269 research outputs found
Model Reduction on the Wnt Pathway Leads to Biological Adaptation
Get PDFComplex systems are an unavoidable problem in the field of biology. One of the ways that scientists have tried to overcome this problem is by building mathematical models—manageable representations designed to look at specific physical phenomena. The Wnt Signaling Pathway is a complex system known to regulate cell-to-cell interactions, play a crucial role in Embryonic Development, and has been implicated in the study of cancer. Typically, the Wnt signal is observed through the behavior of a protein called beta-Catenin (β-Catenin). In 2003, Lee et al. built a model of the Wnt pathway which caused β-Catenin to increase over time. However, in 2010, Jensen et al. built a different model of the Wnt pathway which caused β-Catenin to oscillate over time. This project called for model reduction on the Jensen et al. model to identify the phenomenological parameter combinations that determined features of the Wnt oscillations. The method used to reduce the model is called the Manifold Boundary Approximation Method, which is a geometric, parameter-independent method of reducing the model one parameter at a time. Reduction of the model showed that there were 5 variables and 8 parameters which drove the oscillating behavior of the system. After comparing our results to the Lee et al. reduced model of the Wnt pathway done by student Dane Bjork, a minimal model was constructed which predicted a novel class behavior of the Wnt system: biological adaptation
Herbig Ae/Be Stars in the Magellanic Bridge
Get PDFWe have found Herbig Ae/Be star candidates in the western region of the
Magellanic Bridge. Using the near infrared camera SIRIUS and the 1.4 m
telescope IRSF, we surveyed about 3.0 deg x 1.3 deg (24 deg < RA < 36 deg, -75
deg < Dec. < -73.7 deg) in the J, H, and Ks bands. On the basis of colors and
magnitudes, about 200 Herbig Ae/Be star candidates are selected. Considering
the contaminations by miscellaneous sources such as foreground stars and
early-type dwarfs in the Magellanic Bridge, we estimate that about 80 (about
40%) of the candidates are likely to be Herbig Ae/Be stars. We also found one
concentration of the candidates at the young star cluster NGC 796, strongly
suggesting the existence of pre-main-sequence (PMS) stars in the Magellanic
Bridge. This is the first detection of PMS star candidates in the Magellanic
Bridge, and if they are genuine PMS stars, this could be direct evidence of
recent star formation. However, the estimate of the number of Herbig Ae/Be
stars depends on the fraction of classical Be stars, and thus a more precise
determination of the Be star fraction or observations to differentiate between
the Herbig Ae/Be stars and classical Be stars are required.Comment: 22 pages, 6 figures. Accepted for publication in Ap
An atlas of 2.4 to 4.1 microns ISO/SWS spectra of early-type stars
Get PDFWe present an atlas of spectra of O- and B-type stars, obtained with the
Short Wavelength Spectrometer (SWS) during the Post-Helium program of the
Infrared Space Observatory (ISO). This program is aimed at extending the Morgan
& Keenan classification scheme into the near-infrared. Later type stars will be
discussed in a seperate publication. The observations consist of 57 SWS
Post-Helium spectra from 2.4 to 4.1 microns, supplemented with 10 spectra
acquired during the nominal mission with a similar observational setting. For
B-type stars, this sample provides ample spectral converage in terms of subtype
and luminosity class. For O-type stars,the ISO sample is coarse and therefore
is complemented with 8 UKIRT L'-band observations. In terms of the presence of
diagnostic lines, the L'-band is likely the most promising of the near-infrared
atmospheric windows for the study of the physical properties of B stars.
Specifically, this wavelength interval contains the Brackett alpha, Pfund
gamma, and other Pfund lines which are probes of spectral type, luminosity
class and mass loss. Here, we present simple empirical methods based on the
lines present in the 2.4 to 4.1 microns interval that allow the determination
of: the spectral type of B dwarfs and giants to within two subtypes; the
luminosity class of B stars to within two classes; the mass-loss rate of O
stars and B supergiants to within 0.25 dex.Comment: 19 pages, 11 Postscript figures, accepted by A&
BCD spectrophotometry for massive stars in transition phases
Get PDFContext. Stars in transition phases, like those showing the B[e] phenomenon
and luminous blue variables (LBVs), undergo strong, often irregular mass
ejection events. The prediction of these phases in stellar evolution models is
therefore extremely difficult if not impossible. As a result, their effective
temperatures, their luminosities and even their true nature are not fully
known.
Aims. A suitable procedure to derive the stellar parameters of these types of
objects is to use the BCD spectrophotometric classification system, based on
the analysis of the Balmer discontinuity. The BCD parameters ({\lambda}_1, D)
are independent of interstellar extinction and circumstellar contributions.
Methods. We obtained low-resolution spectra for 14 stars with the B[e]
phenomenon and LBVs. Using the BCD method, we derived the stellar and physical
parameters. The study was complemented with the information provided by the JHK
colour-colour diagram.
Results. For each star, the BCD system gives a complete set of fundamental
parameters and related quantities such as luminosity and distance. We confirmed
HK Ori, HD 323771 and HD 52721 as pre-main sequence HAe/B[e], AS 202 and HD
85567 as FS CMa-type, and HD 62623 as sgB[e] stars. We classified Hen 3-847,
CD-24 5721, and HD 53367 as young B[e] stars or FS CMa-type candidates, and HD
58647 as a slightly evolved B[e] star. In addition, Hen 3-1398 is an sgB[e] and
MWC 877, CPD-59 2854 and LHA 120-S 65 are LBV candidates. The stellar
parameters of the latter two LBVs are determined for the first time.
Conclusions. Our results emphasise that the BCD system is a highly valuable
tool to derive stellar parameters and physical properties of B-type stars in
transition phases. This method can be combined with near-IR colour-colour
diagrams to determine or confirm the evolutionary stage of emission-line stars
with dust disks
Hubble Space Telescope Photometry of Hodge 301: An "Old" Star Cluster in 30 Doradus
Full text linkWe present Hubble Space Telescope Planetary Camera UVI data for the
little-studied cluster Hodge 301 3' northwest of 30 Doradus' central ionizing
cluster R136. The average reddening of Hodge 301 is found to be =
(0.28+-0.05) mag from published infrared and ultraviolet photometry. Using two
different sets of evolutionary models, we derive an age of ~ 20-25 Myr for
Hodge 301, which makes it roughly 10 times as old as R136. Hodge 301 is the
most prominent representative of the oldest population in the 30 Dor starburst
region; a region that has undergone multiple star formation events. This range
of ages is an important consideration for the modelling of starburst regions.
Hodge 301 shows a widened upper main sequence largely caused by Be stars. We
present a list of Be star candidates. The slope of the initial mass function
for intermediate-mass main sequence stars ranging from 10 to 1.3 solar masses
is found to be -1.4+-0.1 in good agreement with a Salpeter law. There is no
indication for a truncation or change of slope of the IMF within this mass
range. In accordance with the age of Hodge 301 no obvious pre-main-sequence
stars are seen down to 1 solar mass. We estimate that up to 41+-7 stars with
more than 12 solar masses may have turned into supernovae since the formation
of the cluster. Multiple supernova explosions are the most likely origin of the
extremely violent gas motions and the diffuse X-ray emission observed in the
cluster surroundings.Comment: To appear in the Astronomical Journal (Feb 2000 issue). 16 pages in
two-column style. 9 separate figures, in part in significantly reduced
resolution for space reasons (bitmapped postscript or jpg
On the Binarity of LBV Stars
Get PDFWe report on the binarity of luminous blue variable stars observed with a set of techniques and instruments. Among them, observations at high angular resolution with the VLT-NACO, the VLTI-AMBER and with spectrographs such as the VLT-XSHOOTER allowed us to find several LBV stars as binaries or having a potential companion. In particular the LBV Pistol Star clearly presents radial velocity variations and line profiles modifications (double peak appearance). In addition, the absorption component of the P Cygni lines varies as well with the time indicating a potential wind structure variability. Our observations also show directly for the first time a companion to at least one of the observed LBVs (HD 168625). This one seems to affect the environment of the system. This system is known to be surrounded by several rings similar to those of SN1987A, possibly indicating a future supernova occurrence for this Galactic object. These results show that Eta Car is no longer unique
The VLT-FLAMES Tarantula Survey XVIII. Classifications and radial velocities of the B-type stars
Get PDFWe present spectral classifications for 438 B-type stars observed as part of the VLT-FLAMES Tarantula Survey (VFTS) in the 30 Doradus region of the Large Magellanic Cloud. Radial velocities are provided for 307 apparently single stars, and for 99 targets with radial-velocity variations which are consistent with them being spectroscopic binaries. We investigate the spatial distribution of the radial velocities across the 30 Dor region, and use the results to identify candidate runaway stars. Excluding potential runaways and members of two older clusters in the survey region (SL 639 and Hodge 301), we determine a systemic velocity for 30 Dor of 271.6 ± 12.2 kms-1 from 273 presumed single stars. Employing a 3σ criterion we identify nine candidate runaway stars (2.9% of the single stars with radial-velocity estimates). The projected rotational velocities of the candidate runaways appear to be significantly different to those of the full B-type sample, with a strong preference for either large (≥345 kms-1) or small (≤65 kms-1) rotational velocities. Of the candidate runaways, VFTS 358 (classified B0.5: V) has the largest differential radial velocity (−106.9 ± 16.2 kms-1), and a preliminary atmospheric analysis finds a significantly enriched nitrogen abundance of 12 + log (N/H) ≳ 8.5. Combined with a large rotational velocity (ve sin i = 345 ± 22 kms-1), this is suggestive of past binary interaction for this star
CRIRES-POP: A library of high resolution spectra in the near-infrared
Full text linkNew instrumental capabilities and the wealth of astrophysical information
extractable from the near-infrared wavelength region have led to a growing
interest in the field of high resolution spectroscopy at 1-5 mu. We aim to
provide a library of observed high-resolution and high signal-to-noise-ratio
near-infrared spectra of stars of various types throughout the
Hertzsprung-Russell diagram. This is needed for the exploration of spectral
features in this wavelength range and for comparison of reference targets with
observations and models.
High quality spectra were obtained using the CRIRES near-infrared
spectrograph at ESO's VLT covering the range from 0.97 to 5.3 mu at high
spectral resolution. Accurate wavelength calibration and correction for of
telluric lines were performed by fitting synthetic transmission spectra for the
Earth's atmosphere to each spectrum individually. We describe the observational
strategy and the current status and content of the library which includes 13
objects. The first examples of finally reduced spectra are presented. This
publication will serve as a reference paper to introduce the library to the
community and explore the extensive amount of material.Comment: accepted for publication in A&A; see also the project webpage
http://www.univie.ac.at/crirespo
Properties of the Scorpii Circumstellar Disk from Continuum Modeling
Full text linkWe present optical and infrared photometric observations of the
Be binary system Sco, obtained in 2000--2005, mid-infrared (10 and m) photometry and optical ( 3200--10500 \AA)
spectropolarimetry obtained in 2001. Our optical photometry confirms the
results of much more frequent visual monitoring of Sco. In 2005, we
detected a significant decrease in the object's brightness, both in optical and
near-infrared brightness, which is associated with a continuous rise in the
hydrogen line strenghts. We discuss possible causes for this phenomenon, which
is difficult to explain in view of current models of Be star disks. The 2001
spectral energy distribution and polarization are succesfully modeled with a
three-dimensional non-LTE Monte Carlo code which produces a self-consistent
determination of the hydrogen level populations, electron temperature, and gas
density for hot star disks. Our disk model is hydrostatically supported in the
vertical direction and radially controlled by viscosity. Such a disk model has,
essentially, only two free parameters, viz., the equatorial mass loss rate and
the disk outer radius. We find that the primary companion is surrounded by a
small (7 ), geometrically-thin disk, which is highly non-isothermal
and fully ionized. Our model requires an average equatorial mass loss rate of
1.5\times 10^{-9} M_{\sun} yr.Comment: 27 pages, 9 figures, submitted to Ap
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