OB Stars & Stellar Bowshocks in Cygnus-X: A Novel Laboratory Estimating Stellar Mass Loss Rates

We use mid-IR images from the Spitzer Cygnus~X Legacy Survey to search for stellar bowshocks, a signature of early type "runaway" stars with high space velocities. We identify ten arc-shaped nebulae containing centrally located stars as candidate bowshocks. New spectroscopic observations of five stars show that all are late O to early B dwarfs. Our morphologically selected sample of bowshock candidates encompasses diverse physical phenomena. Three of the stars appear to be pre-main-sequence objects on the basis of rising SEDs in the mid-IR, and their nebulae may be photon-dominated regions (PDRs). Four objects have ambiguous classification. These may be partial dust shells or bubbles. We conclude that three of the objects are probable bowshocks, based on their morphological similarity to analytic prescriptions. Their nebular morphologies reveal no systematic pattern of orientations that might indicate either a population of stars ejected from or large-scale hydrodynamic outflows from Cyg OB2. The fraction of runaways among OB stars near Cyg OB2 identified either by radial velocity or bowshock techniques is ~0.5%, much smaller than the 8% estimated among field OB stars. We also obtained a heliocentric radial velocity for the previously known bowshock star, BD+43\degr3654, of -66.2+/-9.4 km/s, solidifying its runaway status and implying a space velocity of 77+/-10 km/s. We use the principles of momentum-driven bowshocks to arrive at a novel method for estimating stellar mass loss rates. Derived mass loss rates range between 10^-7 and few x10^-6 solar masses/yr for the three O5V -- ~B2V stars identified as generating bowshocks. These values are at the upper range of, but broadly consistent with, estimates from other methods. (Abridged)Comment: 49 pages, 19 figures; Accepted for publication in ApJ; full-resolution color figure version available at http://physics.uwyo.edu/~chip/Papers/CygXBowshocks; comments invite

A Study of Cyg OB2: Pointing the Way Towards Finding Our Galaxy's Super Star Clusters

New optical MK classification spectra have been obtained for 14 OB star candidates identified by Comeron et al. (2002) and presumed to be possible members of the Cyg OB2 cluster. All 14 candidate OB stars observed are indeed early-type stars, strongly suggesting the remaining 31 candidates identified by Comeron et al. are also early-type stars. However, as many as half of the new stars appear to be significantly older than the previously studied optical cluster, making their membership in Cyg OB2 suspect. Despite this, the recognition of Cyg OB2 being a more massive and extensive star cluster than previously realized, along with the recently recognized candidate super star cluster Westerlund 1 only a few kpc away (Clark & Negueruela 2002), reminds us that we are woefully under-informed about the massive cluster population in our Galaxy. Extrapolations of the locally derived cluster luminosity function indicate 10s to perhaps 100 of these very massive open clusters (Mcl ~ 10^4 M_sun, Mv ~ -11) should exist within our galaxy. Radio surveys will not detect these massive clusters if they are more than a few million years old. Our best hope for remedying this shortfall is through deep infrared searches and follow up near-infrared spectroscopic observations, as was used by Comeron et al. to locate candidate members of the Cyg OB2 association.Comment: 30 pages, 12 figures, ApJ in pres

Optical photometry and spectral classification in the field of the open cluster NGC 6996 in the North America Nebula

We present and discuss broad band CCD $UBV(I)_C$ photometry and low resolution spectroscopy for stars in the region of the open cluster NGC 6996, located in the North America Nebula. The new data allow us to tightly constrain the basic properties of this object. We revise the cluster size, which in the past has been significantly underestimated. The width of the Main Sequence is mainly interpreted in terms of differential reddening, and indeed the stars' color excess $E_{B-V}$ ranges from 0.43 to 0.65, implying the presence of a significant and evenly distributed dust component. We cross-correlate our optical photometry with near infrared from 2MASS, and by means of spectral classification we are able to build up extinction curves for an handful of bright members. We find that the reddening slope and the total to selective absorption ratio $R_V$ toward NGC 6996 are anomalous. Moreover the reddening corrected colors and magnitudes allow us to derive estimates for the cluster distance and age, which turn out to be $760 \pm 70 pc$ ($V_{0}-M_{V} = 9.4 \pm 0.2$) and $\sim 350$ Myr, respectively. Basing on our results, we suggest that NGC 6996 is located in front of the North America Nebula, and does not seem to have any apparent relationship with it.Comment: 19 pages, 12 eps figures, in press in A&

Pre-main sequence stars in the Lagoon Nebula (M8)

We report the discovery of new pre-main sequence (PMS) stars in the Lagoon Nebula (M8) at a distance of 1.25 kpc, based on intermediate resolution spectra obtained with the Boller & Chivens spectrograph at the 6.5-m Magellan I telescope (Las Campanas Observatory, Chile). According to the spectral types, the presence of emission lines and the lithium 6708A absorption line, we are able to identify 27 classical T Tauri stars, 7 weak-lined T Tauri stars and 3 PMS emission objects with spectral type G, which we include in a separated stellar class denominated "PMS Fe/Ge class". Using near-infrared photometry either from 2MASS or from our own previous work we derive effective temperatures and luminosities for these stars and locate them in the Hertzsprung-Russell diagram, in order to estimate their masses and ages. We find that almost all of our sample stars are younger than 3 10^6 years and span over a range of masses between 0.8 and 2.5 Msun. A cross-correlation between our spectroscopic data and the X-ray sources detected with the Chandra ACIS instrument is also presented.Comment: 18 pages, 15 figures, MNRAS, in pres

Spectroscopic distances of 28 nearby star candidates

28 hitherto neglected candidates for the Catalogue of Nearby Stars (CNS) were investigated to verify their classification and to improve their distance estimates. All targets had at least a preliminary status of being nearby dwarf stars based on their large proper motions and relatively faint magnitudes. Better photometric and/or spectroscopic distances were required for selecting stars which are worth the effort of trigonometric parallax measurements. Low-resolution spectra were obtained with NASPEC at the Tautenburg 2m telescope and with CAFOS at the Calar Alto 2.2m telescope. The spectral types of M-type stars were determined by direct comparison of the target's spectra with those of comparison stars of known spectral types observed with the same instrument. The classification of earlier types was done based on comparison with published spectral libraries. The majority were classified as M dwarfs including 11 stars within 25 pc. The fainter component of LDS 1365, previously thought to form a nearby common proper motion pair, is according to our results an unrelated high-velocity background star. For several other nearby common proper motion pairs our distance estimates of the fainter components are in good agreement with Hipparcos distances of the brighter components. (abridged)Comment: 5 pages, 4 figures, accepted by Astron. Astrophy

Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse Interstellar Medium through Observations of H3+

Observations of H3+ in the Galactic diffuse interstellar medium (ISM) have led to various surprising results, including the conclusion that the cosmic-ray ionization rate (zeta_2) is about 1 order of magnitude larger than previously thought. The present survey expands the sample of diffuse cloud sight lines with H3+ observations to 50, with detections in 21 of those. Ionization rates inferred from these observations are in the range (1.7+-1.3)x10^-16 s^-1<zeta_2<(10.6+-8.2)x10^-16 s^-1 with a mean value of zeta_2=(3.5^+5.3_-3.0)x10^-16 s^-1. Upper limits (3 sigma) derived from non-detections of H3+ are as low as zeta_2<0.4x10^-16 s^-1. These low upper-limits, in combination with the wide range of inferred cosmic-ray ionization rates, indicate variations in zeta_2 between different diffuse cloud sight lines. A study of zeta_2 versus N_H (total hydrogen column density) shows that the two parameters are not correlated for diffuse molecular cloud sight lines, but that the ionization rate decreases when N_H increases to values typical of dense molecular clouds. Both the difference in ionization rates between diffuse and dense clouds and the variation of zeta_2 among diffuse cloud sight lines are likely the result of particle propagation effects. The lower ionization rate in dense clouds is due to the inability of low-energy (few MeV) protons to penetrate such regions, while the ionization rate in diffuse clouds is controlled by the proximity of the observed cloud to a site of particle acceleration.Comment: 48 pages, 19 figures, 4 tables, accepted for publication in Ap

The eclipsing, double-lined, Of supergiant binary Cyg OB2-B17

Massive, eclipsing, double-lined, spectroscopic binaries are not common but are necessary to understand the evolution of massive stars as they are the only direct way to determine stellar masses. They are also the progenitors of energetic phenomena such as X-ray binaries and gamma-ray bursts. We present a photometric and spectroscopic analysis of the candidate binary system Cyg OB2-B17 to show that it is indeed a massive evolved binary. We utilise V band and white-light photometry to obtain a light curve and period of the system, and spectra at different resolutions to calculate preliminary orbital parameters and spectral classes for the components. Our results suggest that B17 is an eclipsing, double-lined, spectroscopic binary with a period of 4.0217+/-0.0004 days, with two massive evolved components with preliminary classifications of O7 and O9 supergiants. The radial velocity and light curves are consistent with a massive binary containing components with similar luminosities, and in turn with the preliminary spectral types and age of the association.Comment: 10 pages, 10 figures (1 degraded), accepted for publication in Astronomy and Astrophysic

The Distance and Size of the Red Hypergiant NML Cyg from VLBA and VLA Astrometry

We have measured the annual parallax and proper motion of NML Cyg from multiple epoch VLBA observations of the circulstellar H2O and SiO masers. The measured parallax of NML Cyg is 0.620+/-0.047 mas, corresponding to a distance of 1.61+/-0.12 kpc. The measured proper motion of NML Cyg is mu_x = -1.55+/-0.42 mas/yr eastward and mu_y= -4.59+/-0.41 mas/yr northward. Both Both the distance and proper motion are consistent with that of Cyg OB2, within their joint uncertainty, confirming their association. Taking into consideration molecular absorption signatures seen toward NML Cyg, we suggest that NML Cyg lies on the far side of the Cyg OB2 association. The stellar luminosity revised with our distance brings NML Cyg significantly below the empirical luminosity limit for a red supergiant. Using the VLA observation the radio photosphere and the SiO maser as a phase reference, we partially resolve the radio photosphere of NML Cyg at 43 GHz and find its diameter is about 44 mas, suggesting an optical stellar diameter of 22 mas, if the size of radio photosphere is 2 times the optical photosphere. Based on the position of circumstellar SiO masers relative to the radio photosphere, we estimate the absolute position of NML Cyg at epoch 2008.868 to be R.A. = 20h46m25.5382s +/- 0.0010s, Decl. = 40d06'59.379" +/- 0.015". The peculiar motions of NML Cyg, the average of stars in Cyg OB2, and four other star-forming regions rules out that an expanding "Stroemgren sphere" centered on Cyg OB2 is responsible for the kinematics of the Cygnus X region.Comment: 15 pages, 11 figures, accepted by A&

A multiwavelength investigation of the massive eclipsing binary Cyg OB2 #5

The properties of the early-type binary Cyg OB2 #5 have been debated for many years and spectroscopic and photometric investigations yielded conflicting results. We have attempted to constrain the physical properties of the binary by collecting new optical and X-ray observations. We find that the orbital period of the system slowly changes though we are unable to discriminate between several possible explanations of this trend. The best fit solution of the continuum light curve reveals a contact configuration with the secondary star being significantly brighter and hotter on its leading side facing the primary. The mean temperature of the secondary star turns out to be only slightly lower than that of the primary, whilst the bolometric luminosity ratio is found to be 3.1. The solution of the light curve yields a distance of 925 +/- 25 pc much lower than the usually assumed distance of the Cyg OB2 association. Whilst we confirm the existence of episodes of higher X-ray fluxes, the data reveal no phase-locked modulation with the 6.6 day period of the eclipsing binary nor any clear relation between the X-ray flux and the 6.7 yr radio cycle. The bright region of the secondary star is probably heated by energy transfer in a common envelope in this contact binary system as well as by the collision with the primary's wind. The existence of a common photosphere probably also explains the odd mass-luminosity relation of the stars in this system. Most of the X-ray, non-thermal radio, and possibly gamma-ray emission of Cyg OB2 #5 is likely to arise from the interaction of the combined wind of the eclipsing binary with at least one additional star of this multiple system

A Radial Velocity Survey of the Cygnus OB2 Association

We conducted a radial velocity survey of the Cygnus OB2 Association over a 6 year (1999 - 2005) time interval to search for massive close binaries. During this time we obtained 1139 spectra on 146 OB stars to measure mean systemic radial velocities and radial velocity variations. We spectroscopically identify 73 new OB stars for the first time, the majority of which are likely to be Association members. Spectroscopic evidence is also presented for a B3Iae classification and temperature class variation (B3 - B8) on the order of 1 year for Cygnus OB2 No. 12. Calculations of the intial mass function with the current spectroscopic sample yield Gamma = -2.2 +/- 0.1. Of the 120 stars with the most reliable data, 36 are probable and 9 are possible single-lined spectroscopic binaries. We also identify 3 new and 8 candidate double-lined spectroscopic binaries. These data imply a lower limit on the massive binary fraction of 30% - 42%. The calculated velocity dispersion for Cygnus OB2 is 2.44 +/- km/s, which is typical of open clusters. No runaway OB stars were found.Comment: 56 pages, 23 figures, 5 tables, accepted for publication in the Astrophysical Journa