Interstellar Values Research Articles

Parameterization of stars and determination of interstellar extinction from multicolor photometry

We have developed a method for the determination of stellar parameters and interstellar extinction values from multicolor photometry. The application of this method to a set of stars in a small area in the sky allows us to determine an increase of interstellar extinction with distance in that direction and, consequently, to construct a 3D extinction map of the Milky Way Galaxy.

Deep Search for Molecular Oxygen in TW Hya

The dominant form of oxygen in cold molecular clouds is gas-phase carbon monoxide (CO) and ice-phase water (H2O). Yet, in planet-forming disks around young stars, gas-phase CO and H2O are less abundant relative to their interstellar medium values, and no other major oxygen-carrying molecules have been detected. Some astrochemical models predict that gas-phase molecular oxygen (O2) should be a major carrier of volatile oxygen in disks. We report a deep search for emission from the isotopologue 16O18O (N J = 21 − 01 line at 233.946 GHz) in the nearby protoplanetary disk around TW Hya. We used imaging techniques and matched filtering to search for weak emission but do not detect 16O18O. Based on our results, we calculate upper limits on the gas-phase O2 abundance in TW Hya of (6.4–70) × 10−7 relative to H, which is 2–3 orders of magnitude below solar oxygen abundance. We conclude that gas-phase O2 is not a major oxygen carrier in TW Hya. Two other potential oxygen-carrying molecules, SO and SO2, were covered in our observations, which we also do not detect. Additionally, we report a serendipitous detection of the C15N N J = 25/2 − 13/2 hyperfine transitions, F = 3 − 2 and F = 2 − 1, at 219.9 GHz, which we found via matched filtering and confirm through imaging.

The Mountain in the Sea: Place, Wilderness and History on the Sunset Side of Hoerikwaggo

For six years of my life, I lived in Camps Bay. That’s a bit like saying I lived in Beverley Hills, or Knightsbridge, or on The Peak in Hong Kong. Every city has a suburb that is the local byword for conspicuous consumption, for interstellar property values, for self-indulgent wealth. In Cape Town to say you live in Camps Bay is to say your other car is a Range Rover; that your Pilates instructor comes to your house every morning; that your daughter started using Botox at seventeen.

Infrared Spectroscopic Survey of the Quiescent Medium of Nearby Clouds. II. Ice Formation and Grain Growth in Perseus and Serpens

The properties of dust change during the transition from diffuse to dense clouds as a result of ice formation and dust coagulation, but much is still unclear about this transformation. We present 2–20 μm spectra of 49 field stars behind the Perseus and Serpens Molecular Clouds and establish relationships between the near-infrared continuum extinction (A K) and the depths of the 9.7 μm silicate (τ 9.7) and 3.0 μm H2O ice (τ 3.0) absorption bands. The τ 9.7/A K ratio varies from large, diffuse interstellar medium-like values (∼0.55), to much lower ratios (∼0.26). Above extinctions of A K ∼ 1.2 (A V ∼ 10; Perseus, Lupus, dense cores) and ∼2.0 (A V ∼ 17; Serpens), the τ 9.7/A K ratio is lowest. The τ 9.7/A K reduction from diffuse to dense clouds is consistent with a moderate degree of grain growth (sizes up to ∼0.5 μm), increasing the near-infrared color excess (and thus A K), but not affecting the ice and silicate band profiles. This grain growth process seems to be related to the ice column densities and dense core formation thresholds, highlighting the importance of density. After correction for Serpens foreground extinction, the H2O ice formation threshold is in the range of A K = 0.31–0.40 (A V = 2.6–3.4) for all clouds, and thus grain growth takes place after the ices are formed. Finally, abundant CH3OH ice (∼21% relative to H2O) is reported for 2MASSJ18285266+0028242 (Serpens), a factor of >4 larger than for the other targets.

Do the Geminga, Monogem and PSR J0622+3749 γ -ray halos imply slow diffusion around pulsars?

The HAWC Collaboration has reported the detection of an extended $\gamma$-ray emission around the Geminga and Monogem pulsars of a few degree extension. Very recently, the LHAASO Collaboration released also the data for an extended $\gamma$-ray emission around the pulsar PSR J0622+3749. This flux can be explained with electrons and positrons injected from these sources and their inverse Compton Scattering on the interstellar radiation fields. So far the size of such $\gamma-$ray halos has been interpreted as the result of the diffusion coefficient around the sources being about two orders of magnitude smaller than the average in the Galaxy. However, this conclusion is driven by the assumption that particles propagate diffusively right away after the injection without taking into account the ballistic propagation. The propagation of cosmic-ray leptons in the proximity of the Geminga, Monogem and PSR J0622+3749 pulsars is examined here considering the transition from the quasi-ballistic, valid for the most recently injected particles, to the diffusive transport regime. For typical interstellar values of the diffusion coefficient, the quasi-ballistic regime dominates the lepton distribution up to distances of a few tens of parsec from the pulsar for particle energies above $\sim 10$ TeV. In this regime the resulting $\gamma-$ray source tends to be rather compact, despite particles travel a long distance. Indeed, for larger values of the diffusion coefficient, particles propagate ballistically up to larger distances with the result of a more point-like $\gamma-$ray source. When such transition is taken into account, a good fit to the HAWC and LHAASO $\gamma-$ray data around Geminga, Monogem and PSR J0622+3749 is obtained without the need to invoke a strong suppression of the diffusion coefficient.

On the X-Ray Properties of the Putative Central Compact Object in 1E 0102.2−7219

Abstract We have analyzed the archival Chandra X-ray Observatory observations of the compact feature in the Small Magellanic Cloud supernova remnant 1E 0102.2−7219, which has recently been suggested to be the central compact object remaining after the supernova explosion. In our analysis, we have used appropriate, time-dependent responses for each of the archival observations, have modeled the background instead of subtracting it, and have fit unbinned spectra to preserve the maximal spectral information. The spectrum of this feature is similar to the spectrum of the surrounding regions that have significantly enhanced abundances of O, Ne, and Mg. We find that the previously suggested blackbody model is inconsistent with the data, as Monte Carlo simulations indicate that more than 99% of the simulated data sets have a test statistic value lower than that of the data. The spectrum is described adequately by a nonequilibrium ionization thermal model with two classes of models that fit the data equally well. One class of models has a temperature of kT ∼ 0.79 keV, an ionization timescale of ∼3 × 1011 cm−3 s, and marginal evidence for enhanced abundances of O and Ne, and the other has a temperature of kT ∼ 0.91 keV, an ionization timescale of ∼7 × 1010 cm−3 s, and abundances consistent with local interstellar medium values. We also performed an image analysis and find that the spatial distribution of the counts is not consistent with that of a point source. The hypothesis of a point-source distribution can be rejected at the 99.9% confidence level. Therefore, this compact feature is most likely a knot of O- and Ne-rich ejecta associated with the reverse shock.

Cool stars in the Galactic center as seen by APOGEE

The Galactic center region, including the nuclear disk, has until recently been largely avoided in chemical census studies because of extreme extinction and stellar crowding. Large, near-IR spectroscopic surveys, such as the Apache Point Observatory Galactic Evolution Experiment (APOGEE), allow the measurement of metallicities in the inner region of our Galaxy. Making use of the latest APOGEE data release (DR16), we are able for the first time to study cool Asymptotic Giant branch (AGB) stars and supergiants in this region. The stellar parameters of five known AGB stars and one supergiant star (VR 5-7) show that their location is well above the tip of the red giant branch. We studied metallicities of 157 M giants situated within 150 pc of the Galactic center from observations obtained by the APOGEE survey with reliable stellar parameters from the APOGEE pipeline making use of the cool star grid down to 3200 K. Distances, interstellar extinction values, and radial velocities were checked to confirm that these stars are indeed situated in the Galactic center region. We detect a clear bimodal structure in the metallicity distribution function, with a dominant metal-rich peak of [Fe/H] ∼ +0.3 dex and a metal-poor peak around {Fe/H] = −0.5 dex, which is 0.2 dex poorer than Baade’s Window. The α-elements Mg, Si, Ca, and O show a similar trend to the Galactic bulge. The metal-poor component is enhanced in the α-elements, suggesting that this population could be associated with the classical bulge and a fast formation scenario. We find a clear signature of a rotating nuclear stellar disk and a significant fraction of high-velocity stars with vgal > 300 km s−1; the metal-rich stars show a much higher rotation velocity (∼200 km s−1) with respect to the metal-poor stars (∼140 km s−1). The chemical abundances as well as the metallicity distribution function suggest that the nuclear stellar disk and the nuclear star cluster show distinct chemical signatures and might be formed differently.

Disc and wind in black hole X-ray binary MAXI J1820+070 observed through polarized light during its 2018 outburst

ABSTRACT We describe the first complete polarimetric data set of the entire outburst of a low-mass black hole X-ray binary system and discuss the constraints for geometry and radiative mechanisms it imposes. During the decaying hard state, when the optical flux is dominated by the non-thermal component, the observed polarization is consistent with the interstellar values in all filters. During the soft state, the intrinsic polarization of the source is small, ∼0.15 per cent in B and V filters, and is likely produced in the irradiated disc. A much higher polarization, reaching ∼0.5 per cent in V and R filters, at a position angle of ∼25○ observed in the rising hard state coincides in time with the detection of winds in the system. This angle coincides with the position angle of the jet. The detected optical polarization is best explained by scattering of the non-thermal (hot flow or jet base) radiation in an equatorial wind.

Spectral Classification and Estimation of Distances to the Be/X-Ray Binaries 1H1936+541 and 1H2202+501

The results of the spectral and photometric studies of two Be/X-ray binaries, 1H1936+541 and 1H2202+501, are presented. The spectral class of the optical component of the 1H1936+541 system was determined for the first time, and the spectral class of the optical component of the 1H2202+501 system was refined. According to the results, the optical components of the systems 1H1936+541 and 1H2202+501 are Be stars of the spectral types B1Ve and B3Ve, respectively. These findings, as well as the well-known spectral type distribution of Be stars, can testify to the fact that the object 1H1936+541 belongs to X-ray binary systems. At the same time, taking into account the fact that a very small number of Be/X-ray binaries of the spectral type B3 is known so far, the object 1H2202+501 cannot unambiguously be classified as a Be/X-ray binary. The rotational speeds of the main components of these systems are 246 ± 11 km/s for 1H1936+541 and 111 ± 8 km/s for1H2202+501; the range of the tilt angles of their rotation axes is within 49°–82° and 21°–28°, respectively. Additionally, using the B and V band photometric observations and spectral classes obtained, the interstellar extinction values E(В – V), as well as the distances to the objects, were estimated. In particular, for 1H2202+501, the value of E(В – V) is 0.36 ± 0.03m, and the distance r ranges from 0.8 to 1.6 kpc. For the 1H1936+541 system, the following values were obtained: E(В – V) = 0.23 ± 0.03m and r = 2.1–3.6 kpc. The comparison of the distances to the objects with the data from the GAIA DR2 catalog ( http://gea.esac.esa.int/archive/ ) revealed the coincidence of the corresponding values within the limits of errors.

LBV Candidates in M31 and M33. Overview of 20 Years of the 6-m Telescope: Observations and Results

An ongoing SAO RAS program for the 6-m telescope to search for and study luminous blue variable (LBV) candidates in the M31 and M33 galaxies was started in 1997. This paper summarizes the results of our observations over the last 20 years. Over this time, we have a general lists of candidates and obtained photometry and spectroscopy for approximately one hundred stars from the list. A detailed study and classification of LBV candidates were carried out.We alsomonitored spectrally and photometrically the confirmed LBV stars in these galaxies. Based on these results, the spectral energy distributions (SEDs) of LBVs and LBV candidates were computed, and the interstellar extinction values, temperature, and luminosity were derived. We propose a new method of confirming the nature of LBV stars. It is based on a special property exhibited only by LBV stars—the constancy of the bolometric luminosity, with except of a rare phase of giant eruption, registered for two historical LBVs in our galaxy and about few dozen extragalactic LBVs. Using this method, we determine the fundamental parameters of these stars: temperature, radius, luminosity, and interstellar extinction. We discuss the nature of several individual LBV stars discovered by us in M33 and the classification of the LBV candidates in M31. Thus, for 20 years of observations we have found four LBV stars in M33; in the M31 galaxy with different methods, we have confirmed seven objects as a LBV class.

Evidence for a metal-poor population in the inner Galactic bulge

The inner Galactic Bulge has, until recently, been avoided in chemical evolution studies due to extreme extinction and stellar crowding. Large, near-IR spectroscopic surveys, such as APOGEE, allow for the first time the measurement of metallicities in the inner region of our Galaxy. We study metallicities of 33 K/M giants situated in the Galactic Center region from observations obtained with the APOGEE survey. We selected K/M giants with reliable stellar parameters from the APOGEE/ASPCAP pipeline. Distances, interstellar extinction values, and radial velocities were checked to confirm that these stars are indeed situated in the inner Galactic Bulge. We find a metal-rich population centered at [M/H] = +0.4 dex, in agreement with earlier studies of other bulge regions, but also a peak at low metallicity around $\rm [M/H] = -1.0\,dex$, suggesting the presence of a metal-poor population which has not previously been detected in the central region. Our results indicate a dominant metal-rich population with a metal-poor component that is enhanced in the $\alpha$-elements. This metal-poor population may be associated with the classical bulge and a fast formation scenario.

Neutral and Ionized Hydrides in Star-Forming Regions. Observations with Herschel/HIFI

The cosmic abundance of hydrides depends critically on high-energy UV, X-ray, and particle irradiation. Here we study hydrides in star-forming regions where irradiation by the young stellar object can be substantial, and density and temperature can be much enhanced over interstellar values. Lines of OH, CH, NH, and SH and their ions OH(+), CH(+), NH(+), SH(+), H2O(+), and H3O(+) were observed in star-forming regions by the HIFI spectrometer onboard the Herschel Space Observatory. Molecular column densities are derived from observed ground-state lines, models, or rotational diagrams. We report here on two prototypical high-mass regions, AFGL 2591 and W3 IRS5, and compare them to chemical calculations by making assumptions on the high-energy irradiation. A model assuming no ionizing protostellar emission is compared with (i) a model assuming strong protostellar X-ray emission and (ii) a two-dimensional (2D) model including emission in the far UV (FUV, 6-13.6 eV), irradiating the outflow walls that separate the outflowing gas and infalling envelope material. We confirm that the effect of FUV in two-dimensional models with enlarged irradiated surfaces is clearly noticeable. A molecule that is very sensitive to FUV irradiation is CH(+), enhanced in abundance by more than 5 orders of magnitude. The HIFI observations of CH(+) lines agree with the two-dimensional FUV model by Bruderer et al., which computes abundances, non-LTE excitation, and line radiative transfer.20 It is concluded that CH(+) is a good FUV tracer in star-forming regions. The effect of potential X-ray irradiation is not excluded but cannot be demonstrated by the present data.

Ionization within the Local Cavity by Hot White Dwarfs

ABSTRACTWe present estimates of the contribution from 33 nearby (d < 100 pc) hot DA white dwarfs to the total ionization of the Local Cavity. The EUV spectra of these white dwarfs have been derived using non-LTE stellar atmosphere models (with the inclusion of metals where necessary) to calculate the incident flux at the Earth using up-to-date distances and interstellar column densities. Our model spectra give a very good approximation to the spectra obtained with observations from the NASA Extreme Ultraviolet Explorer satellite. The number of Lyman continuum photons per second emitted by each of the white dwarfs for wavelengths over the 30–912 Å range was then derived. Using these numbers, the respective sizes of the ionized Strömgren spheres of HII were then calculated for each star under the assumption of ambient interstellar electron density values of ne = 0.04 cm-3 and 0.10 cm-3. The location and sizes of these ionized regions have been plotted on a 3D map of the neutral absorption contours of the local ISM, as derived for both in-plane and out of plane directions. A similar procedure was also carried out for the determination of the sizes of the ionized HeII regions surrounding the same 33 white dwarfs. We conclude that only a few of the very hottest DA white dwarfs located within the Local Cavity can possess appreciably sized ionized Strömgren spheres of HII and HeII. The size of these ionized regions can potentially range between 20 pc and 40 pc in diameter, dependent on the ambient value of the electron density of the interstellar medium. However, the volumetric filling factor of these ionized spheres within the Local Cavity is small (∼6%), even at the lowest value of electron density considered in this study (i.e., ne = 0.04 cm-3). Thus, the contribution from DA white dwarfs to the ionization of both the local interstellar cloud complex and other clouds located within the Local Cavity is of secondary importance compared to the ionization contribution provided by the two early B-type stars of ϵ and β CMa.

An old disk still capable of forming a planetary system

From the masses of the planets orbiting the Sun, and the abundance of elements relative to hydrogen, it is estimated that when the Solar System formed, the circumstellar disk must have had a minimum mass of around 0.01 solar masses within about 100 astronomical units of the star. (One astronomical unit is the Earth-Sun distance.) The main constituent of the disk, gaseous molecular hydrogen, does not efficiently emit radiation from the disk mass reservoir, and so the most common measure of the disk mass is dust thermal emission and lines of gaseous carbon monoxide. Carbon monoxide emission generally indicates properties of the disk surface, and the conversion from dust emission to gas mass requires knowledge of the grain properties and the gas-to-dust mass ratio, which probably differ from their interstellar values. As a result, mass estimates vary by orders of magnitude, as exemplified by the relatively old (3-10 million years) star TW Hydrae, for which the range is 0.0005-0.06 solar masses. Here we report the detection of the fundamental rotational transition of hydrogen deuteride from the direction of TW Hydrae. Hydrogen deuteride is a good tracer of disk gas because it follows the distribution of molecular hydrogen and its emission is sensitive to the total mass. The detection of hydrogen deuteride, combined with existing observations and detailed models, implies a disk mass of more than 0.05 solar masses, which is enough to form a planetary system like our own.

RESOLVING THE CO SNOW LINE IN THE DISK AROUND HD 163296

We report Submillimeter Array (SMA) observations of CO (J=2--1, 3--2 and 6--5) and its isotopologues (13CO J=2--1, C18O J=2--1 and C17O J=3--2) in the disk around the Herbig Ae star HD 163296 at ~2" (250 AU) resolution, and interpret these data in the framework of a model that constrains the radial and vertical location of the line emission regions. First, we develop a physically self-consistent accretion disk model with an exponentially tapered edge that matches the spectral energy distribution and spatially resolved millimeter dust continuum emission. Then, we refine the vertical structure of the model using wide range of excitation conditions sampled by the CO lines, in particular the rarely observed J=6--5 transition. By fitting 13CO data in this structure, we further constrain the vertical distribution of CO to lie between a lower boundary below which CO freezes out onto dust grains (T ~ 19 K) and an upper boundary above which CO can be photodissociated (the hydrogen column density from the disk surface is ~ 10^{21} cm-2). The freeze-out at 19 K leads to a significant drop in the gas-phase CO column density beyond a radius of ~155 AU, a "CO snow line" that we directly resolve. By fitting the abundances of all CO isotopologues, we derive isotopic ratios of 12C/13C, 16O/18O and 18O/17O that are consistent with quiescent interstellar gas-phase values. This detailed model of the HD 163296 disk demonstrates the potential of a staged, parametric technique for constructing unified gas and dust structure models and constraining the distribution of molecular abundances using resolved multi-transition, multi-isotope observations.

Formation of Jupiter using opacities based on detailed grain physics

Numerical simulations, based on the core-nucleated accretion model, are presented for the formation of Jupiter at 5.2 AU in three primordial disks with three different assumed values of the surface density of solid particles. The grain opacities in the envelope of the protoplanet are computed using a detailed model that includes settling and coagulation of grains and that incorporates a recalculation of the grain size distribution at each point in time and space. We generally find lower opacities than the 2% of interstellar values used in previous calculations (Hubickyj, O., Bodenheimer, P., Lissauer, J.J. [2005]. Icarus 179, 415–431; Lissauer, J.J., Hubickyj, O., D’Angelo, G., Bodenheimer, P. [2009]. Icarus 199, 338–350). These lower opacities result in more rapid heat loss from and more rapid contraction of the protoplanetary envelope. For a given surface density of solids, the new calculations result in a substantial speedup in formation time as compared with those previous calculations. Formation times are calculated to be 1.0, 1.9, and 4.0 Myr, and solid core masses are found to be 16.8, 8.9, and 4.7 M ⊕, for solid surface densities, σ, of 10, 6, and 4 g cm −2, respectively. For σ = 10 and σ = 6 g cm −2, respectively, these formation times are reduced by more than 50% and more than 80% compared with those in a previously published calculation with the old approximation to the opacity.

Exploring the conditions required to form giant planets via gravitational instability in massive protoplanetary discs

We carry out global three-dimensional radiation hydrodynamical simulations of self-gravitating accretion discs to determine if, and under what conditions, a disc may fragment to form giant planets. We explore the parameter space (in terms of the disc opacity, temperature and size) and include the effect of stellar irradiation. We find that the disc opacity plays a vital role in determining whether a disc fragments. Specifically, opacities that are smaller than interstellar Rosseland mean values promote fragmentation (even at small radii, R < 25AU) since low opacities allow a disc to cool quickly. This may occur if a disc has a low metallicity or if grain growth has occurred. With specific reference to the HR 8799 planetary system, given its star is metal-poor, our results suggest that the formation of its imaged planetary system could potentially have occurred by gravitational instability. We also find that the presence of stellar irradiation generally acts to inhibit fragmentation (since the discs can only cool to the temperature defined by stellar irradiation). However, fragmentation may occur if the irradiation is sufficiently weak that it allows the disc to attain a low Toomre stability parameter.

CCD BV and 2MASS photometric study of the open cluster NGC 1513

We present CCD BV and JHK s 2MASS photometric data for the open cluster NGC 1513. We observed 609 stars in the direction of the cluster up to a limiting magnitude of V∼19 mag. The star-count method showed that the centre of the cluster lies at α 2000=04 h 09 m 36 s , δ 2000=49°28′43″ and its angular size is r=10 arcmin. The optical and near-infrared two-colour diagrams revealed the colour excesses in the direction of the cluster as E(B−V)=0.68±0.06, E(J−H)=0.21±0.02 and E(J−K s )=0.33±0.04 mag. These results are consistent with normal interstellar extinction values. Optical and near-infrared Zero Age Main-Sequences (ZAMS) provided an average distance modulus of (m−M)0=10.80±0.13 mag, which can be translated into a distance of 1440±80 pc. Finally, using Padova isochrones we determined the metallicity and age of the cluster as Z=0.015±0.004 ([M/H]=−0.10±0.10 dex) and log (t/yr)=8.40±0.04, respectively.

FOUR-DIMENSIONAL TRANSPORT OF GALACTIC COSMIC RAYS IN THE OUTER HELIOSPHERE AND HELIOSHEATH

2008 marked the beginning of sunspot cycle 24 in the inner heliosphere. Intensities of galactic hydrogen and helium measured by the Voyagers in 2008 were the highest ever recorded and believed to be approaching the interstellar values. We investigate transport of galactic cosmic ray (GCR) protons in the three-dimensional, asymmetric heliosphere, including the inner heliosheath region, by tracking stochastic phase-space trajectories of Parker equation under steady plasma background conditions. The latter is calculated from a three-dimensional MHD model of the global heliosphere that takes into account the effect of neutral hydrogen atoms. The model is applied to quiet solar wind (SW) conditions appropriate for the 2008-2009 solar minimum. Model-derived cosmic-ray spectra and radial gradients are reviewed in the context of Voyager observations in the heliosheath. It is shown that the heliosheath is an important modulation barrier for lower energy ions. Radial cosmic-ray gradients in the heliosheath are expected to be small in the directions of the Voyagers (1.5%-1.8% per AU at 180 MeV). In our model the termination shock does not accelerate GCR ions very efficiently, and their intensities in the heliosheath never exceed interstellar values. Analysis of cosmic-ray residence times in different parts in the heliosphere shows that, prior to their detection, ions spend 3-6 times longer transiting the heliosheath and the heliotail than they spend in the supersonic SW.

Parameters of galactic early B supergiants

Context. The interstellar extinction, E(B - V), of OB supergiants is usually derived from the observed color index, (B - V), in comparison with the intrinsic one. This method works properly only if the stellar wind does not influence the optical continuum emission. Over the past years, many OB supergiants have been found to have wind-velocity distributions with rather high β values; i.e., their winds are only slowly accelerating, resulting in relative density enhancements within the wind-continuum forming region. It has been found that these kinds of winds might indeed influence the total continuum emission even at optical wavelengths. Aims. We investigate the influence of the wind on the observed color indices of OB supergiants, in order to distinguish between interstellar and circumstellar extinction contributions. Methods. We first tested the influence of the wind on the UBV band fluxes of a model O supergiant star with artificial winds of different β values and a fixed interstellar extinction value. From the UBV magnitudes of these systems, the apparent interstellar extinction was then derived by calculating the values of E(B - V), from the (B - V) as well as from the (U - B) color index. Then we turned to a sample of galactic early-type B supergiants for which the stellar and wind parameters are known. All except one of these stars have β values higher than 1.0, which makes this set the most suitable one for our investigation. We calculated each star's wind contribution to the BV band fluxes. The observed magnitudes were corrected for the wind contributions, and the interstellar extinctions were derived and compared with those derived purely from the (B - V) color index. Results. From our model supergiant, we find that with increasing β the wind starts to influence the observable color indices. The wavelength dependence of the wind contribution is thereby different from that of the interstellar extinction contribution. Thus, the interstellar extinction values derived from the two color indices of our reddened star plus wind systems disagree. This effect is stronger for higher β. In addition, E(B ― V) derived from the (B ― V) color index always overestimates the real interstellar extinction. This trend is also found for the investigated B supergiant sample. Consequently, the luminosities of these stars are systematically overestimated. Our wind model always computes lower limits to the real wind contributions. This means that the real interstellar extinction values, hence the stellar luminosities of the studied B supergiant sample, might well be lower.