Intrinsic properties of carbon stars. I. Effective temperature scale of N-type carbon stars

It is shown that the infrared flux method for determining stellar effective temperatures (Blackwell and Shallis 1977; Blackwell, Petford and Shallis 1980) can be applied to cool carbon stars. Although the spectra of cool carbon stars are highly line blanketed, the spectral region between 3 and 4 μm (L-band in the infrared photometry system) is found to be relatively free from strong line absorption. The ratioR _L of bolometric flux toL flux can then be used as a measure of effective temperature. On the basis of the predicted line-blanketed flux based on model atmospheres, with an empirical correction for the effect of 3 μm absorption due to polyatomic species (HCN, C₂H₂), it is shown thatR _L is roughly proportional to T³ _eff. The high sensitivity ofR _L to T_eff makes it a very good measure of effective temperature, and the usual difficulty due to differential line blanketing effect in the analyses of photometric indices of cool carbon stars can be minimized. It is found that the majority of N-type carbon stars with small variability (SRb and Lb variables) are confined to the effective temperature range between 2400 and 3200 K, in contrast to M-giant stars (M0 III - M6 III, including SRb and Lb variables) that are confined to the effective temperature range between 3200 and 3900 K. The effective temperatures based on the infrared flux method show good agreement with those derived directly from angular diameter measurements of 5 carbon stars. On the basis of the new effective temperature scale for carbon stars, it is shown that the well known C-classification does not represent a temperature sequence. On the other hand, colour temperatures based on various photometric indices all show good correlations with our derived effective temperatures. An erratum to this article is available at .     

Intrinsic properties of carbon stars. II. Spectra,colours, and HR diagram of cool carbon stars

On the basis of the effective temperature scale proposed previously for cool carbon stars (Paper I), other intrinsic properties of them are examined in detail. It is shown that the major spectroscopic properties of cool carbon stars, including those of molecular bands due to polyatomic species (SiC₂, HCN, C₂H₂ etc.), can most consistently be understood on the basis of our new effective temperature scale and the theoretical prediction of chemical equilibrium. Various photometric indices of cool carbon stars also appear to be well correlated with the new effective temperatures. Furthermore, as effective temperatures of some 30 carbon stars are now obtained, the calibration of any photometric index is straightforward, and some examples of such a calibration are given. In general, colour index-effective temperature calibrations for carbon stars are quite different from those for K-M giant stars. It is found that the intrinsic (R —I)₀ colour is nearly the same for N-irregular variables in spite of a considerable spread in effective temperatures, and this fact is used to estimate the interstellar reddening of carbon stars. An observational HR diagram of red giant stars, including carbon stars as well as K-M giant stars, is obtained on the basis of our colour index-effective temperature calibrations and the best estimations of luminosities. It is shown that carbon stars and M giant stars are sharply divided in the HR diagram by a nearly vertical line at aboutT _eff = 3200 K (logT _eff = 3.50) and the carbon stars occupy the upper right region of M giant stars (except for some high luminosity, high temperature J-type stars in the Magellanic Clouds; also Mira variables are not considered). Such an observational HR diagram of red giant stars shows rather a poor agreement with the current stellar evolution models. Especially, a more efficient mixing process in red giant stars, as compared with those ever proposed, is required to explain the formation of carbon stars.     

20颗依巴谷碳星的近红外测光

对20颗依巴谷(Hipparcos)卫星所观测的碳星作了近红外JHK测光，由近红外观测结果估算了其在K波段的热改正BCK和视热星等mbpl以及有效温度Te，结合依巴谷卫星所得视差，得到其中一些星的绝对热星等Mbol。     

Spectrophotometric studies of Am stars

The relationships among the various physical parameters-namely, the effective temperatures, radii and bolometric magnitudes, determined on the basis of the energy distribution curves of 25 Am stars — have been studied. Their effective temperatures are in the range of 7200 K to 9700 K; the radii, 1.5R to 2.5R ; the bolometric magnitudes, 0.75 mag. to 2.25 mag.; and the masses, 1.5M to 2.25M . The Am stars in general, appear redder than their normal counterparts, the blanketing in the blue andUV regions being the major cause. For the relatively cooler stars, the (B-V) colours are found to be less affected by blanketing. They are located in the neighbourhood of the upper edge of the zero-age Main Sequence band and show a fairly wide range in the evolutionary status among themselves. The bolometric corrections which are independent of the uncertainties in the parallax measurements, follow the same trend as that of the Ap stars, with reference to the temperature.     

X-ray Observations of Eight Young Open Star Clusters: I. Membership and X-ray Luminosity

We present a detailed investigation of X-ray source contents of eight young open clusters with ages between 4 to 46 Myr using archival X-ray data from XMM-Newton. The probable cluster memberships of the X-ray sources have been established on the basis of multi-wavelength archival data, and samples of 152 pre-main sequence (PMS) low mass (<2M _⊙), 36 intermediate mass (2–10M _⊙) and 16 massive (>10M _⊙) stars have been generated. X-ray spectral analyses of high mass stars reveal the presence of high temperature plasma with temperature <2 keV, and mean L _X/L _bol of 10^???6.9. In the case of PMS low mass stars, the plasma temperatures have been found to be in the range of 0.2 keV to 3 keV with a median value of ~1.3 keV, with no significant difference in plasma temperatures during their evolution from 4 to 46 Myr. The X-ray luminosity distributions of the PMS low mass stars have been found to be similar in the young star clusters under study. This may suggest a nearly uniform X-ray activity in the PMS low mass stars of ages ~4–14 Myr. These observed values of L _X/L _bol are found to have a mean value of 10^??3.6±0.4, which is below the X-ray saturation level. The L _X/L _bol values for the PMS low mass stars are well correlated with their bolometric luminosities, that implies its dependence on the internal structure of the low mass stars. The difference between the X-ray luminosity distributions of the intermediate mass stars and the PMS low mass stars has not been found to be statistically significant. Their L _X/L _bol values, however have been found to be significantly different from each other with a confidence level greater than 99.999% and the strength of X-ray activity in the intermediate mass stars is found to be lower compared to the low mass stars. However, the possibility of X-ray emission from the intermediate mass stars due to a low mass star in close proximity of the intermediate mass star can not be ruled out.     

Cooling of neutron stars: Two types of triplet neutron superfluidity

We consider the cooling of neutron stars with superfluid cores composed of neutrons, protons, and electrons (for singlet proton pairing and triplet neutron pairing). The emphasis is on triplet neutron pairing with the component of the total moment of neutron pairs along the quantization axis |m _J | = 2. This case stands out in that it leads to power-law rather than exponential suppression of the main neutrino processes by neutron superfluidity. For the chosen critical neutron temperatures T _cn, the cooling with |m _J | = 2 proceeds either almost in the same way as the commonly considered cooling with m _J =0 or appreciably faster. The cooling with variable (over the core) critical temperatures T _cn(ρ) and T _cp(ρ) can generally be described by the cooling with some effective constant temperatures T _cn and T _cp. The hypothesis of strong neutron superfluidity with |m _J | = 2 is in conflict with the observational data on the thermal radiation from isolated neutron stars; the hypothesis of weak neutron superfluidity of any type is consistent with the observations.     

Asymptotic Giant Branch Stars as Tracers of Star Formation Histories: the GAIA Context

We discuss a possible use of the asymptotic giant branch (AGB) stars for tracing star formation histories on the Galactic and extragalactic distance scales with the ESA's astrometric space mission GAIA. Extensive numerical simulations demonstrate that metallicities (Δ [M/H] ≲ 0.3) can be obtained for the AGB stars with GAIA up to the distances of ∼ 200 kpc, if no interstellar extinction is present. Reliable population ages can be also obtained from the AGB stars if their T _eff are constrained precisely. We show that precise effective temperatures can be obtained by fitting observed spectral energy distributions of the AGB stars with theoretical fluxes calculated from the synthetic spectra. A combination of the derived effective temperatures with the bolometric luminosities allows to derive precise population ages for a wide range of ages and metallicities over the large distance scales. This demonstrates that AGB stars can be employed very effectively for tracing star formation histories with GAIA, allowing to refine the global evolutionary scenarios of stellar populations in the Milky Way and the galaxies beyond. This revised version was published online in July 2006 with corrections to the Cover Date.     

Radii of some Ap,Am and A stars

Based on the observed energy curves of nine Ap stars, three Am stars, four normal A stars and one F0 V magnetic star, their radii have been estimated.Thence, the bolometric magnitudesM _bo1 have been obtained and a plot between logT _e andM _bo1 of these stars shows that a majority of Ap and Am stars are a little above the zero-age Main-Sequence, suggesting that they are slightly more evolved as compared to the normal A stars.The bolometric corrections derived from the aboveM _bo1 are much closer to those computed by Mihalas than to the ones given by Davis and Webb, the latter being about O ^m 1 more negative than the former.     

Effective temperatures,radii and bolometric magnitudes of Ap and Am stars

The effective temperatures radii and bolometric magnitudes of Ap, Am and normal A stars have been estimated from their energy distribution curves between 478 nm and 680 nm. All the Am stars and one Ap star (i.e. CrB) were found to be affected by line blanketing, a rough estimation of which in the respective (B-V) colours has been found out in each case.The range in effective temperature is 0.45–0.60 in terms of (=5040/T _e), while it is 1.8–4.8R in the case of radius, that in bolometric magnitude being from-0^m.67 to+1^m.61. An approximate estimate of the masses shows that they are between 1.5 and 3.0M . All these estimates are in agreement with those of the normal A stars. The Ap and Am stars are found to be slightly evolved and, therefore, are probably in the hydrogen shell-burning phase.     

Fundamental stellar parameters derived from the evolutionary tracks

The surface gravities and radii of stars are calculated for different MK spectral types using the masses of stars determined from their evolutionary tracks in the HR diagram and the most reliable values of effective temperatures and absolute bolometric magnitudes. MK spectral types are calibrated in absolute visual magnitudes using the studies ofM _v published since 1965. The calibration of MK types in temperatures is based on the newest investigations including the results both from the ultraviolet and the infrared. The obtained masses, gravities, and the mass-luminosity relationship show reasonable agreement with independent observational data.     

Two new LBV candidates in the M 33 galaxy

We present two new luminous blue variable (LBV) candidate stars discovered in the M33 galaxy. We identified these stars as massive star candidates at the final stages of evolution, presumably with a notable interstellar extinction. The candidates were selected from the Massey et al. catalog based on the following criteria: emission in H _α , V<18^./m 5 and 0_.^m 35 < (B - V) < 1_.^m 2. The spectra of both stars reveal a broad and strong H _α emission with extended wings (770 and 1000 kms⁻¹). Based on the spectra we estimated the main parameters of the stars. Object N45901 has a bolometric luminosity log(L/L_⊙) = 6.0–6.2 with the value of interstellar extinction A _V = 2.3 ± 0.1. The temperature of the star’s photosphere is estimated as T⋆ ∼ 13000–15000 K, its probable mass on the Zero Age Main Sequence is M∼ 60–80 M_⊙. The infrared excess in N 45901 corresponds to the emission of warm dust with the temperature Twarm ∼ 1000 K, and amounts to 0.1%of the bolometric luminosity. A comparison of stellar magnitude estimates from different catalogs points to the probable variability of the object N45901. Bolometric luminosity of the second object, N125093, is log(L/L_⊙) = 6.3 − 6.6, the value of interstellar extinction is A _V = 2.75 ± 0.15. We estimate its photosphere’s temperature as T⋆∼ 13000–16000K, the initial mass as M ∼ 90–120 M_⊙. The infrared excess in N125093 amounts to 5–6% of the bolometric luminosity. Its spectral energy distribution reveals two thermal components with the temperatures T_warm ∼ 1000K and T_cold ∼ 480 K. The [Ca II] λλ7291, 7323 lines, observed in LBV-like stars Var A and N93351 in M33 are also present in the spectrum of N 125093. These lines indicate relatively recent gas eruptions and dust activity linked with them. High bolometric luminosity of these stars and broad H α emissions allow classifying the studied objects as LBV candidates.     

Graphite grain formation in the atmospheres of R coronae borealis stars

The results of calculations of graphite grain formation in the atmospheres of R CrB stars are given. The parameters for the models wereM=1M _⊙,M _bol=?6 mag. The effective temperature ranged from 5300K to 8300K. The chemical composition corresponded to the hydrogen-deficient carbon rich mixture:X=0,Y=0.9,Z _c=0.1. The results obtained show the existence of a critical mass loss rate which is ranged fromM _*≈10^?6 M _⊙yr^?1 forT _eff=5300 K toM _*≈10^?5 M _⊙ yr^?1 forT _eff=8300 K. As soon as the rate of mass loss exceedsM _* by 3–5 times the degree of condensation of carbon changes from 0 to 0.7. The finite radii of grains are about from 0.01 μm to 0.6 μm depending on the density near the condensation point, the velocity of matter outflow, and the stellar effective temperature. The duration of grain growth should amount to some dozens of days. It is supposed that the most probable explanation of dust-shell formation around R CrB stars is graphite condensation behind a shock wave arising from nonlinear stellar pulsation.     

Spectrophotometric studies of Ap stars

Based on the observed energy distribution curves of about a hundred Ap stars, the various relationships among their physical parameters: namely, the temperature, colour index, bolometric correction and bolometric magnitude have been studied. The hotter Ap stars have been found to be apparently bluer than their normal counterparts, which is possibly due to the broad continuum features at 4200 and 5200 that are generally present in Ap stars only. The bolometric corrections are independent of parallax measurements; the Ap stars as well as the normal stars follow the same sequence of bolometric corrections when related to temperature. The Ap stars appear to beslightly evolved and their position in the HR diagram indicates the hydrogen shell burning phase. The mass range of Ap stars is similar to that of normal A stars.     

Infrared photometry of Sakurai’s object (V4334 Sgr) in 1996–1999

The infrared photometric observations of V4334 Sgr in 1996–1999 are presented. Together with optical data, they have allowed us to accurately estimate the bolometric flux from this star and to investigate the structure of its dust envelope over the above period. The star is shown to have passed through four well-defined stages in these four years as it moved backward along the post-AGB track, and it now appears to have started moving forward after a halt. At the first stage (1996), there was no dust in the star’s envelope. Its visual brightness slightly increased, and it reddened in the entire observed spectral range. The bolometric flux also gradually rose. At the second stage (1997), an optically thick dust envelope condensed around the star, which, however, essentially did not manifest itself at optical wavelengths. The bolometric flux continued to rise through an increase in the star’s infrared brightness alone; the rate of its rise also increased. At the third stage (1998–March 1999), V4334 Sgr entered the R CrB phase. First two shallow minima and then two deep minima were observed at optical wavelengths. The star appreciably reddened during the deep minima. The bolometric flux ceased to rise and began to gradually fall in the second half of 1998. At the fourth stage (since March 1999 up until now), V4334 Sgr has been at a protracted deep minimum, which is atypical of the R CrB stars. The bolometric flux between March and October underwent no significant variations. We describe the structure of the dust envelope around V4334 Sgr since its formation. From June 1997 until July 1998, the optical depth of the dust shell, its inner and outer radii, and its mass increased by factors of ～2.2, ～2.0, 2.3, and ～10, respectively. In July 1998, τ(V)≈2.3, R _{d, in}≈7.4×10¹⁴ cm, R _{d, in}/R _{d, out}≈0.7(R _{d, in}/R _*≈47), and M _dust≈1.6×10^?7 M _⊙.     

Symmetry of the neutron and proton superfluidity effects in cooling neutron stars

We investigate the combined effect of neutron and proton superfluidities on the cooling of neutron stars whose cores consist of nucleons and electrons. We consider the singlet state paring of protons and the triplet pairing of neutrons in the cores of neutron stars. The critical superfluid temperatures T _c are assumed to depend on the matter density. We study two types of neutron pairing with different components of the total angular momentum of a Cooper pair along the quantization axis (|m _J |=0 or 2). Our calculations are compared with the observations of thermal emission from isolated neutron stars. We show that the observations can be interpreted by using two classes of superfluidity models: (1) strong proton superfluidity with a maximum critical temperature in the stellar core T _c ^max ?4×10⁹ K and weak neutron superfluidity of any type (T _c ^max ?2×10⁸ K); (2) strong neutron superfluidity (pairing with m _J =0) and weak proton superfluidity. The two types of models reflect an approximate symmetry with respect to an interchange of the critical neutron and proton pairing temperatures.     

Scanner observations of the classical cepheids RT Aur and T Vul

The effective temperatures of the classical Cepheids RT Aur and T Vul have been determined by a comparison of their spectral scans with appropriate model atmospheres. The radii of the stars have been determined through the Wesselink method. Using these temperatures and the Wesselink radii, the luminosities of the stars have been determined. These radii estimates, including the radii of SU Cas (Joshi & Rautela 1980) andζ Gem (unpublished) fit better in the theoretical period-radius relationship given by Cogan (1978), as compared to earlier determinations of Wesselink radii. The pulsation masses and evolutionary masses of the stars have been calculated. The pulsation to evolutionary mass ratio is derived to be 0.85. Based on the effective temperatures obtained by us at different phases of the stars aθ _c ? (B-V)₀ relationship is found of the form, \(\begin{gathered} \theta _e = 0.274 (B - V)_0 + 0.637 \\ \pm 0.011 \pm 0.007 \\ \end{gathered} \)      

Generation of a Near Infra-Red Guide Star Catalog for Thirty-Meter Telescope Observations

The requirements for the production of a near Infra-Red Guide Star Catalog (IRGSC) for Thirty Meter Telescope (TMT) observations are identified and presented. A methodology to compute the expected J band magnitude of stellar sources from their optical (g, r, i) magnitudes is developed. The computed and observed J magnitudes of sources in three test fields are compared and the methodology developed is found to be satisfactory for the magnitude range, J_Vega = 16–22 mag. From this analysis, we found that for the production of final TMT IRGSC (with a limiting magnitude of J_Vega = 22 mag), we need g, r, i bands optical data which go up to i _AB ~ 23 mag. Fine tuning of the methodology developed, such as using Spectral Energy Distribution (SED) template fitting for optimal classification of stars in the fainter end, incorporating spectral libraries in the model, to reduce the scatter, and modification of the existing colour–temperature relation to increase the source density are planned for the subsequent phase of this work.     

Analysis of HST ultraviolet spectra for T Tauri stars: Estimating the interstellar extinction and the contribution from an accretion shock to the emission-continuum formation

We analyzed the spectra of eight T Tauri stars (T Tau, RY Tau, CO Ori, EZ Ori, GW Ori, GX Ori, V1044 Ori, and SU Aur) in the wavelength range from 1200 to 3100 Å taken with the STIS spectrograph from the Hubble Space Telescope. For each star, we found an upper limit on the interstellar extinction A _v , which proved to be lower than the values obtained by different authors from optical observations. For T Tau and RY Tau, we found the upper limits on their luminosities, masses, and radii as well as the bolometric luminosity of the excess emission continuum. The latter is most likely associated with mass accretion from a protoplanetary disk. We show that the bulk of the emission continuum is radiated in the infrared. For these stars, we determined the ratio of the flux in the C IV 1550 doublet lines to the excess-continuum flux. This ratio proved to be two orders of magnitude lower than its values predicted by the accretion-shock (AS ) models developed by Lamzin (1998) and Calvet and Gullbring (1998). This result leads us to believe that for T Tau and RY Tau, the emission continuum originates in the accretion disk and/or in the boundary layer rather than in the AS, as has been assumed previously. This implies that in these stars, only a small fraction of the accreted matter passes through the AS, while the bulk of this matter settles in the equatorial plane of the star, passing through the boundary layer.     

Accurate fundamental parameters for lower main-sequence stars

We derive an empirical effective temperature and bolometric luminosity calibration for G and K dwarfs, by applying our own implementation of the Infrared Flux Method to multiband photometry. Our study is based on 104 stars for which we have excellent   BV ( RI )_C JHK _S  photometry, excellent parallaxes and good metallicities.
Colours computed from the most recent synthetic libraries (ATLAS9 and MARCS) are found to be in good agreement with the empirical colours in the optical bands, but some discrepancies still remain in the infrared. Synthetic and empirical bolometric corrections also show fair agreement.
A careful comparison to temperatures, luminosities and angular diameters obtained with other methods in the literature shows that systematic effects still exist in the calibrations at the level of a few per cent. Our Infrared Flux Method temperature scale is 100-K hotter than recent analogous determinations in the literature, but is in agreement with spectroscopically calibrated temperature scales and fits well the colours of the Sun. Our angular diameters are typically 3 per cent smaller when compared to other (indirect) determinations of angular diameter for such stars, but are consistent with the limb-darkening corrected predictions of the latest 3D model atmospheres and also with the results of asteroseismology.
Very tight empirical relations are derived for bolometric luminosity, effective temperature and angular diameter from photometric indices.
We find that much of the discrepancy with other temperature scales and the uncertainties in the infrared synthetic colours arise from the uncertainties in the use of Vega as the flux calibrator. Angular diameter measurements for a well-chosen set of G and K dwarfs would go a long way to addressing this problem.