21 research outputs found
Asteroseismological Observations of the Central Star of the Planetary Nebula NGC 1501
We report on a global CCD time-series photometric campaign to decode the
pulsations of the nucleus of the planetary nebula NGC1501. The star is hot and
hydrogen-deficient, similar to the pre-white-dwarf PG 1159 stars. NGC1501 shows
pulsational brightness variations of a few percent with periods ranging from 19
to 87 minutes. The variations are very complex, suggesting a pulsation spectrum
that requires a long unbroken time series to resolve. Our CCD photometry of the
star covers a two-week period in 1991 November, and used a global network of
observatories. We obtained nearly continuous coverage over an interval of one
week in the middle of the run. We have identified 10 pulsation periods, ranging
from 5235 s down to 1154 s. We find strong evidence that the modes are indeed
nonradial g-modes. The ratios of the frequencies of the largest-amplitude modes
agree with those expected for modes that are trapped by a density discontinuity
in the outer layers. We offer a model for the pulsation spectrum that includes
a common period spacing of 22.3 s and a rotation period of 1.17 days; the
period spacing allows us to assign a seismological mass of 0.55+/-0.03 Msun.Comment: 12 pages, AASTEX, 7 tables, 6 EPS figures, to appear in AJ, 12/96
Corrected version repairs table formatting and adds missing Table
More evidence for an intracluster planetary nebulae population in the Virgo cluster
We surveyed a 50 sq arcmin region in the Virgo cluster core to search for
intergalactic planetary nebulae, and found 11 candidates in the surveyed area.
The measured fluxes of these unresolved sources are consistent with these
objects being planetary nebulae from an intracluster population of stars. We
compute the cumulative luminosity function of these 11 planetary nebula
candidates. If we assume that they belong to the Virgo cluster, their
cumulative luminosity function is in good agreement with planetary nebula
luminosity function simulations. This comparison allows us to estimate the
surface mass density of the intracluster stellar population at the surveyed
field in the cluster core.Comment: ApJ Letters, in press. A .ps file is also available at:
http://www.usm.uni-muenchen.de:8001/people/mendez/preprints/preprints.htm
Hubble Space Telescope Photometry of the Dwarf Spheroidal Galaxy ESO 410-G005
We present HST WFPC2 imaging of the nearby low-surface-brightness dwarf
spheroidal galaxy ESO 410-G005, which has been resolved into stars for the
first time. The resulting color-magnitude diagram for about 2500 stars shows a
red giant branch branch with a tip at I=(22.4+-0.15), which yields a distance
of D=(1.9+-0.2) Mpc. ESO 410-G005 is found to be metal-poor with a mean
metallicity of (-1.8+-0.4) dex estimated from its red giant branch. Upper
asymptotic giant branch stars appear to be present near the center of the
galaxy, indicative of a substantial, centrally concentrated intermediate-age
population, unless these objects are artifacts of crowding. Previous studies
did not detect ESO 410-G005 in H alpha or in HI. ESO 410-G005 is a probable
member of the Sculptor group. Its linear separation from the nearest spiral,
NGC 55, is 230 kpc on the sky. The deprojected separation ranges from 340 to
615 kpc depending on the assumed distance of NGC 55. ESO 410 G005 appears to be
a relatively isolated dSph within the Sculptor group. Its absolute magnitude,
Mv = (-12.1+-0.2) mag, its central surface brightness, mu_V = (22.7+-0.1)
mag/arcsec^2, and its mean metallicity, [Fe/H] = (-1.8+-0.4) dex, follow the
trend observed for dwarf galaxies in the Local Group. (abridged)Comment: Accepted for publication in the Astrophysical Journal, Vol. 542 (Oct
20). 23 pages in AASTEX style, 9 figures, partially in gif format to save
spac
UBVI Surface Photometry of the Spiral Galaxy NGC 300 in the Sculptor Group
We present UBVI surface photometry for 20.'5 X 20.'5 area of a late-type
spiral galaxy NGC 300. In order to understand the morphological properties and
luminosity distribution characteristics of NGC 300, we have derived isophotal
maps, surface brightness profiles, ellipticity profiles, position angle
profiles, and color profiles. By merging the I-band data of our surface
brightness measurements with those of Boeker et al. (2002) based on Hubble
Space Telescope observations, we have made combined I-band surface brightness
profiles for the region of 0."02 < r < 500" and decomposed the profiles into
three components: a nucleus, a bulge, and an exponential disk.Comment: 16 pages(cjaa209.sty), Accepted by the Chinese J. Astron. Astrophys.,
Fig 2 and 8 are degraded to reduce spac
The Stellar Content and Star Formation History of the late-type spiral galaxy NGC 300 from Hubble Space Telescope observations
We present the first WFPC2 V, I photometry for the Sculptor Group galaxy NGC
300 in four fields ranging from the centre to the outer edge. We have made the
first measurement of the star formation histories in two disk fields: the
oldest stars were born at similar epochs and formation activity increased but
at different mean rates. The main disk stellar population is predominantly old,
consisting of RGB and AGB stars, based on a synthetic colour magnitude diagram
analysis. Z is found to have been more metal poor than 0.006 (or 0.33Zsolar)
with no evidence for significant change in the mean Z value over time in both
disk fields. In the central region, we find a dearth of bright stars with
respect to the two disk fields that cannot be explained by observational
effects. Taken at face value, this finding would agree with the Davidge (1998)
report of suppressed star formation there during the past 1Gyr with respect to
his disk fields at larger radii; but the possibility of significant central
extinction affecting our finding remains. We have also determined the first
distance modulus estimate based on the tip of the red giant branch method: on
the Cepheid distance scale of Ferrarese et al. (2000) we find (m-M)o =
26.56+/-0.07 (+/-0.13) mag; and a similar value from the Cepheid-independent
empirical method by Lee et al. (1993), both in good agreement with the Cepheid
distance determined by Freedman et al. (2001). A discrepancy between this and
the theoretical calibration of the red giant branch tip magnitude method
remains. Finally, we report a newly detected young (up to about 10Myr) stellar
association of about average size (~140pc) in one of the disk fields.Comment: 47 pages, 12 figures (the quality of Figures 1 and 6 has been
degraded), Corrected minor textual errors and two reference omissions to
match the version to be published in The Astronomical Journa
Planetary Nebulae as standard candles XI. Application to Spiral Galaxies
We report the results of an [O III] lambda 5007 survey for planetary nebulae
(PN) in three spiral galaxies: M101 (NGC 5457), M51 (NGC 5194/5195) and M96
(NGC 3368). By comparing on-band/off-band [O III] lambda 5007 images with
images taken in H-alpha and broadband R, we identify 65, 64 and 74 PN
candidates in each galaxy, respectively. From these data, an adopted M31
distance of 770 kpc, and the empirical planetary nebula luminosity function
(PNLF), we derive distances to M101, M51, and M96 of 7.7 +/- 0.5, 8.4 +/- 0.6,
and 9.6 +/- 0.6 Mpc. These observations demonstrate that the PNLF technique can
be successfully applied to late-type galaxies, and provide an important overlap
between the Population I and Population II distance scales. We also discuss
some special problems associated with using the PNLF in spiral galaxies,
including the effects of dust and the possible presence of [O III] bright
supernova remnants.Comment: 38 pages, TeX, with tables included but not figures. Uses epsf.tex
and kpnobasic.tex. To be published in the Astophysical Journal. Full paper is
available at http://www.astro.psu.edu/users/johnf/Text/research.htm
Planetary Nebulae as Standard Candles. XII. Connecting the Population I and Population II Distance Scales
We report the results of [OIII] lambda 5007 surveys for planetary nebulae
(PNe) in NGC 2403, 3115, 3351, 3627, 4258, and 5866. Using on-band/off-band
[OIII] and H-alpha images, we identify samples of PNe in these galaxies and
derive distances using the planetary nebula luminosity function (PNLF). We then
combine these measurements with previous data to compare the PNLF, Cepheid, and
surface brightness fluctuation (SBF) distance scales. We use a sample of 13
galaxies to show that the absolute magnitude of the PNLF cutoff is fainter in
small, low-metallicity systems, but the trend is well modelled theoretically.
When this dependence is removed, the scatter between the Cepheid and PNLF
distances becomes consistent with the internal errors of the methods and
independent of any obvious galaxy parameter. We then use the data to
recalibrate the zero point of the PNLF distance scale. We use a sample of 28
galaxies to show that the scatter between the PNLF and SBF distance
measurements agrees with that predicted from the techniques' internal errors,
and that no systematic trend exists between the distance residuals and stellar
population. However, we find the PNLF and SBF methods have a significant scale
offset: Cepheid-calibrated PNLF distances are, on average, ~0.3 mag smaller
than Cepheid-calibrated SBF distances. We discuss the possible causes of this
offset, and suggest that internal extinction in the bulges of the SBF
calibration galaxies is the principle cause of the discrepancy. If this is
correct, the SBF-based Hubble Constant must be increased by ~7%. We use our
distance to NGC 4258 to argue that the short distance scale to the LMC is
correct, and that the global Hubble Constant inferred from the HST Key Project
should be increased by 8 +/- 3% to H_0 = 78 +/- 7 km/s/Mpc. (abridged)Comment: 38 pages, 9 figures included, accepted for publication in the
Astrophysical Journa
Oxygen and nitrogen abundances in nearby galaxies. Correlations between oxygen abundance and macroscopic properties
We performed a compilation of more than 1000 published spectra of HII regions
in spiral galaxies. The oxygen and nitrogen abundances in each HII region were
recomputed in a homogeneous way, using the P-method. The radial distributions
of oxygen and nitrogen abundances were derived. The correlations between oxygen
abundance and macroscopic properties are examined. There is a significant
difference between the L-Z relationship obtained here and that based on the
oxygen abundances determined through the R_23-calibrations. The oxygen
abundance of NGC 5457 recently determined using direct measurements of Te
(Kennicutt, Bresolin & Garnett 2003) agrees with the L-Z relationship derived
here, but is in conflict with the L-Z relationship derived with the R_23-based
oxygen abundances. The obtained L-Z relation for spirals is compared to that
for irregulars. Our sample of galaxies shows evidence that the slope of the
O/H-M_B relationship for spirals is slightly more shallow than that for
irregulars. The effective oxygen yields were estimated for spiral and irregular
galaxies. The effective oxygen yield increases with increasing luminosity from
M_B=-11 to M_B=-18 (or with increasing rotation velocity from Vrot=10 km/s to
Vrot=100 km/s) and then remains approximately constant. Irregular galaxies from
our sample have effective oxygen yields lowered by a factor of 3 at maximum,
i.e. irregular galaxies usually keep at least 1/3 of the oxygen they
manufactured during their evolution.Comment: Accepted for publication in Astronomy and Astrophysics (Figures 2-5,
Tables 2,6 and Appendix will only be published in the electronic version of
the Journal