54 research outputs found
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 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 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 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 () and 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
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