807 research outputs found
Hunting for Extremely Faint Planetary Nebulae in the SDSS Spectroscopic Database
Using ~1,700,000 target- and sky-fiber spectra from the SDSS, we have carried
out a systematic search for Galactic planetary nebulae (PNe) via detections of
the [OIII] 4959, 5007 lines. Thanks to the excellent sensitivity of the SDSS
spectroscopic surveys, this is by far the deepest search for PNe ever taken,
reaching a surface brightness of the [OIII] 5007 line down to about 29.0
mag./arcsec^2. The search recovers 13 previously known PNe in the Galactic
Caps. In total, 44 new PN candidates are identified, including 7 candidates of
multiple detections and 37 candidates of single detection. The 7 candidates of
multiple detections are all extremely large (between 21' - 154') and faint,
located mostly in the low Galactic latitude region and with a kinematics
similar to disk stars. After checking their images in Ha and other bands, three
of them are probably HII regions, one is probably associated with a new
supernova remnant, another one is possibly a true PN, and the remaining two
could be either PNe or supernova remnants. Based on sky positions and
kinematics, 7 candidates of single detection probably belong to the halo
population. If confirmed, they will increase the number of known PNe in the
Galactic halo significantly. All the newly identified PN candidates are very
faint, with a surface brightness of the [OIII] 5007 line between 27.0 - 30.0
mag./arcsec^2, very challenging to be discovered with previous techniques and
thus may greatly increase the number of "missing" faint PNe. Our results
demonstrate the power of large scale fiber spectroscopy in hunting for
ultra-faint PNe and other types of emission line nebulae. Combining the large
spectral databases provided by the SDSS and other on-going projects (e.g. the
LAMOST Galactic surveys), it is possible to build a statistically meaningful
sample of ultra-faint, large, evolved PNe, thus improving the census of
Galactic PNe.Comment: 23 pages, 1 table and 16 figures. MNRAS accepted. High resolution
version and online-only material may be found at
http://kiaa.pku.edu.cn/DSSGAC/YL13_pne_sdss.pd
Stellar loci I. Metallicity dependence and intrinsic widths
Stellar loci are widely used for selection of interesting outliers, reddening
determinations, and calibrations. However, hitherto the dependence of stellar
loci on metallicity has not been fully explored and their intrinsic widths are
unclear. In this paper, by combining the spectroscopic and re-calibrated
imaging data of the SDSS Stripe 82, we have built a large, clean sample of
dwarf stars with accurate colors and well determined metallicities to
investigate the metallicity dependence and intrinsic widths of the SDSS stellar
loci. Typically, one dex decrease in metallicity causes 0.20 and 0.02 mag
decrease in colors u-g and g-r, and 0.02 and 0.02 mag increase in colors r-i
and i-z, respectively. The variations are larger for metal-rich stars than for
metal-poor ones, and for F/G/K stars than for A/M ones. Using the sample, we
have performed two dimensional polynomial fitting to the u-g, g-r, r-i, and i-z
colors as a function of color g-i and metallicity [Fe/H]. The residuals, at the
level of 0.029, 0.008, 0.008 and 0.011 mag for the u-g, g-r, r-i, and i-z
colors, respectively can be fully accounted for by the photometric errors and
metallicity uncertainties, suggesting that the intrinsic widths of the loci are
at maximum a few mmag. The residual distributions are asymmetric, revealing
that a significant fraction of stars are binaries. In a companion paper, we
will present an unbiased estimate of the binary fraction for field stars. Other
potential applications of the metallicity dependent stellar loci are briefly
discussed.Comment: 6 pages, 4 figures, 1 table, ApJ in pres
Stellar loci III: Photometric metallicities for half million FGK stars of Stripe 82
We develop a method to estimate photometric metallicities by simultaneously
fitting the dereddened colors u-g, g-r, r-i and i-z from the SDSS with those
predicted by the metallicity-dependent stellar loci. The method is tested with
a spectroscopic sample of main-sequence stars in Stripe 82 selected from the
SDSS DR9 and three open clusters. With 1 per cent photometry, the method is
capable of delivering photometric metallicities precise to about 0.05, 0.12,
and 0.18 dex at metallicities of 0.0, -1.0, and -2.0, respectively, comparable
to the precision achievable with low-resolution spectroscopy at a
signal-to-noise ratio of 10. We apply this method to the re-calibrated Stripe
82 catalog and derive metallicities for about 0.5 million stars of colors 0.3 <
g-i < 1.6 mag and distances between 0.3 -- 18 kpc. Potential systematics in the
metallicities thus derived, due to the contamination of giants and binaries,
are investigated. Photometric distances are also calculated. About 91, 72, and
53 per cent of the sample stars are brighter than r = 20.5, 19.5, and 18.5 mag,
respectively. The median metallicity errors are around 0.19, 0.16, 0.11, and
0.085 dex for the whole sample, and for stars brighter than r = 20.5, 19.5, and
18.5 mag, respectively. The median distance errors are 8.8, 8.4, 7.7, and 7.3
per cent for the aforementioned four groups of stars, respectively. The data
are publicly available. Potential applications of the data in studies of the
distribution, (sub)structure, and chemistry of the Galactic stellar
populations, are briefly discussed. The results will be presented in future
papers.Comment: 10 pages, 10 figures, ApJ accepte
Information-Coupled Turbo Codes for LTE Systems
We propose a new class of information-coupled (IC) Turbo codes to improve the
transport block (TB) error rate performance for long-term evolution (LTE)
systems, while keeping the hybrid automatic repeat request protocol and the
Turbo decoder for each code block (CB) unchanged. In the proposed codes, every
two consecutive CBs in a TB are coupled together by sharing a few common
information bits. We propose a feed-forward and feed-back decoding scheme and a
windowed (WD) decoding scheme for decoding the whole TB by exploiting the
coupled information between CBs. Both decoding schemes achieve a considerable
signal-to-noise-ratio (SNR) gain compared to the LTE Turbo codes. We construct
the extrinsic information transfer (EXIT) functions for the LTE Turbo codes and
our proposed IC Turbo codes from the EXIT functions of underlying convolutional
codes. An SNR gain upper bound of our proposed codes over the LTE Turbo codes
is derived and calculated by the constructed EXIT charts. Numerical results
show that the proposed codes achieve an SNR gain of 0.25 dB to 0.72 dB for
various code parameters at a TB error rate level of , which complies
with the derived SNR gain upper bound.Comment: 13 pages, 12 figure
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