769 research outputs found
All-electric single electron spin initialization
We propose a nanodevice for single-electron spin initialization. It is based
on a gated planar semiconductor heterostructure with a quantum well and with
potentials generated by voltages applied to local gates. Initially we insert an
electron with arbitrary spin into the nanodevice. Next we perform a sequence of
spin manipulations, after which the spin is set in a desired direction (e.g.,
the growth direction). The operations are done all-electrically, do not require
any external fields and do not depend on the initial spin direction.Comment: 6 pages, 4 figure
Multi-Mode and Non-Standard Classical Cepheids in the Magellanic System
We present a sample of the most interesting classical Cepheids selected from
the OGLE collection of classical Cepheids in the Magellanic System. The main
selection criterion for this sample was the presence of non-standard, unique
pulsational properties.
The sample contains the first known double-mode Cepheid pulsating in the
second- and third-overtone modes and a large number of objects with non-radial
modes excited. We also found Cepheids revealing Blazhko-like light curve
modulation, objects ceasing pulsations or showing atypical shapes of their
light curves. Additionally, the status of several triple mode classical
Cepheids is updated based on OGLE-IV photometry extending the time baseline to
15 years.Comment: 11 pages, 7 figure
Over 38000 RR Lyrae Stars in the OGLE Galactic Bulge Fields
We present the most comprehensive picture ever obtained of the central parts
of the Milky Way probed with RR Lyrae variable stars. This is a collection of
38257 RR Lyr stars detected over 182 square degrees monitored photometrically
by the Optical Gravitational Lensing Experiment (OGLE) in the most central
regions of the Galactic bulge. The sample consists of 16804 variables found and
published by the OGLE collaboration in 2011 and 21453 RR Lyr stars newly
detected in the photometric databases of the fourth phase of the OGLE survey
(OGLE-IV). 93% of the OGLE-IV variables were previously unknown. The total
sample consists of 27258 RRab, 10825 RRc, and 174 RRd stars. We provide OGLE-IV
I- and V-band light curves of the variables along with their basic parameters.
About 300 RR Lyr stars in our collection are plausible members of 15 globular
clusters. Among others, we found the first pulsating variables that may belong
to the globular cluster Terzan 1 and the first RRd star in the globular cluster
M54. Our survey also covers the center and outskirts of the Sagittarius Dwarf
Spheroidal Galaxy enabling studies of the spatial distribution of the old
stellar population from this galaxy.
A group of double-mode RR Lyr stars with period ratios around 0.740 form a
stream in the sky that may be a relic of a cluster or a dwarf galaxy tidally
disrupted by the Milky Way. Three of our RR Lyr stars experienced a pulsation
mode switching from double-mode to single fundamental mode or vice versa. We
also present the first known RRd stars with large-amplitude Blazhko effect.Comment: 21 pages, 9 figures, submitted to Acta Astronomic
The OGLE Collection of Variable Stars. Classical Cepheids in the Magellanic System
We present here a nearly complete census of classical Cepheids in the
Magellanic System. The sample extends the set of Cepheids published in the past
by the Optical Gravitational Lensing Experiment (OGLE) to the outer regions of
the Large (LMC) and Small Magellanic Cloud (SMC). The entire collection
consists of 9535 Cepheids of which 4620 belong to the LMC and 4915 are members
of the SMC. We provide the I- and V-band time-series photometry of the
published Cepheids, their finding charts, and basic observational parameters.
Based on this unique OGLE sample of Cepheids we present updated
period--luminosity relations for fundamental, first, and second mode of
pulsations in the I- and V-bands and for the W_I extinction-free Wesenheit
index. We also show the distribution of classical Cepheids in the Magellanic
System. The OGLE collection contains several classical Cepheids in the
Magellanic Bridge - the region of interaction between the Magellanic Clouds.
The discovery of classical Cepheids and their estimated ages confirm the
presence of young stellar population between these galaxies.Comment: 16 pages, 5 figure
The OGLE Collection of Variable Stars. Anomalous Cepheids in the Magellanic Clouds
We present a collection of 250 anomalous Cepheids (ACs) discovered in the
OGLE-IV fields toward the Large (LMC) and Small Magellanic Cloud (SMC). The LMC
sample is an extension of the OGLE-III Catalog of ACs published in 2008, while
the SMC sample contains the first known bona fide ACs in this galaxy. The total
sample is composed of 141 ACs in the LMC and 109 ACs in the SMC. All these
stars pulsate in single modes: fundamental (174 objects) or first overtone (76
objects). Additionally, we report the discovery of four ACs located in the
foreground of the Magellanic Clouds. These are the first fundamental-mode ACs
known in the Galactic field.
We demonstrate that the coefficients phi_21 and phi_31 determined by the
Fourier light curve decomposition are useful discriminators between classical
Cepheids and ACs, at least in the LMC and in the field of the Milky Way. In the
SMC, the light curve shapes and mean magnitudes of short-period classical
Cepheids make them similar to ACs, which is a source of difficulties in the
discrimination of both classes of pulsators. The presence of unidentified ACs
in the catalogs of classical Cepheids may be partly responsible for the
observed non-linearity of the period-luminosity relation observed for
short-period Cepheids in the SMC. We compare spatial distributions of ACs,
classical Cepheids and RR Lyr stars. We show that the distribution of ACs
resembles that of old stars (RR Lyr variables), although in the LMC there are
visible structures typical for young population (classical Cepheids): the bar
and spiral arms. This may suggest that ACs are a mixture of relatively young
stars and mergers of very old stars.Comment: 18 pages, 9 figures, submitted to Acta Astronomic
Blazhko-type modulation in the double-mode RR Lyrae stars of the OGLE Galactic bulge collection
We present the analysis of Blazhko-type modulation in double-mode RR Lyrae
(RRd) stars found in the OGLE photometry of the Galactic bulge. Modulation is
detected in 15 stars. Most of them have non-typical period ratio of the radial
modes. In the Petersen diagram, at a given period of the fundamental mode, they
are located significantly below or above the sequence formed by the majority of
RRd stars.
Multi-periodic modulation is very frequent; two or three modulation periods
are detected in 8 stars. Modulation periods vary from ~20 to more than 300
days. Radial mode amplitudes can be modulated by a few to nearly hundred per
cent. Both radial modes may be modulated with the same period. More commonly
however, dominant modulation for the fundamental mode has different period than
dominant modulation for the first overtone. Quite often modulation of only one
mode is detected in the data. We find a clear feedback between pulsation
amplitude of the dominant mode and mean stellar brightness: lower the pulsation
amplitude, brighter the star. At phases of low pulsation amplitude, the mode
periods are prone to fast changes. All the stars share the common feature:
their pulsation properties are non-stationary. Amplitudes and phases of the
radial modes vary irregularly on a long time-scale of a few hundred or thousand
days. The short-term modulations are also irregular.
One of the stars has switched the pulsation mode recently: from single-mode
fundamental mode pulsation to RRd state. In other star the non-radial mode with
characteristic ~0.61 period ratio to the first overtone is detected. This
non-radial mode is likely modulated with the same period as the radial modes.Comment: 21 pages, 20 figures, accepted for publication in MNRAS; tables
A1-A15 to be published on-line onl
Period-Luminosity Relations for Ellipsoidal Binary Stars in the OGLE-III Fields of the Large Magellanic Cloud
We report the discovery of two distinct types of ellipsoidal binary systems
occupying, so called, sequence E on the period-luminosity (P-L) diagram. We
propose that steeper P-L relation is composed of giant-dwarf binaries, while
the other consists of giant-giant binary systems. Analysis is based on a sample
of 5334 objects, which we select from the OGLE-III survey data toward the the
Large Magellanic Cloud. We show that one of the components of ellipsoidal
binaries is typically either a Red Clump or a Red Giant Branch star, which
leads to clear separation split of the sequence E at P = 40 d. In its
short-period part, we identify two subsequences corresponding to the two types
of binary systems (E2 and E3), while in the longer-period part the two groups
merge forming a single subsequence E1. We extract a group of 271 ellipsoidal
systems with eccentric orbits, from our sample. We present the
period-luminosity relation they follow.Comment: 16 pages, 15 figures, submitted to Acta Astronomic
Deciphering the 3D structure of the old Galactic bulge from the OGLE RR Lyrae stars
We have analyzed a sample of 27,258 fundamental-mode RR Lyrae variable stars
(type RRab) detected recently toward the Galactic bulge by the Optical
Gravitational Lensing Experiment (OGLE) survey. The data support our earlier
claim that these metal-poor stars trace closely the barred structure formed of
intermediate-age red clump giants. The distance to the Galactic center (GC)
inferred from the bulge RR Lyrae stars is R_0=8.27+/-0.01(stat)+/-0.40(sys)
kpc. We show that their spatial distribution has the shape of a triaxial
ellipsoid with an major axis located in the Galactic plane and inclined at an
angle of i=20+/-3 deg to the Sun-GC line of sight. The obtained scale-length
ratio of the major axis to the minor axis in the Galactic plane and to the axis
vertical to the plane is 1:0.49(2):0.39(2). We do not see the evidence for the
bulge RR Lyrae stars forming an X-shaped structure. Based on the light curve
parameters, we derive metallicities of the RRab variables and show that there
is a very mild but statistically significant radial metallicity gradient. About
60% of the bulge RRab stars form two very close sequences on the
period-amplitude (or Bailey) diagram, which we interpret as two major old bulge
populations: A and B. Their metallicities likely differ. Population A is about
four times less abundant than the slightly more metal-poor population B. Most
of the remaining stars seem to represent other, even more metal-poor
populations of the bulge. The presence of multiple old populations indicates
that the Milky Way bulge was initially formed through mergers.Comment: ApJ, in pres
Intriguing triple-mode RR Lyrae star with period doubling
We report the discovery of an intriguing triple-mode RR Lyrae star found in
the OGLE Galactic bulge collection, OGLE-BLG-RRLYR-24137. In the OGLE catalog
the star was identified as RRd star - double-mode pulsator, pulsating
simultaneously in the fundamental and in the first overtone modes. We find that
third mode is excited and firmly detect its period doubling. Period ratios are
not far from that expected for triple-mode - fundamental, first and third
overtone - pulsation. Unfortunately, we cannot reproduce period ratios of the
three modes with a consistent set of pulsation models. Therefore the other
interpretation, that additional mode is non-radial, is also likely.Comment: 8 pages, 7 figures, accepted for publication in MNRA
OGLE-ing the Magellanic System: Stellar populations in the Magellanic Bridge
We report the discovery of a young stellar bridge, that forms a continuous
connection between the Magellanic Clouds. This finding is based on number
density maps for stellar populations found in data gathered by OGLE-IV, that
fully cover over 270 deg^2 of the sky in the Magellanic Bridge area. This is
the most extensive optical survey of this region up to date. We find that the
young population is present mainly in the western half of the MBR, which,
together with the newly discovered young population in the eastern Bridge, form
a continuous stream of stars connecting both galaxies along Dec ~ -73.5 deg.
The young population distribution is clumped, with one of the major densities
close to the SMC, and the other, fairly isolated and located approximately
mid-way between the Clouds, which we call the OGLE island. These overdensities
are well matched by HI surface density contours, although the newly found young
population in the eastern Bridge is offset by ~2 deg north from the highest HI
density contour. We observe a continuity of red clump stars between the
Magellanic Clouds, which represent an intermediate-age population. Red clump
stars are present mainly in the southern and central parts of the Magellanic
Bridge, below its gaseous part, and their presence is reflected by a strong
deviation from the radial density profiles of the two galaxies. This may
indicate either a tidal stream of stars, or that the stellar halos of the two
galaxies overlap. On the other hand, we do not observe such an overlap within
an intermediate-age population represented by the top of the red giant branch
and the asymptotic giant branch stars. We also see only minor mixing of the old
populations of the Clouds in the southern part of the Bridge, represented by
the lowest part of the red giant branch.Comment: 18 pages, 14 Figures. Published in ApJ. On-line data available from
http://ogle.astrouw.edu.p
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