2,078 research outputs found
Stokes' Phenomenon arising from the confluence of two simple poles
We study certain confluences of equations with two Fuchsian singularities which produce an irregular singularity of Poincaré rank one. We demonstrate a method to understand how to pass from solutions with power-like behavior which are analytic in neighbourhoods to solutions with exponential behavior which are analytic in sectors and have divergent asymptotic behavior. We explicitly calculate the Stokes' matrices of the confluent system in terms of the monodromy data, specifically the connection matrices, of the original system around the merging singularities. The confluence of Gauss' hypergeometric equation gives an excellent opportunity to show our approach with a concrete example. We explicitly show how the Stokes' data arise in the confluences of the isomonodromic deformation problems for the Painlevé equations PVI to PV and PV to PIII(D6)
A study of T Tauri stars with multiple spectral types and their surface temperatures
This thesis describes the results of two spectral studies of T Tauri stars. T Tauri stars
are young, low mass, stars which have yet to evolve to the point where Hydrogen fusion
occurs in their cores. They are normally found in groups associated with gas clouds in
locations called star forming regions. Much work has been done to explain the bizarre
spectral features of T Tauri stars, and this has led to the development of a standard
model for their structures. Classical T Tauri stars are thought to be surrounded by large
discs of accreting material, which slowly adds to the mass of the star. This accretion
process results in large U.V. and I.R. fluxes for the stars when compared to their main
sequence counterparts. Weak-line T Tauri stars do not display any accretion features
and are thought to be more evolved than Classical T Tauri stars. Observations show
that both types of T Tauri star have active chromospheres and large cool spots, which
are both thought to be a result of strong magnetic fields. This thesis is motivated
by two studies, one spectroscopic, one photometric, both of which suggest that some
T Tauri stars display photospheric features from regions of different temperatures, or
multiple spectral types (MST).The first spectral study discussed is of near-I.R. data taken with UKIRT. The
spectra are of high resolution data from the region around 1.63 fim, and are of T Tauri,
giant and dwarf stars of known spectral type. It is shown that a ratio of selected OH
to Fe lines gives a good trend with the optically measured Te//> derived from spectral
types, for both dwarfs and giants. Whilst some of the T Tauri stars fit this trend, it is
found that 4 stars clearly lie above it. This is taken as an indication of cooler regions
in the spectra than had previously been detected, an MST effect.The second spectral study discussed is of optical data taken with the FLAIR system
on the UKST. The spectra span the entire optical region observable with this instrument, and are intended to show variation in temperature sensitive features between
the blue and red optical regions. Spectra are presented from 2 consecutive nights of
observations and previously discovered MST stars are found to show deeper G bands
than would be predicted from the TiO bands in the red spectra. It is found that the
G band of one object has varied both on a night to night basis and since the previous
observations from the mid-1980âs. It is also shown that no variation has occurred in
the red spectraAll of the information on MST stars is then compared in order to determine the best
physical model for the effect. It is shown that none of the regions in the standard model
of Classical T Tauri stars can produce the MTS features. The first model considered
that could display MTS features is of binary systems of stars with differing spectral
type, but equivalent flux in the optical. It is found that these systems would be too
rare to explain all the MTS results. The large cool spots seen on T Tauri stars are also
considered as the cause of the MST effect. It is shown that the size of spots required
is much greater than the size of observed cool regions. The current theories of solar
active regions are discussed, revealing that the dominant photospheric regions are in
fact the small, bright faculae. Whilst the spectra of solar faculae are still not fully
understood it is found that a faculae+photosphere+cool spot model of T Tauri spectra
can produce all the observed MST effects.Finally, the systematic errors in the H-R diagram introduced by the MST effect
are considered. By comparison with research on dwarf stars it is found that the MST
effects do not significantly alter the stellar Tef f and luminosity. The largest errors
are introduced if only a small wavelength region is considered when determining the
spectral type of the star. Studies that derive a Te/ / from an apparent spectral type
and ignore the MST effect can both over and underestimate Te/ / ; leading to a possible
factor of 2 error in the stellar mass. As T Tauri spectral parameters are also used to
derive the overall features of star forming regions, this can lead to the miscalculation
of the mass function and ages of these regions
Line and Continuum Variability in Active Galaxies
We compared optical spectroscopic and photometric data for 18 AGN galaxies
over 2 to 3 epochs, with time intervals of typically 5 to 10 years. We used the
Multi-Object Double Spectrograph (MODS) at the Large Binocular Telescope (LBT)
and compared the spectra to data taken from the SDSS database and the
literature. We find variations in the forbidden oxygen lines as well as in the
hydrogen recombination lines of these sources. For 4 of the sources we find
that, within the calibration uncertainties, the variations in continuum and
line spectra of the sources are very small. We argue that it is mainly the
difference in black hole mass between the samples that is responsible for the
different degree of continuum variability. In addition we find that for an
otherwise constant accretion rate the total line variability (dominated by the
narrow line contributions) reverberates the continuum variability with a
dependency . Since
this dependency is prominently expressed in the narrow line emission it implies
that the luminosity dominating part of the narrow line region must be very
compact with a size of the order of at least 10 light years. A comparison to
literature data shows that these findings describe the variability
characteristics of a total of 61 broad and narrow line sources.Comment: 30 pages including the appendix, 18 figures including the appendix.
Accepted 2015 September 3. Received 2015 August 24; in original form 2015
July 3 in Monthly Notices of the Royal Astronomical Societ
SUBARU and e-Merlin observations of NGC3718. Diaries of an SMBH recoil?
NGC3718 is a LINER galaxy, lying at a distance of about
Mpc away from earth and its similarities with NGC5128 often award it the name
"northern Centaurus A". We use high angular resolution ( mas) e-Merlin
radio and SUBARU NIR ( mas) data, to take a detailed view of the
processes taking place in its central region. In order to preserve some
objectivity in our interpretation, we combine our results with literature
values and findings from previous studies. Our NIR maps suggest, on one hand,
that towards the stellar bulge there are no large scale absorption phenomena
caused by the apparent dust lane and, on the other, that there is a significant
(local) contribution from hot ( K) dust to the nuclear NIR emission.
The position where this takes place appears to be closer to the offset compact
radio emission from our e-Merlin cm map, lying offset by pc from
the center of the underlying stellar bulge. The shape of the radio map suggests
the presence of one (or possibly two, forming an X-shape) bipolar structure(s)
() arcsec across, which combined with the balance between the
gas and the stellar velocity dispersions and the presence of hard X-ray
emission, point towards effects expected by AGN feedback. We also argue that
NGC3718 has a "core" in its surface brightness profile, despite the fact that
it is a gas-rich galaxy and we discuss its mixed photometric and spectroscopic
characteristics. The latter combined with the observed spatial and radio
offsets, the relative redshift between the broad and the narrow
line, the limited star formation activity and AGN
feedback, strongly imply the existence of an SMBH recoil. Finally, we discuss a
possible interpretation, that could naturally incorporate all these findings
into one physically consistent picture.Comment: 18 pages, 18 figures, 3 tables, accepted for publications in A&
Sequential star formation in IRAS 06084-0611 (GGD 12-15): From intermediate-mass to high-mass stars
Context. The formation and early evolution of high- and intermediate-mass
stars towards the main sequence involves the interplay of stars in a clustered
and highly complex environment. To obtain a full census of this interaction,
the Formation and Early evolution of Massive Stars (FEMS) collaboration studies
a well-selected sample of 10 high-mass star-forming regions. Aims. In this
study we examine the stellar content of the high-mass star-forming region
centered on IRAS 06084-0611 in the Monoceros R2 cloud. Methods. Using the
near-infrared H- and K-band spectra from the VLT/SINFONI instrument on the ESO
Very Large Telescope (VLT)and photometric near-infrared NTT/SOFI, 2MASS and
Spitzer/IRAC data, we were able to determine the spectral types for the most
luminous stars in the cluster. Results. Two very young and reddened massive
stars have been detected by SINFONI: a massive Young Stellar Object (YSO) con-
sistent with an early-B spectral type and a Herbig Be star. Furthermore, stars
of spectral type G and K are detected while still in the Pre-Main Sequence
(PMS) phase. We derive additional properties such as temperatures, extinctions,
radii and masses. We present a Hertzsprung-Russell diagram and find most
objects having intermediate masses between \sim1.5-2.5 M\odot. For these stars
we derive a median cluster age of \sim4 Myr. Conclusions. Using Spitzer/IRAC
data we confirm earlier studies that the younger class 0/I objects are
centrally located while the class II objects are spread out over a larger area,
with rough scale size radii of \sim0.5 pc and \sim1.25 pc respectively.
Moreover, the presence of a massive YSO, an ultracompact H ii region and highly
reddened objects in the center of the cluster suggest a much younger age of < 1
Myr. A possible scenario for this observation would be sequential star
formation along the line of sight; from a cluster of intermediate-mass to
high-mass stars.Comment: 14 pages, 10 figures, 2 tables. Astronomy and Astrophysic
Peer-review in a world with rational scientists: Toward selection of the average
One of the virtues of peer review is that it provides a self-regulating
selection mechanism for scientific work, papers and projects. Peer review as a
selection mechanism is hard to evaluate in terms of its efficiency. Serious
efforts to understand its strengths and weaknesses have not yet lead to clear
answers. In theory peer review works if the involved parties (editors and
referees) conform to a set of requirements, such as love for high quality
science, objectiveness, and absence of biases, nepotism, friend and clique
networks, selfishness, etc. If these requirements are violated, what is the
effect on the selection of high quality work? We study this question with a
simple agent based model. In particular we are interested in the effects of
rational referees, who might not have any incentive to see high quality work
other than their own published or promoted. We find that a small fraction of
incorrect (selfish or rational) referees can drastically reduce the quality of
the published (accepted) scientific standard. We quantify the fraction for
which peer review will no longer select better than pure chance. Decline of
quality of accepted scientific work is shown as a function of the fraction of
rational and unqualified referees. We show how a simple quality-increasing
policy of e.g. a journal can lead to a loss in overall scientific quality, and
how mutual support-networks of authors and referees deteriorate the system.Comment: 5 pages 4 figure
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