51 research outputs found
H and K maps of two star-forming regions: S 140 and CEP A OB3
The molecular clouds near S 140 and Cep A OB3 both contain regions that emit strongly in the infrared but which have relatively little or no emission in radio wavelengths. The lack of radio emission is usually interpreted to mean that little ionization has taken place, and that the IR emission comes from dust heated by a central pre-main sequence object. We have made high resolution maps of these two areas with H and K broadband filters with the 2.3m telescope of the Wyoming Infrared Observatory (WIRO). Our observations were made with an InSb detector using the standard WIRO mapping technique of multiple scanning to construct a series of 64x64 pictures. Our scanning was made in one arcsec steps with a 7 inch aperture to produce a resolution of about 6 sec FWHM. Observations of S 140 by Blair et al (1978) with moderate resolution in J, H, and K, revealed a prominent IR source that they called S 140IR. We have scanned an area surrounding their S 140IR region and have obtained total IR fluxes which are in close agreement with Blair et al. Additional detail corresponds to that seen by Dinerstein et al (1979) at 0.9 microns and Campbell (1986) at 1.0 microns. The large (H-K) values for all components of S 140IR indicates that they are probably embedded in the parent molecular cloud with A(v) = 25 mag and A(K) = 2.2 mag. Recent radio studies of Cep A by Hughes and Wouterloot (1984) and FIR studies by Evans et al (1981) have shown the similarity of Cep A to S 140. There is, however, some emission detected from ionized regions by the radio observations, indicating a slightly more advanced stage of evolution for Cep A. We observed an area around the strongest source in Cep A in both H and K, and the K map is presented. We have identified three sources separate from the main source
Mid-infrared spectra of late-type stars: Long-term evolution
Recent ground-based mid-infrared spectra of 29 late-type stars, most with
substantial dust shells, are compared to ground-based spectra of these stars
from the 1960s and 1970s and to IRAS-LRS spectra obtained in 1983. The spectra
of about half the stars show no detectable changes, implying that their
distributions of circumstellar material and associated dust grain properties
have changed little over this time interval. However, many of the stars with
strong silicate features showed marked changes. In nearly all cases the
silicate peak has strengthened with respect to the underlying continuum,
although there is one case (VY~CMa) in which the silicate feature has almost
completely disappeared. This suggests that, in general, an oxygen-rich star
experiences long periods of gradual silicate feature strengthening, punctuated
by relatively rare periods when the feature weakens. We discuss various
mechanisms for producing the changes, favoring the slow evolution of the
intrinsic dust properties (i.e., the chemical composition or grain structure).
Although most IRAS spectra agree well with ground-based spectra, there are a
number of cases where they fall well outside the expected range of uncertainty.
In almost all such cases the slopes of the red and blue LRS spectra do not
match in their region of overlap.Comment: Accepted in ApJ, 20 pages, 5 figures, 1 tabl
A correlation between infrared excess and period for Mira variables
The [8.7]-[11.4] infrared color is found to increase monotonically with increasing period for a sample of 41 Mira variables. We conclude that mass loss is not a stochastic process and that for any Mira variable the rate of mass loss is directly linked to the parameters which dictate the period of that variable
New Results for Two Optically Faint Low Mass X-Ray Binary Systems
We present optical photometry of the low mass X-ray binary systems GX 349+2
and Ser X-1. Extensive VRI photometry of the faint optical counterpart (V=18.4)
to GX 349+2 reveals a period of 22.5 +/- 0.1 h and half-amplitude 0.2 mag. This
result confirms and extends our previously reported 22 h period. No color
change is detected over the orbit, although the limits are modest. We also
report the discovery of two new variable stars in the field of GX 349+2,
including a probable W UMa system. Ser X-1 is one of the most intense
persistent X-ray burst sources known. It is also one of only three burst
systems for which simultaneous optical and X-ray bursts have been observed. The
faint blue optical counterpart MM Ser (B~19.2) has long been known to have a
companion 2.1" distant. Our images indicate that MM Ser is itself a further
superposition of two stars, separated by only 1". At the very least, the ratio
of inferred burst to quiescent optical flux is affected by the discovery of
this additional component. In the worst case, the wrong object may have
previously been assumed as the optical counterpart.Comment: 16 pages including 10 figures and 3 tables; Uses AASTeX 4.0; Accepted
for publication in The Astrophysical Journal, Volume 490, November 20, 199
The effect of stellar evolution on SiC dust grain sizes
Stars on the asymptotic giant branch (AGB) produce dust in their
circumstellar shells. The nature of the dust-forming environment is influenced
by the evolution of the stars, in terms of both chemistry and density, leading
to an evolution in the nature of the dust that is produced. Carbon-rich AGB
stars are known to produce silicon carbide (SiC). Furthermore, observations of
the ~11um SiC feature show that the spectral features change in a sequence that
correlates with stellar evolution. We present new infrared spectra of amorphous
SiC and show that the ~9um feature seen in both emission and absorption, and
correlated with trends in the ~11um feature, may be due to either amorphous SiC
or to nano-crystalline diamond with a high proportion of Si substituting for C.
Furthermore, we identify SiC absorption in three ISO spectra of extreme carbon
stars, in addition to the four presented by Speck et al. (1997). An accurate
description of the sequence in the IR spectra of carbon stars requires
accounting for both SiC emission and absorption features. This level of detail
is needed to infer the role of dust in evolution of carbon stars. Previous
attempts to find a sequence in the infrared spectra of carbon stars considered
SiC emission features, while neglecting SiC absorption features, leading to an
interpretation of the sequence inadequately describes the role of dust. We show
that the evolutionary sequence in carbon star spectra is consistent with a
grain size evolution, such that dust grains get progressively smaller as the
star evolves. The evolution of the grain sizes provides a natural explanation
for the shift of the ~11um SiC feature in emission and in absorption. Further
evidence for this scenario is seen in both post-AGB star spectra and in
meteoritic studies of presolar grains.Comment: accepted by ApJ 8 figure
Mid-infrared spectroscopy of carbon stars in the Small Magellanic Cloud
We have observed a sample of 36 objects in the Small Magellanic Cloud (SMC)
with the Infrared Spectrometer on the Spitzer Space Telescope. Nineteen of
these sources are carbon stars. An examination of the near- and mid-infrared
photometry shows that the carbon-rich and oxygen-rich dust sources follow two
easily separated sequences. A comparison of the spectra of the 19 carbon stars
in the SMC to spectra from the Infrared Space Observatory (ISO) of carbon stars
in the Galaxy reveals significant differences. The absorption bands at 7.5 um
and 13.7 um due to C2H2 are stronger in the SMC sample, and the SiC dust
emission feature at 11.3 um is weaker. Our measurements of the MgS dust
emission feature at 26-30 um are less conclusive, but this feature appears to
be weaker in the SMC sample as well. All of these results are consistent with
the lower metallicity in the SMC. The lower abundance of SiC grains in the SMC
may result in less efficient carbon-rich dust production, which could explain
the excess C2H2 gas seen in the spectra. The sources in the SMC with the
strongest SiC dust emission tend to have redder infrared colors than the other
sources in the sample, which implies more amorphous carbon, and they also tend
to show stronger MgS dust emission. The weakest SiC emission features tend to
be shifted to the blue; these spectra may arise from low-density shells with
large SiC grains.Comment: Accepted by ApJ 21 March, 2006, 13 pages (emulateapJ), 12 figure
An ISO/SWS study of the dust composition around S stars
We investigate the composition of the solid-state materials in the winds
around S-type AGB stars. The S stars produce dust in their wind that bears a
resemblance to the dust produced in some O-rich AGB stars. However, the
reported resemblance is mostly based on IRAS/LRS spectra with limited spectral
resolution, sensitivity, and wavelength coverage. We investigate the dust
composition around S stars using ISO/SWS data that surpass the previous studies
in terms of spectral resolution and wavelength coverage. We compare the dust
spectra from the 9 sources with the O-rich AGB spectra and a subset of M
super-giants. We constructed average dust emission spectra of the different
categories. We report the discovery of several previously unreported dust
emission features in the S star spectra. The long wavelength spectra of W Aql
and pi1 Gru exhibit the "30" micrometer feature attributed to MgS. Two sources
exhibit a series of emission bands between 20 and 40 micrometer that we
tentatively ascribe to Diopside. We show that the 10-20 micrometer spectra of
the S stars are significantly different from the O-rich AGB stars. The O-rich
stars exhibit a structured emission feature that is believed to arise from
amorphous silicate and aluminium-oxide. The S stars lack the substructure found
in the O-rich stars. Instead they show a smooth peak with a varying
peak-position from source to source. We suggest that this feature is caused by
a family of related material, whose exact composition determines the peak
position. The observed trend mimics the laboratory trend of non-stoichiometric
silicates. In this scenario the degree of non-stoichiometry is related to the
Mg to SiO4 ratio, in other words, to the amount of free O available during the
dust grain growth.Comment: 9 pages, 7 figures, accepted by A&
Interstellar Grains -- The 75th Anniversary
The year of 2005 marks the 75th anniversary since Trumpler (1930) provided
the first definitive proof of interstellar grains by demonstrating the
existence of general absorption and reddening of starlight in the galactic
plane. This article reviews our progressive understanding of the nature of
interstellar dust.Comment: invited review article for the "Light, Dust and Chemical Evolution"
conference (Gerace, Italy, 26--30 September 2004), edited by F. Borghese and
R. Saija, 2005, in pres
Orbitally modulated dust formation by the WC7+O5 colliding-wind binary WR140
We present high-resolution infrared (2--18 micron) images of the archetypal
periodic dust-making Wolf-Rayet binary system WR140 (HD 193793) taken between
2001 and 2005, and multi-colour (J -- [19.5]) photometry observed between 1989
and 2001. The images resolve the dust cloud formed by WR140 in 2001, allowing
us to track its expansion and cooling, while the photometry allows tracking the
average temperature and total mass of the dust. The combination of the two
datasets constrains the optical properties of the dust. The most persistent
dust features, two concentrations at the ends of a `bar' of emission to the
south of the star, were observed to move with constant proper motions of
324+/-8 and 243+/-7 mas/y. Longer wavelength (4.68-micron and 12.5-micron)
images shows dust emission from the corresponding features from the previous
(1993) periastron passage and dust-formation episode. A third persistent dust
concentration to the east of the binary (the `arm') was found to have a proper
motion ~ 320 mas/y. Extrapolation of the motions of the concentrations back to
the binary suggests that the eastern `arm' began expansion 4--5 months earlier
than those in the southern `bar', consistent with the projected rotation of the
binary axis and wind-collision region (WCR) on the sky. Comparison of model
dust images and the observations constrain the intervals when the WCR was
producing sufficiently compressed wind for dust nucleation in the WCR, and
suggests that the distribution of this material was not uniform about the axis
of the WCR, but more abundant in the following edge in the orbital plane.Comment: 21 pages, 10 figures, accepted for MNRAS. A version with higher
resolution figures is available at
ftp://ftp.roe.ac.uk/pub/pmw/wr140dust.ps.g
- âŠ