348 research outputs found
Cepheid Calibration of the Peak Brightness of SNe Ia. X. SN 1991T in NGC 4527
Repeated imaging observations have been made of NGC 4527 with the Hubble
Space Telescope between April and June 1999, over an interval of 69 days.
Images were obtained on 12 epochs in the F555W band and on five epochs in the
F814W band. The galaxy hosted the type Ia supernova SN1991T, which showed
relatively unusual behavior by having both an abnormal spectrum near light
maximum, and a slower declining light curve than the proto-typical Branch
normal SNe Ia.
A total of 86 variables that are putative Cepheids have been found, with
periods ranging from 7.4 days to over 70 days. From photometry with the DoPHOT
program, the de-reddened distance modulus is determined to be (m-M)_0 = 30.67
+/- 0.12 (internal uncertainty) using a subset of the Cepheid data whose
reddening and error parameters are secure. A parallel analysis of the Cepheids
using photometry with ROMAFOT yields (m -M)_0 =30.82 +/- 0.11. The final
adopted modulus is (m -M)_0 =30.74 +/- 0.12 +/- 0.12 (d=14.1 +/- 0.8 +/- 0.8
Mpc).
The photometric data for SN1991T are used in combination with the Cepheid
distance to NGC 4527 to obtain the absolute magnitude for this supernova of
M_V^0(max) = -19.85 +/- 0.29. The relatively large uncertainty is a result of
the range in estimates of the reddening to the supernova. Thus SN1991T is seen
to be only moderately brighter (by ~ 0.3 mag) than the mean for
spectroscopically normal supernovae, although magnitude differences of up to
0.6 mag cannot be ruled out.Comment: 46 pages, LATEX using aaspp4.sty, including 9 embedded tables, 19
figures (gif and jpg files), a full-resolution version (ps files) is
available at http://www.astro.unibas.ch/forschung/ll/cepheid.shtml, accepted
for publication in the Astrophysical Journa
The volume densities of giant molecular clouds in M83
Using observed GALEX far-ultraviolet (FUV) fluxes and VLA images of the 21-cm
HI column densities, along with estimates of the local dust abundances, we
measure the volume densities of a sample of actively star-forming giant
molecular clouds (GMCs) in the nearby spiral galaxy M83 on a typical resolution
scale of 170 pc. Our approach is based on an equilibrium model for the cycle of
molecular hydrogen formation on dust grains and photodissociation under the
influence of the FUV radiation on the cloud surfaces of GMCs. We find a range
of total volume densities on the surface of GMCs in M83, namely 0.1 - 400 cm-3
inside R25, 0.5 - 50 cm-3 outside R25 . Our data include a number of GMCs in
the HI ring surrounding this galaxy. Finally, we discuss the effects of
observational selection, which may bias our results.Comment: 9 pages, 11 figure
The stellar mass to light ratio in the isolated spiral NGC 4414
We present high resolution CO(1-0) interferometric observations and deep HST
B-V-I images of the flocculent isolated Sc type spiral NGC 4414. The goal is to
determine the stellar mass-to-light (M/L) ratio in a galactic disk. The stars
are seen without a dust screen, the central gas mass is very low (undetected),
and we show that the dark matter is negligible in the central regions. We have
developed an axisymmetric analytical gravitational potential model to account
for the central light (mass) profile, the dynamics of the molecular gas in the
highly obscured molecular ring, and the stellar light profile outside the
highly obscured region. The contribution of dark matter is constrained by the
extremely extended HI rotation curve and is small, possibly negligible, at
distances less than 5 -- 7 kpc from the center. Furthermore, the M/L ratios we
derive are low, about 1.5 in I band and 0.5 in K' band. The B and V band M/L
ratios vary greatly due to absorption by dust, reaching 4 in the molecular ring
and decreasing to about 1.6 -- 1.8 at larger radii. This unequivocally shows
that models, like most maximum disk models, assuming constant M/L ratios in an
optical waveband, simply are not appropriate. We illustrate this by making mock
maximum disk models with a constant V band M/L ratio. The key is having the
central light distribution unobscured such that it can be used to trace the
mass. A primitive attempt to determine the intrinsic M/L ratio yields values
close to unity in the B,V, and I bands and slightly below 0.5 in K'.Comment: 12 pages, 10 figures, accepted in A&
Terahertz oscillations in an In<sub>0.53</sub>Ga<sub>0.47</sub>As submicron planar gunn diode
The length of the transit region of a Gunn diode determines the natural frequency at which it operates in fundamental mode – the shorter the device, the higher the frequency of operation. The long-held view on Gunn diode design is that for a functioning device the minimum length of the transit region is about 1.5μm, limiting the devices to fundamental mode operation at frequencies of roughly 60 GHz. Study of these devices by more advanced Monte Carlo techniques that simulate the ballistic transport and electron-phonon interactions that govern device behaviour, offers a new lower bound of 0.5μm, which is already being approached by the experimental evidence that has shown planar and vertical devices exhibiting Gunn operation at 600nm and 700nm, respectively. The paper presents results of the first ever THz submicron planar Gunn diode fabricated in In<sub>0.53</sub>Ga<sub>0.47</sub>A on an InP substrate, operating at a fundamental frequency above 300 GHz. Experimentally measured rf power of 28 µW was obtained from a 600 nm long ×120 µm wide device. At this new length, operation in fundamental mode at much higher frequencies becomes possible – the Monte Carlo model used predicts power output at frequencies over 300 GHz
Cepheids and Long Period Variables in NGC 4395
Repeated imaging observations of NGC 4395 were made with the WIYN 3.5 m and
the KPNO 2.1 m telescopes. From the photometry of the resolved brighter stars
in this galaxy eleven Cepheids with periods ranging between 12 and 90 days have
been identified. The true distance modulus has been derived from the apparent
distance moduli in g, r and i. The distance modulus is 28.02 +/- 0.18 based on
the LMC P-L relation by Sandage et al. 2003; this corresponds to a distance of
4.0 +/- 0.3 Mpc. Using the P-L relation from Madore & Freedman 1991, the
distance modulus is 28.15 +/- 0.18; which corresponds to a distance of 4.3 +/-
0.4 Mpc. The reddening is calculated to be E(g-r) = 0.06 +/- 0.08 and E(r-i) =
0.10 +/- 0.08, again from the distance moduli mu_g, mu_r and mu_i. In addition,
37 other variables have been detected, the majority of which have definite
periods. They are probably all red long period variables.Comment: 54 pages, 8 figures, 8 tables, accepted for publication in the
Astronomical Journa
On-chip interrogator based on Fourier Transform spectroscopy
In this paper, the design and the characterization of a novel interrogator
based on integrated Fourier transform (FT) spectroscopy is presented. To the
best of our knowledge, this is the first integrated FT spectrometer used for
the interrogation of photonic sensors. It consists of a planar spatial
heterodyne spectrometer, which is implemented using an array of Mach-Zehnder
interferometers (MZIs) with different optical path differences. Each MZI
employs a 33 multi-mode interferometer, allowing the retrieval of the
complex Fourier coefficients. We derive a system of non-linear equations whose
solution, which is obtained numerically from Newton's method, gives the
modulation of the sensor's resonances as a function of time. By taking one of
the sensors as a reference, to which no external excitation is applied and its
temperature is kept constant, about 92 of the thermal induced phase drift
of the integrated MZIs has been compensated. The minimum modulation amplitude
that is obtained experimentally is 400 fm, which is more than two orders of
magnitude smaller than the FT spectrometer resolution.Comment: 15 pages, 6 figure
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