Stars Of Late Spectral Type Research Articles

Silicate Extinction Profile Based on the Stellar Spectrum by Spitzer/IRS

The 9.7 and 18 μm interstellar spectral features, arising from the Si–O stretching and O–Si–O bending modes of amorphous silicate dust, are the strongest extinction features in the infrared. Here we use the “pair method” to determine the silicate extinction profile by comparing the Spitzer/IRS spectra of 49 target stars with obvious extinction with those of unreddened stars of the same spectral type. The 9.7 μm extinction profile is determined from all the 49 stars and the 18 μm profile is determined from six stars. It is found that the profile has a peak wavelength at ∼9.2–9.8 and ∼18–22 μm, respectively. The peak wavelength of the 9.7 μm feature seems to become shorter for stars of late spectral types; meanwhile the FWHM seems irrelevant to the spectral type, which may be related to circumstellar silicate emission. The silicate optical depth at 9.7 μm, Δτ 9.7, mostly increases with the color excess in J − K S ( EJKS ). The mean ratio of the visual extinction to the 9.7 μm silicate absorption optical depth is A V /Δτ 9.7 ≈ 17.8, in close agreement with that of the solar-neighborhood diffuse interstellar medium. When EJKS>4 , this proportionality changes. The correlation coefficient between the peak wavelength and the FWHM of the 9.7 μm feature is 0.4, which indicates a positive correlation considering the uncertainties of the parameters. The method is compared with replacing the reference star by an atmospheric model spectral energy distribution and no significant difference is observed.

On the Influence of ‘‘Red Leak’’ of Light Filters on the Brightness Estimates of Stars of Late Spectral Types Illustrated by the Observations of Rapid Variability of Symbiotic Stars

On the Influence of ‘‘Red Leak’’ of Light Filters on the Brightness Estimates of Stars of Late Spectral Types Illustrated by the Observations of Rapid Variability of Symbiotic Stars

Follow-up observations of X-ray emitting hot subdwarf stars: the compact He-poor sdO star Feige 34

We report on results obtained with theXMM-Newtonobservation of Feige 34 carried out in April 2018. This is the first spectroscopic X-ray observation of a compact and helium-poor hot subdwarf star. The source was detected at a flux levelfX = 3.4 × 10−14erg cm−2s−1in the energy range 0.2–3 keV, which implies an X-ray-to-bolometric flux ratiofX/fbol ≃ 10−6.5. The source spectrum can be described with the sum of two thermal-plasma components with subsolar abundances at temperatures of ≃0.3 and 1.1 keV. These properties are similar to what is observed in early-type main-sequence stars, where the X-ray emission is attributed to turbulence and shocks in the stellar wind. Therefore, the same phenomenon could explain the X-ray properties of Feige 34. However, it is not possible to reproduce the observed spectrum with a thermal-plasma model if the elemental abundances are fixed at the values obtained from the optical and UV spectroscopy. Moreover, we show that the X-ray luminosity and spectrum are consistent with those expected from a young main-sequence star of late spectral type. Therefore, we discuss the possibility that the observed X-ray emission is due to the companion star of M0 spectral type, whose presence is suggested by the IR excess in the spectral energy distribution of Feige 34.

MODEL ATMOSPHERES OF IRRADIATED EXOPLANETS: THE INFLUENCE OF STELLAR PARAMETERS, METALLICITY, AND THE C/O RATIO

Many parameters constraining the spectral appearance of exoplanets are still poorly understood. We therefore study the properties of irradiated exoplanet atmospheres over a wide parameter range including metallicity, C/O ratio and host spectral type. We calculate a grid of 1-d radiative-convective atmospheres and emission spectra. We perform the calculations with our new Pressure-Temperature Iterator and Spectral Emission Calculator for Planetary Atmospheres (PETIT) code, assuming chemical equilibrium. The atmospheric structures and spectra are made available online. We find that atmospheres of planets with C/O ratios $\sim$ 1 and $T_{\rm eff}$ $\gtrsim$ 1500 K can exhibit inversions due to heating by the alkalis because the main coolants CH$_4$, H$_2$O and HCN are depleted. Therefore, temperature inversions possibly occur without the presence of additional absorbers like TiO and VO. At low temperatures we find that the pressure level of the photosphere strongly influences whether the atmospheric opacity is dominated by either water (for low C/O) or methane (for high C/O), or both (regardless of the C/O). For hot, carbon-rich objects this pressure level governs whether the atmosphere is dominated by methane or HCN. Further we find that host stars of late spectral type lead to planetary atmospheres which have shallower, more isothermal temperature profiles. In agreement with prior work we find that for planets with $T_{\rm eff}$ $<$ 1750 K the transition between water or methane dominated spectra occurs at C/O $\sim$ 0.7, instead of $\sim$ 1, because condensation preferentially removes oxygen.

Optical counterparts of ROSAT X-ray sources in two selected fields at low vs. high Galactic latitudes

The optical identification of large number of X-ray sources such as those from the ROSAT All-Sky Survey is challenging with conventional spectroscopic follow-up observations. We investigate two ROSAT All-Sky Survey fields of size 10 * 10 degrees each, one at galactic latitude b = 83 deg (Com), the other at b = -5 deg (Sge), in order to optically identify the majority of sources. We used optical variability, among other more standard methods, as a means of identifying a large number of ROSAT All-Sky Survey sources. All objects fainter than about 12 mag and brighter than about 17 mag, in or near the error circle of the ROSAT positions, were tested for optical variability on hundreds of archival plates of the Sonneberg field patrol. The present paper contains probable optical identifications of altogether 256 of the 370 ROSAT sources analysed. In particular, we found 126 AGN (some of them may be misclassified CVs), 17 likely clusters of galaxies, 16 eruptive double stars (mostly CVs), 43 chromospherically active stars, 65 stars brighter than about 13 mag, 7 UV Cet stars, 3 semiregular resp. slow irregular variable stars of late spectral type, 2 DA white dwarfs, 1 Am star, 1 supernova remnant and 1 planetary nebula. X-ray emission is, expectedly, tightly correlated with optical variability, and thus our new method for optically identifying X-ray sources is demonstrated to be feasible.

Investigation of the stellar content in the western part of the Carina nebula

We obtained deep $UBVRI$ H$\alpha$ photometric data of the field situated to the west of the main Carina nebula and centered on WR~22. Medium-resolution optical spectroscopy of a subsample of X-ray selected objects along with archival data sets from $Chandra$, $XMM-Newton$ and 2MASS surveys were used for the present study. Different sets of color-color and color-magnitude diagrams are used to determine reddening for the region and to identify young stellar objects (YSOs) and estimate their age and mass. Our spectroscopic results indicate that the majority of the X-ray sources are late spectral type stars. The region shows a large amount of differential reddening with minimum and maximum values of $E(B-V)$ as 0.25 and 1.1 mag, respectively. Our analysis reveals that the total-to-selective absorption ratio $R \rm_{V}$ is $\sim$3.7 $\pm$ 0.1, suggesting an abnormal grain size in the observed region. We identified 467 YSOs and studied their characteristics. The ages and masses of the 241 optically identified YSOs range from $\sim$0.1 to 10 Myr and $\sim$0.3 to 4.8M$_\odot$, respectively. However, the majority of them are younger than 1 Myr and have masses below 2 M$_\odot$. The high mass star WR 22 does not seem to have contributed to the formation of YSOs in the CrW region. The initial mass function slope, $\Gamma$, in this region is found to be -1.13 $\pm$ 0.20 in the mass range of 0.5 < M/M$_\odot$ < 4.8. The $K$-band luminosity function slope ($\alpha$) is also estimated as 0.31 $\pm$ 0.01. We also performed minimum spanning tree analysis of the YSOs in this region, which reveals that there are at least ten YSO cores associated with the molecular cloud, and that leads to an average core radius of 0.43 pc and a median branch length of 0.28 pc.

The character of solar-type activity and the depth of the convective zone

The origin of solar-type activity for low-mass stars of late spectral types is considered. Spectroscopic data were used to study the dependence of the activity level logR′ HK on the lithium abundance logA(Li) and axial rotation rate. A close correlation between logA(Li) and logR′ HK is found for two groups of G stars, hotter and cooler than the Sun. This relation is most clearly expressed in the case of high activity, and is somewhat more strongly expressed for G6-K3 dwarfs, which includes many BY Dra variables, than for F8-G5 stars. It is confirmed that, for stars with high activity, both the lithium abundance and the activity level are determined by the rotation rate, which depends on the age. The lithium abundance exhibits different dependences on the chromospheric activity, depending on the level of this activity. Cooler stars, with detectable lithium and solar-like chromospheres, possess much stronger coronas. This change in the relationship between the relative luminosities of the chromosphere and corona can be reliably traced using larger datasets. The different ratios between the activity of the choromosphere and corona for cooler and hotter G stars may reflect the fact that their convective zones become deeper or shallower than some critical value. This is consistent with observations of parameters describing rotational modulation and the correlation and anti-correlation of chromospheric and photospheric activity indices for stars hotter and cooler than the Sun. Physically, this means that the character of the activity could be related to a changing contribution of the large-scale and local magnetic fields to the generation of the activity. The results of this study confirm the earlier idea that there may be different evolution paths associated with solar-type activity. The results can be used to refine methods for estimating ages of stars from their activity levels (gyrochronology).

Plasma heating during the parametric excitation of acoustic waves in coronal magnetic loops

We examine plasma heating due to the dissipation of acoustic waves excited in coronal magnetic loops by parametric resonance with the five-minute oscillations in the velocity of the photospheric convection. The energy of acoustic waves excited in the coronal magnetic loop, rate of dissipation of acoustic waves, and rate of heating of the coronal plasma are determined. The maximum temperature predicted for the apex of the loop is calculated as a function of the velocity of photospheric oscillations, length of the loop, and electric current in the loop. It is shown that the mechanism proposed can explain the origin of quasi-stationary X-ray loops with temperatures of 3–6 MK. The lengths of these loops are resonant for acoustic waves excited by the five-minute photospheric oscillations. The use of the proposed mechanism to explain heating of the X-ray loops expected to be on stars of late spectral types is discussed.

A MILLIMETER-WAVE INTERFEROMETRIC STUDY OF DUST AND CO DISKS AROUND LATE SPECTRAL TYPE STARS IN TAURUS-AURIGA

We present a new millimeter survey of 23 pre-main-sequence stars in the Taurus-Auriga star-forming region. The main goal of the project was to identify circumstellar disks around stars of late spectral types (M {sub *} {approx}< 0.7 M {sub sun}). We used the IRAM Plateau de Bure Interferometer to obtain observations at 1.3 mm, 2.7 mm, and the CO J = 1-0 and J = 2-1 lines. We detected eight sources in continuum emission, with new detections of CIDA-1 and CIDA-8 at 1.3 mm. Overall, we find that circumstellar disks around stars with spectral types later than M2 tend to have smaller millimeter continuum fluxes than those around earlier type stars. We also present four detections of disks in their CO line emission (LkH{alpha} 358, GO Tau, Haro 6-13, IRAS 04385+2550). By mapping the rotation of the resolved CO emission from the circumstellar disks, we obtain estimates of the dynamical mass of the central stars.

K-band spectroscopy of pre-cataclysmic variables

Aims. There exists now substantial evidence for abundance anomalies in a number of cataclysmic variables (CVs), indicating that the photosphere of the secondary star incorporates thermonuclear processed material. However, the spectral energy distribution in CVs is usually dominated by the radiation produced by the accretion process, severely hindering an investigation of the stellar components. On the other hand, depending on how the secondary star has acquired such material, the above mentioned abundance anomalies could also be present in pre-CVs, i.e. detached white/red dwarf binaries that will eventually evolve into CVs, but have not yet started mass transfer, and therefore allow for an unobstructed view on the secondary star at infrared wavelengths. Methods. We have taken K-band spectroscopy of a sample of 13 pre-CVs in order to examine them for anomalous chemical abundances. In particular, we study the strength of the 12CO and 13CO absorption bands that have been found diminished and enhanced, respectively, in similar studies of CVs. Results. All our systems show CO abundances that are within the range observed for single stars. The weakest 12CO bands with respect to the spectral type are found in the pre-CV BPM 71214, although on a much smaller scale than observed in CVs. Furthermore there is no evidence for enhanced 13CO. Taking into account that our sample is subject to the present observational bias that favours the discovery of young pre-CVs with secondary stars of late spectral types, we can conclude the following: 1) our study provides observational proof that the CO anomalies discovered in certain CVs are not due to any material acquired during the common envelope phase, and 2) if the CO anomalies in certain CVs are not due to accretion of processed material during nova outburst, then the progenitors of these CVs are of a significantly different type than the currently known sample of pre-CVs.

First Byurakan Spectral Sky Survey. Stars of Late-Spectral Types. XIV. Zone +13° ≤ ≤ +33°

First Byurakan Spectral Sky Survey. Stars of Late-Spectral Types. XIV. Zone +13° ≤ ≤ +33°

Butterfly diagram for starspots on LQ Hya

Information on the latitude distribution of starspots and changes in this distribution from year to year is very important for our understanding of the nature of stellar activity and for developing dynamo theory. The concept of butterfly diagrams is introduced for highly spotted stars of late spectral types, by analogy to the Maunder diagrams for the Sun. Our approach is based on the zonal spottedness models constructed by Alekseev and Gershberg. A detailed analysis is given for the single active star LQ Hya, and a comparison is made to similar analyses for several stars with two well-separated spot belts—EK Dra, VY Ari, V775 Her, and V833 Tau. The lower boundary of the butterfly diagram drifts toward the equator during the activity-rise phase, i.e., during years when the relative spotted area increases. This effect is clearly expressed for LQ Hya and other stars whose orientation enables observation of both hemispheres and virtually vanishes for V833 Tau, which is viewed nearly pole-on. The upper boundary of the diagram is virtually unchanged for all the considered spotted stars except V775 Her, for which it moves toward the pole. The drift rate of the lower boundary is −1 to −2 deg/yr, a factor of two to three smaller in magnitude than the corresponding solar value. Our analysis provides an independent confirmation of the occurrence of high-latitude spots on stars that are younger than the Sun and whose activity is high but less regular than the solar activity; it also enables the identification of the starting times of stellar cycles.

Point X-ray sources in the SNR G 315.4-2.30 (MSH 14-63, RCW 86)

We report the results of a search for a point X-ray source (stellar remnant) in the southwest protrusion of the supernova remnant G 315.4-2.30 (MSH 14-63, RCW 86) using the archival data of the Chandra X-ray Observatory. The search was motivated by a hypothesis that G 315.4-2.30 is the result of an off-centered cavity supernova explosion of a moving massive star, which ended its evolution just near the edge of the main-sequence wind-driven bubble. This hypothesis implies that the southwest protrusion in G 315.4-2.30 is the remainder of a pre-existing bow shock-like structure created by the interaction of the supernova progenitor's wind with the interstellar medium and that the actual location of the supernova blast center is near the center of this hemispherical structure. We have discovered two point X-ray sources in the proper place. One of the sources has an optical counterpart with the photographic magnitude $13.38\pm0.40$, while the spectrum of the source can be fitted with an optically thin plasma model. We interpret this source as a foreground active star of late spectral type. The second source has no optical counterpart to a limiting magnitude $\sim 21$. The spectrum of this source can be fitted almost equally well with several simple models (power law: photon index $=1.87$; two-temperature blackbody: $kT_1 =0.11$ keV, $R_1 =2.34 $ km and $kT_2 =0.71$ keV, $R_2 =0.06$ km; blackbody plus power law: $kT =0.07$ keV, photon index $=2.3$). We interpret this source as a candidate stellar remnant (neutron star), while the photon index and non-thermal luminosity of the source (almost the same as those of the Vela pulsar and the recently discovered pulsar PSR J 0205+6449 in the supernova remnant 3C 58) suggest that it can be a young ordinary pulsar.

On the nature of radio emission from stars of late spectral type

On the nature of radio emission from stars of late spectral type

O prirode radioizlucheniya zvezd pozdnikh spektral'nykh klassov

O prirode radioizlucheniya zvezd pozdnikh spektral'nykh klassov

A search for peculiar objects in young open clusters

A serendipitous sky eld in Vulpecula was observed with Johnson BVR photometry and slitless spec- troscopy in the 5600{8000 A range, looking for emission line and late spectral type stars. We discovered that the B2V star LS II +32 9, most probably a member of the Vul OB4 association, is now an H emitter. Furthermore, we identied the optical counterpart of the IRAS source IRAS 19564+3224 as a C star. Photometric study of the surrounding stellar eld revealed that this object lies inside the sky area of a previously undetected open cluster of intermediate age. We discuss the possible cluster membership of the Carbon star and its implications. Some hints about the galactic structure along the line of sight of the observed eld are also given

Statistical analysis of intrinsic polarization, IR excess and projected rotational velocity distributions of classical Be stars

We present the results of statistical analyses of a sample of 627 Be stars. The parameters of intrinsic polarization $(p_{\ast})$, projected rotational velocity $(v \sin{i})$, and near IR excesses have been investigated. The values of $p_{\ast}$ have been estimated for a much larger and more representative sample of Be stars ( ≈ 490 objects) than previously. We have confirmed that most Be stars of early spectral type have statistically larger values of polarization and IR excesses in comparison with the late spectral type stars. It is found that the distributions of $ p_{\ast} $ diverge considerably for the different spectral subgroups. In contrast to late spectral types (B5-B9.5), the distribution of $ p_{\ast} $ for B0-B2 stars does not peak at the value $p_{\ast}=0$% . Statistically significant differences in the mean projected rotational velocities ($\overline{v\sin{i}}$) are found for different spectral subgroups of Be stars in the sense that late spectral type stars (V luminosity class) generally rotate faster than early types, in agreement with previously published results. This behaviour is, however, not obvious for the III-IV luminosity class stars. Nevertheless, the calculated values of the ratio $v_{\rm t}/v_{\rm c}$ of the true rotational velocity, v t , to the critical velocity for break-up, v c , is larger for late spectral type stars of all luminosity classes. Thus, late spectral type stars appear to rotate closer to their break-up rotational velocity. The distribution of near IR excesses for early spectral subgroups is bi-modal, the position of the second peak displaying a maximum value $E(V-L)\approx 1 .\!\!\!^{\rm m}3$ for O-B1.5 stars, decreasing to $E(V-L)\approx0.\!\!\!^{\rm m}8$ for intermediate spectral types (B3-B5). It is shown that bi-modality disappears for late spectral types (B6-B9.5). No correlations were found between $p_{\ast}$ and near IR excesses and between $E(V-L)$ and $v\sin{i}$ for the different subgroups of Be stars. In contrast to near IR excesses, a relation between $p_{\ast}$ and far IR excesses at 12 μ m is clearly seen. A clear relation between $p_{\ast}$ and $v\sin{i}$ (as well as between $p_{\ast}$ and $\overline{v\sin{i}}/v_{\rm c}$) is found by the fact that plots of these parameters are bounded by a triangular distribution of $p_{\ast}$ : $v\sin{i}$, with a decrease of $p_{\ast}$ towards very small and very large $v\sin{i}$ (and $\overline{v\sin{i}}/v_{\rm c}$) values. The latter behaviour can be understood in the context of a larger oblateness of circumstellar disks for the stars with a rapid rotation. From the analysis of correlations between different observational parameters we conclude that circumstellar envelopes for the majority of Be stars are optically thin disks with the range of the half-opening angle of $10\degr< \Theta< 40\degr$.

Research Note Radial velocities of giant M stars near the ecliptic

A search dedicated to the identification of new, dwarf and cold stars close to the Sun has allowed us to measure the radial velocities of a set of stars of late spectral type. The stars selected were M-type stars present in the Hipparcos Input Catalogue located in a strip of ± 20° around the Ecliptic and observables from La Silla. Our final set contains 22 M-type stars whose radial velocities V r have been measured with an accuracy of ~0.5 km s -1 during a single observing run. From their velocities, parallaxes and magnitudes we deduce that all the stars observed are red giants, with space velocities relative to the Sun mostly lower than 100 km s -1 .

Optical identifications of iras point sources based on low-dispersion fbs spectra. Stars. IV

A fourth list of point sources from the IRAS Point Source Catalog (PSC) that are optically identified with stars of late spectral types is given. The list contains data on 41 objects. The identifications were based on the Digital Sky Survey (DSS), the First Byurakan Survey, blue and red maps of the Palomar Observatory Sky Survey, and infrared fluxes at the wavelengths 12, 25, 60, and 100 μm in the region of +65° ≤ δ ≤ +69° and 05h10m ≤ α ≤ 18h10m. Of the 41 objects, which are given in the IRAS PSC as unidentified sources of infrared emission, 9 are associated with known stars in existing catalogs while 32 sources remained unidentified in the optical range, one of which is also a source in the deep IRAS survey (IRAS Serendipitous Source Catalog). The optical coordinates, their departures from the IR coordinates, the V stellar magnitudes, the color indices CI, and the preliminary spectral subtypes have been determined. The objects have optical magnitudes in the range of 8m.5-14m.5. Finder charts from the DSS are given for 32 of the objects.

On the Initial Mass Function of Be Stars and the Missing Be Stars of Late Spectral Types

AbstractThis is an attempt at determining the IMF (Initial Mass Function) of Be stars relative to that of B stars in the vicinity of the Sun. We have represented the IMF as a power function of the stellar mass Mβ, so that the relative IMF is then proportional to Mβ(Be)–β(B). Allowing for systematic differences in the counting of Be stars due to their apparent overluminosity, a difference β(Be) – β(B) ~ 0 is found, which may indicate that there are no huge intrinsic differences between the two types of objects. In these calculations changes of spectral types due to high rotation were not taken into account. This effect may still strongly affect the results obtained. By extrapolating the MβBe) – β(B) curve to spectral types later than B7, we reckon that in a volume limited to the apparent magnitude V = 7 there may be about 150 still undetected Be stars of late spectral types.