Cyg OB2 Research Articles

Chemical Abundances of OB Stars with High Projected Rotational Velocities

Elemental abundances of carbon, nitrogen, oxygen, magnesium, aluminum, and silicon are presented for a sample of 12 rapidly rotating OB star (v sin i > 60 km s-1) members of the Cep OB2, Cyg OB3, and Cyg OB7 associations. The abundances are derived from spectrum synthesis, using both LTE and non-LTE calculations. As found in almost all previous studies of OB stars, the average abundances are slightly below solar, by about 0.1 to 0.3 dex. In the case of oxygen, even with the recently derived low solar abundances, the OB stars are closer to, but still below, the solar value. Results for the nine Cep OB2 members in this sample can be combined with results published previously for eight Cep OB2 stars with low projected rotational velocities to yield the most complete set of abundances, to date, for this particular association. These abundances provide a clear picture of both the general chemical and individual stellar evolution that has occurred within this association. By placing the Cep OB2 stars studied in an HR-diagram we identify the presence of two distinct age subgroups, with both subgroups having quite uniform chemical abundances. Two stars are found in the older subgroup that show significant N/O overabundances, with both stars being two of the most massive, the most evolved, and the most rapidly rotating of the members studied in Cep OB2. These characteristics of increased N abundances being tied to high mass, rapid rotation, and an evolved phase are those predicted from models of rotating stars that undergo rotationally driven mixing.

Interstellar C$_\mathsf{2}$ and CN toward the Cyg OB2 association

An analysis of deep optical echelle spectra towards six stars in the Cyg OB2 (VI Cygni) association is presented. Interstellar absorption lines up to J 00 = 18 in the (2,0) and (3,0) bands of the C2 A 1 u {X 1+ system are detected towards Cyg OB2 No. 12. The large number of rotational lines accurately constrains the gas-kinetic temperature T and the density n to T =3 5 K andn = (600 100) cm 3 . The inferred C2 column density is N(C2 )=( 20 +4 2 )1 0 13 cm 2 . The detection of various lines in the (1,0) and (2,0) band of the CN A 2 u {X 2+ red system suggest a column density of N (CN) =( 8 13) 10 13 cm 2 .C 2 absorption lines are also detected towards Cyg OB2 No. 5 and No. 9. Inferred parameters are T =5 0 K,n = (600 200) cm 3 , N(C2) = (10 +3:5 1:5) 10

Chemical Abundances of OB Stars in Five OB Associations

We present LTE abundances of magnesium, aluminum, sulfur, and iron and non-LTE abundances of carbon, nitrogen, oxygen, and silicon for a sample of 15 slowly rotating B stars belonging to five OB associations: Cyg OB3, Cyg OB7, Lac OB1, Vul OB1, and Cep OB3. These OB associations lie on the Galactic plane and are situated within 3 kpc of the Sun. Of the eight elements sampled, non-LTE abundances for C, N, O, and Si, as well as LTE abundances for Al and Fe, generally show subsolar abundances, with typical underabundances of ~0.2-0.4 dex. The LTE abundances for Mg and S tend to fall closer to solar values in the five associations. Whether the somewhat larger abundances derived for Mg and S, relative to the other six elements studied, are significantly different will require further work, while the modest, but persistent, underabundances (relative to solar) found for the other elements confirm a number of previous studies of young disk OB stars lying relatively near to the Sun. The five associations studied here do not span a significant range of Galactocentric distances; however, their derived abundances agree with what would be expected based upon previous studies that have mapped abundance versus Galactocentric distance and measured abundance gradients in the Milky Way disk.

Circumstellar Carbonaceous Material Associated with Late-Type Dusty WC Wolf-Rayet Stars

We have studied the 5-8.5 μm infrared spectra of the late-type Wolf-Rayet stars WR 118, WR 112, and WR 104, the WN star WR 147, the B5 hypergiant Cygnus OB2 No. 12, and the Galactic center luminous blue variable Pistol Star using the Short Wavelength Spectrometer on the Infrared Space Observatory. We attribute an absorption feature at 6.2 μm in the spectra of WC stars to amorphous carbon dust. This absorption feature is not detected in the diffuse interstellar medium toward the WR 147, Cyg OB2 No. 12, or the Pistol Star, and therefore we suggest that it is circumstellar in nature. In addition, we detect a broad absorption feature extending from approximately 6.5 to 8 μm. We tentatively attribute this absorption to the C–C stretching modes that accompany the 6.2 μm band in aromatic materials. Our analysis of the 6.2 μm absorption profile suggests that the dust grains have to be rather large (~1 μm) and point toward dense clumps as the sites of dust formation.

Observational Constraints on the Abundance and Evolution of “XCN” in Interstellar Grain Mantles

The 4.62 μm "XCN" absorption feature, attributed to CN-bearing molecules in solids, is potentially an important diagnostic of the evolution of organic matter in the interstellar medium and the envelopes of newly formed stars. We report quantitative limits on the strength of this feature in the diffuse interstellar medium toward the reddened B-type star Cyg OB2 No. 12 and in the dark cloud toward the young stellar object R CrA IRS 2. On the basis of an assumed band strength for the carrier species, we estimate that less than 0.3% and less than 0.1% of the elemental nitrogen is in CN bonds along these lines of sight, respectively; if they are typical of diffuse and dense environments, it follows that the carrier of XCN is no more than a trace constituent of either organic-refractory or icy interstellar grain mantles. Appreciable XCN abundances seem to occur only in the envelopes of certain young stellar objects (YSOs), most notably the high-mass objects W33A and AFGL 7009S. We confirm the presence of XCN in the spectrum of the low-mass YSO R CrA IRS 7. The strengths of the XCN absorptions in R CrA IRS 7 and other low-mass YSOs indicate mean XCN concentrations relative to H2O in the ices of ~1%, comparable with the abundance of CN-bearing species in comets.

An analysis of HST UV spectra of Cyg OB2 stars

As a first step of a vigorous program to investigate the Wind Momentum - Luminosity Relationship (WLR) of Galactic O-stars by analyzing stars belonging to the same cluster we present UV HST observations of six supergiants and one giant in the Cyg OB2 association. Terminal and turbulent wind velocities, velocity laws and metal ion column densities are derived and mean ionization fractions are estimated. Turbulent velocities are mostly in the range 10-14% of $v_\infty$. The terminal velocities agree well with the average $v_\infty$ vs. spectral class relationship compiled by Kudritzki & Puls ([CITE]). We compare the observed $v_\infty$ vs. escape velocity (depending on the diagnostics of the stellar mass) correlation with the predictions of the radiatively driven wind theory and find better agreement with the spectroscopic masses rather than with the evolutionary ones. The β velocity field exponents are in the range 0.7-0.8, without any trend towards larger values. We show that for a single luminosity class there is a tight relationship between T eff and $ $ (the mean density at the point in the stellar wind, where half the wind terminal velocity is reached). In consequence, the ionization fractions show the same trend with both, T eff and $ $: we find that Nv increases with T eff , Siiv decreases and Civ does not clearly correlate. As a byproduct, we also derive interestellar Hi column densities towards Cyg OB2, which turn out to be quite large. For one object (Cyg OB2$\#$2) we find inconsistencies making the association membership questionable.

Can the gamma-ray source 3EG J2033+4118 be produced by the stellar system Cygnus OB2 No. 5?

We discuss the possibility that the stellar system Cyg OB2 No. 5 can be a gamma-ray source in the light of recent EGRET and radio data. This system is formed by an O7 Ia + Ofpe/WN9 contact binary. A third star, probably a B0 V star also associated with the system, is located at ∼ 1700 AU from the primary. We estimate the expected gamma-ray luminosity from the colliding winds region, the terminal shock of the wind, and the unstable zone at the base of the wind, and conclude that, under very reasonable assumptions, Cyg OB2 No. 5 can generate about a half of the gamma-ray flux detected from the positionally coincident source 3EG J2033+4118. We suggest, then, that other O stars belonging to the association, also placed within the 95% probability EGRET location contour, could contribute to the observed gamma-ray flux.

ROSAT HRI observations of P Cyg and surrounding area

We present a detailed analysis of deep ROSAT HRI observations of the luminous blue variable P Cyg and its surrounding radio nebula. The HRI image provides a point source at the position of P Cyg. However, we show that this emission can be attributed to the ultraviolet leak of the ROSAT HRI. The X-ray flux upper limit derived from the HRI data of this star is discussed in the context of X-ray emission from hot stars. Furthermore, we present a search for diffuse X-ray emission possibly associated with the radio nebula surrounding P Cyg. We compare our results to model predictions and X-ray fluxes observed for shells around other hot stars. Additionally, we detect 10 X-ray sources in the field of view. All but one of these X-ray emitters have stellar counter parts in the Palomar Sky Survey. We suggest that they are active late-type stars possibly belonging to Cyg OB1.

Observations of Diffuse Interstellar Bands Attributed to C \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $^{-}_{7}$ \end{document}

Recent advances in laboratory gas-phase spectroscopy of large molecules and their ions permit a direct comparison between the diUuse interstellar bands (DIBs) and proposed carriers. On the basis of gas-phase data, Tulej et al. recently suggested that —ve electronic transitions of the linear A2% u ^ X2% g carbon-chain anion match with DIBs. We have obtained high-resolution visible spectra of four C 7 reddened stars (HD 46711, HD 50064, HD 183143, and Cyg OB2 12) to make a detailed comparison with the laboratory data. Our data show that three of the bands at 6270.2 at 6064.0 C 7

Spectroscopic analysis of OB stars in Cygnus OB2

We obtain the stellar parameters of eleven OB stars in Cyg OB2 using model atmospheres. Each of the analysed stars shows an enhanced Helium abundance. We releate this is to the young agae of the OB association and/or the low rotational velocity of the stars.

A [ITAL]Hipparcos[/ITAL] Census of the Nearby OB Associations

A comprehensive census of the stellar content of the OB associations within 1 kpc from the Sun is presented, based on Hipparcos positions, proper motions, and parallaxes. It is a key part of a long-term project to study the formation, structure, and evolution of nearby young stellar groups and related star-forming regions. OB associations are unbound moving groups, which can be detected kinematically because of their small internal velocity dispersion. The nearby associations have a large extent on the sky, which traditionally has limited astrometric membership determination to bright stars (V 6 mag), with spectral types earlier than ~B5. The Hipparcos measurements allow a major improvement in this situation. Moving groups are identified in the Hipparcos Catalog by combining de Bruijne's refurbished convergent point method with the Spaghetti method of Hoogerwerf & Aguilar. Astrometric members are listed for 12 young stellar groups, out to a distance of ~650 pc. These are the three subgroups Upper Scorpius, Upper Centaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as Vel OB2, Tr 10, Col 121, Per OB2, α Persei (Per OB3), Cas–Tau, Lac OB1, Cep OB2, and a new group in Cepheus, designated as Cep OB6. The selection procedure corrects the list of previously known astrometric and photometric B- and A-type members in these groups and identifies many new members, including a significant number of F stars, as well as evolved stars, e.g., the Wolf-Rayet stars γ2 Vel (WR 11) in Vel OB2 and EZ CMa (WR 6) in Col 121, and the classical Cepheid δ Cep in Cep OB6. Membership probabilities are given for all selected stars. Monte Carlo simulations are used to estimate the expected number of interloper field stars. In the nearest associations, notably in Sco OB2, the later-type members include T Tauri objects and other stars in the final pre–main-sequence phase. This provides a firm link between the classical high-mass stellar content and ongoing low-mass star formation. Detailed studies of these 12 groups, and their relation to the surrounding interstellar medium, will be presented elsewhere. Astrometric evidence for moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, Cam OB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive. OB associations do exist in many of these regions, but they are either at distances beyond ~500 pc where the Hipparcos parallaxes are of limited use, or they have unfavorable kinematics, so that the group proper motion does not distinguish it from the field stars in the Galactic disk. The mean distances of the well-established groups are systematically smaller than the pre-Hipparcos photometric estimates. While part of this may be caused by the improved membership lists, a recalibration of the upper main sequence in the Hertzsprung-Russell diagram may be called for. The mean motions display a systematic pattern, which is discussed in relation to the Gould Belt. Six of the 12 detected moving groups do not appear in the classical list of nearby OB associations. This is sometimes caused by the absence of O stars, but in other cases a previously known open cluster turns out to be (part of) an extended OB association. The number of unbound young stellar groups in the solar neighborhood may be significantly larger than thought previously.

X‐Ray and Radio Observations of the Cygnus OB2 Association

Several OB stars in the Cygnus OB2 association are among the strongest stellar X-ray and radio sources in the Galaxy. The radio emission is particularly unusual, displaying a high level of variability and nonthermal behavior. We obtained two ROSAT PSPC observations, a ROSAT HRI observation, and three VLA observations of the association during a 2 yr time span. Our study will focus on four stars, Cyg OB2 No. 5, No. 8A, No. 9, and No. 12. Three of the four (Cyg No. 5, No. 9, and No. 12) were relatively constant in their X-ray emission over the 2 yr time frame. The fourth, Cyg OB2 No. 8A, increased in intensity by ~34%. No short-term (hourly) variability was detected. The observed X-ray characteristics (e.g., luminosity, temperature) are found to be consistent with the X-ray properties of other OB stars. The exception is Cyg OB2 No. 12, whose X-ray characteristics are found to be inconsistent with its spectral classification. Detailed spectral analyses of the PSPC data are presented for two absorption models: (1) ISM (cold absorber) and (2) Wind + ISM (warm absorber). The spectral fits suggest that the X-ray sources are located within the stellar wind, and estimates of the X-ray locations are presented. Adopting the radio-derived mass-loss rates, these X-ray locations are found to be consistent with the shock scenario proposed for OB stars. As expected, the radio emission has continued to be highly variable. Nonthermal characteristics are observed in Cyg OB2 No. 8A and No. 12. One of the most unusual nonthermal radio sources, Cyg OB2 No. 9, was found to be thermal in one of our observations. An observation of Cyg OB2 No. 5 also displayed a thermal radio spectrum. A comparison of the observed and intrinsic X-ray fluxes with the observed radio fluxes suggests that these quantities are anticorrelated; the strongest X-ray source is the weakest radio source. This is contrary to normal expectations for a wind-generated model of X-ray and radio emission. We investigate the long-term temporal behavior of both the X-ray and radio emission by comparing our newly acquired data with the previous X-ray (IPC) and radio data over the past 15 yr. Except for three events observed in Cyg OB2 No. 5, which displayed significant increases in its X-ray emission, the X-ray emission has remained relatively constant with a variability level less than 20% over this time span, whereas the radio emission has stayed highly variable with various levels of nonthermal behavior. We present a model to investigate the case in which the X-ray and radio emission are controlled by stellar wind properties and find that the predicted variability should be comparable in both emission processes. This is not observed. It is very intriguing that every time we observe the radio emission, it is different, whereas the X-ray emission always appears to be constant. If the stellar wind is as variable as suggested by the radio data, we believe it is highly implausible that we just happened to miss all periods of X-ray variability. We also investigate the implications of the observed nonthermal radio spectrum of Cyg OB2 No. 8A. Using the synchrotron emission model of White, we find that this nonthermal radio spectrum predicts a mass-loss rate almost 2 orders of magnitude less than that expected for a thermal radio spectrum. This lower mass-loss rate is consistent with an X-ray source located at the base of the stellar wind, contrary to the basic shock scenario. Since these stars show evidence of changing from thermal to nonthermal radio characteristics, it is difficult to understand how such a large change in mass loss did not produce a significant change in the observed X-rays.

The 10- m profile of molecular-cloud and diffuse ISM silicate dust

High signal-to-noise ratio spectra are presented of the 10-μm silicate absorption feature in lines of sight towards Elias 16 and 18 in the Taurus dark cloud, and towards the heavily reddened supergiant Cyg OB2 no. 12. The observations are fitted with laboratory and astronomical spectra to produce intrinsic absorption profiles. These features, which represent molecular-cloud and diffuse ISM dust respectively, are better fitted with emissivity spectra of the Trapezium and μ Cephei than they are with those of laboratory, terrestrial, or other observations of circumstellar silicates. The difference in width between the silicate band in the two environments can be almost entirely ascribed to a broad excess absorption in the long-wavelength wing of the profiles, which is much stronger in the molecular-cloud lines of sight, and possibly reflects grain growth in the denser environment. Limits are placed on the strength of fine spectral structure; if there is a crystalline silicate component in these spectra, it is most likely to be serpentine. Column-density upper limits for methanol and the photolysis product hexamethylenetetramine (HMT) are less than a few per cent of those of water ice and silicates.

Spectroscopic Analysis of Stars in Cyg OB2

Spectroscopic Analysis of Stars in Cyg OB2

Infrared Spectroscopy of Dust in the Diffuse Interstellar Medium toward Cygnus OB2 No. 12

Observations made with the short-wavelength spectrometer of the Infrared Space Observatory are used to investigate the composition of interstellar dust in the line of sight to Cygnus OB2 No. 12, commonly taken as representative of the diffuse (low-density) interstellar medium. Results are compared with data for the Galactic center source Sgr A*. Nondetections of the 3.0 and 4.27 μm features of H2O and CO2 ices in Cyg OB2 No. 12 confirm the absence of dense molecular material in this line of sight, whereas the presence of these features in Sgr A* indicates that molecular clouds may contribute as much as 10 mag of visual extinction toward the Galactic center. The spectrum of Cyg OB2 No. 12 is dominated by the well-known 9.7 μm silicate feature; detection of a shallow feature near 2.75 μm indicates that the silicates are at least partially hydrated, with composition possibly similar to that of terrestrial phyllosilicates such as serpentine or chlorite. However, the 2.75 μm feature is not seen in the Galactic center spectrum, suggesting that silicates in this line of sight are less hydrated or of different composition. The primary spectral signatures of C-rich dust in the diffuse ISM are weak absorptions at 3.4 μm (the aliphatic C=H stretch) and 6.2 μm (the aromatic C=C stretch). We conclude, based on infrared spectroscopy, that the most probable composition of the dust toward Cyg OB2 No. 12 is a mixture of silicates and carbonaceous solids in a volume ratio of approximately 3:2, with the carbonaceous component primarily in an aromatic form such as amorphous carbon.

Hipparcos, VLA, and CCD Observations of Cygnus OB2 No. 5: Solving the Mysteryof the Radio “Companion”

We present accurate optical and radio astrometry for the unresolved contact binary system Cyg OB2 No. 5 and its weaker component to the northeast. While the radio and optical positions of the primary component (taken to be the unresolved contact binary) agree within observational error (~70 mas), we find that the weaker radio component does not agree in position with the second optical component of the system, but falls in between the two optical components. We conclude that the weaker radio component of this system is not associated with the secondary star but appears to be synchrotron emission produced at the shock interaction zone between the winds of the stars. Differential B- and V-band CCD and Hipparcos photometry was used to derive the approximate spectral type (B0 V-B2 V) of the secondary. Ram pressure arguments in the colliding-wind model give a mass-loss rate for the secondary star consistent with the expected one, considering the earliest spectral type.

Neutral Hydrogen Observations of the Three Wolf-Rayet Stars and one Supernova Remnant Towards the CYG OB1 Association

Neutral Hydrogen Observations of the Three Wolf-Rayet Stars and one Supernova Remnant Towards the CYG OB1 Association

The Cygnus X region XXI. Radio emission at 1420 MHz from Cyg OB2 stars and two probable cometary HII regions

AbstractWe have searched our previously published radio surveys of the Cygnus X region for faint radio point sources that may be associated with luminous stars of the Cyg OB2 association. Five positional coincidences have been found between stars and 1420 MHz radio sources. A particularly interesting example is the Wolf‐Rayet star VCLS 146, which has shown a rapid change in 1420 MHz flux density. In addition, sensitive upper limits have been derived for the emission from 14 early‐type stars, which help establish the time history of their non‐thermal radio emission. Two radio features have been detected which have the properties of cometary HII regions, except that they are several arcminutes in size. Their detection provides evidence of recent star formation in Cyg OB2.

The Cygnus X region XXI. Radio emission at 1420 MHz from Cyg OB2 stars and two probable cometary HII regions

Astronomische NachrichtenVolume 317, Issue 2 p. 159-159 ErratumFree Access The Cygnus X region XXI. Radio emission at 1420 MHz from Cyg OB2 stars and two probable cometary HII regions H. J. Wendker, H. J. Wendker Hamburger Sternwarte Hamburg, GermanySearch for more papers by this authorL. A. Higgs, L. A. Higgs Dominion Radio Astrophysical Observatory, Penticton, B.C., CanadaSearch for more papers by this authorT. L. Landecker, T. L. Landecker Dominion Radio Astrophysical Observatory, Penticton, B.C., CanadaSearch for more papers by this author H. J. Wendker, H. J. Wendker Hamburger Sternwarte Hamburg, GermanySearch for more papers by this authorL. A. Higgs, L. A. Higgs Dominion Radio Astrophysical Observatory, Penticton, B.C., CanadaSearch for more papers by this authorT. L. Landecker, T. L. Landecker Dominion Radio Astrophysical Observatory, Penticton, B.C., CanadaSearch for more papers by this author First published: 1996 https://doi.org/10.1002/asna.2113170214AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume317, Issue21996Pages 159-159 RelatedInformation

The Initial Mass Function and Massive Star Evolution in the OB Associations of the Northern Milky Way

We investigate the massive star content of Milky Way clusters and OB associations in order to answer three questions: (1) How coeval is star formation? (2) How constant is the initial mass function (IMF)? (3) What is the progenitor mass of Wolf-Rayet stars? Our sample includes NGC 6823/Vul OB1, NGC 6871/Cyg OB3, Berkeley 86/Cyg OB1, NGC 6983/Cyg OB1, NGC 7235, NGC 7380/Cep OB1, Cep OB5, IC 1805/Gas OB6, NGC 1893/Aug OB2, and NGC 2244/Mon OB2. Large-field CCD imaging and multiobject, fiber spectroscopy has resulted in UBV photometry for >14000 stars and new spectral types for approximate to 200 stars. These data are used to redetermine distances and reddenings for these regions and to help exclude probable nonmembers in constructing the H-R diagrams. We reanalyze comparable data previously published on Cyg OB2, Tr 14/16, and NGC 6611 and use all of these to paint a picture of star formation and to measure the IMFs. We find the following: (1) Most of the massive stars are born during a period Delta tau 7 M. A comparison with similarly studied OB associations in the Magellanic Clouds reveals no difference in IMF slope, and hence we conclude that starformation of massive stars in clusters proceeds independently of metallicity, at least between z = 0.02 and z = 0.002. The masses of the highest mass stars are approximately equal in the Milky Way, LMC, and SMC associations, contrary to the expectation that this value should vary by a factor of 3 over this metallicity range. We conclude that radiation pressure on grains must not limit the mass of the highest mass star that can form, in accord with the suggestion of Wolfire & Cassinelli that the mere existence of massive stars suggests that shocks or other mechanisms have disrupted grains in star-forming events. (3) The four Wolf-Rayet stars in our sample have come from stars more massive than 40 M.; one WC star and one late-type WN star each appear to have come from very massive (approximate to 100 M.) progenitors.