05 mag) and is near the luminosity–temperature regime where M dwarf stars transition from partially to fully convective, i.e., near spectral type M3.0V. This gap provides a new feature in the HRD that hints at an underlying astrophysical cause, and we propose that it is linked to the onset of full convection in M dwarfs.

We report on multiwavelength observations of nova Small Magellanic Cloud Nova 2016-10a. The present observational set is one of the most comprehensive for any nova in the Small Magellanic Cloud, including low-, medium-, and high-resolution optical spectroscopy and spectropolarimetry from Southern African Large Telescope, Folded Low-Order Yte-Pupil Double-Dispersed Spectrograph, and Southern Astrophysical Research; long-term Optical Gravitational Lensing Experiment V- and I-bands photometry dating back to 6 yr before eruption; Small and Moderate Aperture Research Telescope System optical and near-IR photometry from ∼11 d until over 280 d post-eruption; Swift satellite X-ray and ultraviolet observations from ∼6 d until 319 d post-eruption. The progenitor system contains a bright disc and a main sequence or a sub-giant secondary. The nova is very fast with t2 ≃ 4.0 ± 1.0 d and t3 ≃ 7.8 ± 2.0 d in the V band. If the nova is in the SMC, at a distance of ∼61 ± 10 kpc, we derive MV, max ≃ −10.5 ± 0.5, making it the brightest nova ever discovered in the SMC and one of the brightest on record. At day 5 post-eruption the spectral lines show a He/N spectroscopic class and an Full Width at Half Maximum of ∼3500 km s−1, indicating moderately high ejection velocities. The nova entered the nebular phase ∼20 d post-eruption, predicting the imminent super-soft source turn-on in the X-rays, which started ∼28 d post-eruption. The super-soft source properties indicate a white dwarf mass between 1.2 and 1.3 M⊙ in good agreement with the optical conclusions.

The Sloan Digital Sky Survey (SDSS) and Two Micron All Sky Survey (2MASS) are rich resources for studying stellar astrophysics and the structure and formation history of the Galaxy. As new surveys and instruments adopt similar filter sets, it is increasingly important to understand the properties of the ugrizJHK_s stellar locus, both to inform studies of "normal" main-sequence stars and enable robust searches for point sources with unusual colors. Using a sample of ~600,000 point sources detected by SDSS and 2MASS, we tabulate the position and width of the ugrizJHK_s stellar locus as a function of g - i color, and provide accurate polynomial fits. We map the Morgan-Keenan spectral type sequence to the median stellar locus by using synthetic photometry of spectral standards and by analyzing 3000 SDSS stellar spectra with a custom spectral typing pipeline, described in the Appendix to this paper. We develop an algorithm to calculate a point source's minimum separation from the stellar locus in a seven-dimensional color space, and use it to robustly identify objects with unusual colors, as well as spurious SDSS/2MASS matches. Analysis of a final catalog of 2117 color outliers identifies 370 white-dwarf/M dwarf (WDMD) pairs, 93 QSOs, and 90 M giant/carbon star candidates, and demonstrates that WDMD pairs and QSOs can be distinguished on the basis of their J - K_s and r - z colors. We also identify a group of objects with correlated offsets in the u - g versus g - r and g - r versus r - i color-color spaces, but subsequent follow-up is required to reveal the nature of these objects. Future applications of this algorithm to a matched SDSS-UKIDSS catalog may well identify additional classes of objects with unusual colors by probing new areas of color-magnitude space.