120 research outputs found
Calibration of the Distance Scale from Cepheids
We have used the infrared surface brightness technique to obtain a new
absolute calibration of the Cepheid PL relation in optical and near-infrared
bands from improved data on Galactic stars. The infrared surface brightness
distances to the Galactic variables are consistent with direct interferometric
Cepheid distance measurements, and with the PL calibration coming from
Hipparcos parallaxes of nearby Cepheids, but are more accurate than these
determinations. We find that in all bands, the Galactic Cepheid PL relation
appears to be slightly, but significantly steeper than the corresponding
relation defined by the LMC Cepheids. Since the slope of our LMC Cepheid sample
is clearly better defined than the one of the much smaller Galactic sample, we
fit the LMC slopes to our Galactic calibrating Cepheid sample (which introduces
only a small uncertainty) to obtain our final, adopted and improved absolute
calibrations of the Cepheid PL relations in the VIWJHK bands. Comparing the
absolute magnitudes of 10-day period Cepheids in both galaxies which are only
slightly affected by the different Galactic and LMC slopes of the PL relation,
we derive values for the LMC distance modulus in all these bands which can be
made to agree extremely well under reasonable assumptions for both, the
reddening law, and the adopted reddenings of the LMC Cepheids. This yields, as
our current best estimate from Cepheid variables, a LMC distance modulus of
18.55 +- 0.06.Comment: to be published in: "Stellar Candles", Lecture Notes in Physics
(http://link.springer.de/series/lnpp
WISEA J064750.85-154616.4: a new nearby L/T transition dwarf
Aims: Our aim is to detect and classify previously overlooked brown dwarfs in
the solar neighbourhood. Methods: We performed a proper motion search among
bright sources observed with the Wide-field Infrared Survey Explorer (WISE)
that are also seen in the Two Micron All Sky Survey (2MASS). Our candidates
appear according to their red colours as nearby late-L dwarf
candidates. Low-resolution near-infrared (NIR) classification spectroscopy in
the band allowed us to get spectroscopic distance and tangential velocity
estimates. Results: We have discovered a new L9.5 dwarf, WISEA
J064750.85-154616.4, at a spectroscopic distance of about 14 pc and with a
tangential velocity of about 11 km/s, typical of the Galactic thin disc
population. We have confirmed another recently found L/T transition object at
about 10 pc, WISEA J140533.13+835030.7, which we classified as L8 (NIR).Comment: Research Note accepted for publication in Astronomy and Astrophysics,
5 pages, 3 figure
An overlooked brown dwarf neighbour (T7.5 at d~5pc) of the Sun and two additional T dwarfs at about 10pc
Although many new brown dwarf (BD) neighbours have recently been discovered
thanks to new sky surveys in the mid- and near-infrared (MIR, NIR), their
numbers are still more than five times lower than those of stars in the same
volume. Our aim is to detect and classify new BDs to eventually complete their
census in the immediate Solar neighbourhood. We combined multi-epoch data from
sky surveys at different wavelengths to detect BD neighbours of the Sun by
their high proper motion (HPM). We concentrated at relatively bright MIR
(w2<13.5) BD candidates from WISE expected to be so close to the Sun that they
may also be seen in older NIR (2MASS, DENIS) or even red optical (SDSS i- and
z-band, SSS I-band) surveys. With low-resolution NIR spectroscopy we classified
the new BDs and estimated their distances and velocities. We have discovered
the HPM (pm~470mas/yr) T7.5 dwarf, WISE J0521+1025, which is at d=5.0+-1.3pc
from the Sun the nearest known T dwarf in the northern sky, and two early-T
dwarfs, WISE J0457-0207 (T2) and WISE J2030+0749 (T1.5), with proper motions of
~120 and ~670mas/yr and distances of 12.5+-3.1pc and 10.5+-2.6pc, respectively.
The last one was independently discovered and also classified as a T1.5 dwarf
by Mace and coworkers. All three show thin disk kinematics. They may have been
overlooked in the past owing to overlapping images and because of problems with
matching objects between different surveys and measuring their proper motions.Comment: 7 pages, incl. 6 figures and 1 table, accepted for publication in
Astronomy and Astrophysics, minor changes in title, abstract, and conclusion
Globular Cluster Calibration of the Peak Brightness of the Type Ia Supernova 1992A and the Value of Ho
We have determined the absolute magnitude at maximum light of SN 1992A by
using the turn-over magnitude of the Globular Cluster Luminosity Function of
its parent galaxy, NGC 1380. A recalibration of the peak of the turn-over
magnitude of the Milky Way clusters using the latest HIPPARCOS results has been
made with an assessment of the complete random and systematic error budget. The
following results emerge: a distance to NGC 1380 of 18.6 +/- 1.4 Mpc,
corresponding to (m-M)=31.35 +/- 0.16, and an absolute magnitude of SN 1992A at
maximum of M_B(max)= -18.79 +/- 0.16. Taken at face value, SN 1992A seems to be
more than half a magnitude fainter than the other SNeI-a for which accurate
distances exist. We discuss the implications of this result and present
possible explanations. We also discuss the Phillips'(1993) relationship between
rate of decline and the absolute magnitude at maximum, on the basis of 9
SNeI-a, whose individual distances have been obtained with Cepheids and the
Globular Cluster Luminosity Function. The new calibration of this relationship,
applied to the most distant SNe of the Calan-Tololo survey, yields Ho=62 +/- 6
km/s/Mpc.Comment: 12 pages (MNRAS style, two columns, including 6 figures), accepted
for publication in the MNRAS, a full resolution version of Fig.1 is available
at http://www.ucolick.org/~mkissle
Evidence for a Universal Slope of the Period-Luminosity Relation from Direct Distances to Cepheids in the LMC
We have applied the infrared surface brightness (ISB) technique to derive
distances to 13 Cepheid variables in the LMC which have periods from 3-42 days.
The corresponding absolute magnitudes define PL relations in VIWJK bands which
agree exceedingly well with the corresponding Milky Way relations obtained from
the same technique, and are in significant disagreement with the observed LMC
Cepheid PL relations, by OGLE-II and Persson et al., in these bands. Our data
uncover a systematic error in the p-factor law which transforms Cepheid radial
velocities into pulsational velocities. We correct the p-factor law by
requiring that all LMC Cepheids share the same distance. Re-calculating all
Milky Way and LMC Cepheid distances with the revised p-factor law, we find that
the PL relations from the ISB technique both in LMC and in the Milky Way agree
with the OGLE-II and Persson et al. LMC PL relations, supporting the conclusion
of no metallicity effect on the slope of the Cepheid PL relation in
optical/near infrared bands.Comment: 4 pages, to appear in the proceedings of the "Stellar Pulsation and
Evolution" conference, Monte Porzio Catone, June 200
Infrared Surface Brightness Distances to Cepheids: a comparison of Bayesian and linear-bisector calculations
We have compared the results of Bayesian statistical calculations and
linear-bisector calculations for obtaining Cepheid distances and radii by the
infrared surface brightness method. We analyzed a set of 38 Cepheids using a
Bayesian Markov Chain Monte Carlo method that had been recently studied with a
linear-bisector method. The distances obtained by the two techniques agree to
1.5 \pm 0.6% with the Bayesian distances being larger. The radii agree to 1.1%
\pm 0.7% with the Bayesian determinations again being larger. We interpret this
result as demonstrating that the two methods yield the same distances and
radii. This implies that the short distance to the LMC found in recent
linear-bisector studies of Cepheids is not caused by deficiencies in the
mathematical treatment. However, the computed uncertainties in distance and
radius for our dataset are larger in the Bayesian calculation by factors of
1.4-6.7. We give reasons to favor the Bayesian computations of the
uncertainties. The larger uncertainties can have a significant impact upon
interpretation of Cepheid distances and radii obtained from the infrared
surface brightness method.Comment: 27 pages with 9 figure
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