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High Proper Motion Objects toward the Inner Milky Way: Characterization of Newly Identified Nearby Stars from the VISTA Variables in the Vía Láctea Survey

Gromadzki M., Kurtev R., Beamán J. C., Tekola A., Ramphul R., Ivanov V. D., Minniti D., Folkes S. L., Vaisanen P., Kniazev A. Y., Borissova J., Parsons S. G., Villanueva V.

Acta Astronomica

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2016

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Vol. 66, No. 3

293--306

EN

The census of the Solar neighborhood is still incomplete, as demonstrated by recent discoveries of many objects within 5-10 pc from the Sun. The area around the mid-plane and bulge of the Milky Way presents the most difficulties in searches for such nearby objects, and is therefore deficient in the known population. This is largely due to high stellar densities encountered. Spectroscopic, photometric and kinematic characterization of these objects allows better understanding of the local mass function, the binary fraction, and provides new interesting targets for more detailed studies. We report the spectroscopic follow-up and characterization of twelve bright high proper motion objects, identified from the VISTA Variables in Vía Láctea survey (VVV). We used the 1.9-m telescope of the South African Astronomical Observatory (SAAO) for low-resolution optical spectroscopy and spectral classification, and the MPG/ESP 2.2-m telescope Fiber-fed Extended Range Optical Spectrograph (FEROS) high-resolution optical spectroscopy to obtain the radial and space velocities for three of them. Six of our objects have co-moving companions. We derived optical spectral types and photometric distances, and classified all of them as K and M dwarfs within 27-264 pc from the Sun. Finally, we found that one of the sources, VVV J141421.23-602326.1 (a co-moving companion of VVV J141420.55-602337.1), appears to be a rare massive white dwarf located close to the ZZ Cet instability strip in the CMD and CC diagrams. Many of the objects in our list are interesting targets for exoplanet searches.

2

Ultra-Short-Period Binary Systems in the OGLE Fields Toward the Galactic Bulge

Soszyński I., Stępień K., Pilecki B., Mróz P., Udalski A., Szymański M. K., Pietrzyński G., Wyrzykowski Ł., Ulaczyk K., Poleski R., Kozłowski S., Pietrukowicz P., Skowron J., Pawlak M.

Acta Astronomica

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2015

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Vol. 65, No. 1

39--62

EN

We present a sample of 242 ultra-short-period (Porb <0.22 d) eclipsing and ellipsoidal binary stars identified in the OGLE fields toward the Galactic bulge. Based on the light curve morphology, we divide the sample into candidates for contact binaries and non-contact binaries. In the latter group we distinguish binary systems consisting of a cool main-sequence star and a B-type subdwarf (HW Vir stars) and candidates for cataclysmic variables, including five eclipsing dwarf novae. One of the detected eclipsing binary systems - OGLE-BLG-ECL-000066 - with the orbital period below 0.1 d, likely consists of M dwarfs in a nearly contact configuration. If confirmed, this would be the shortest-period M-dwarf binary system currently known. We discuss possible evolutionary mechanisms that could lead to the orbital period below 0.1 d in an M-dwarf binary.

3

Mt. Suhora Survey - Searching for Pulsating M Dwarfs. III

Baran A. S., Winiarski M., Siwak M., Fox-Machado L., Kozieł-Wierzbowska D., Krzesinski J., Dróżdz M., Winans A.

Acta Astronomica

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2013

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Vol. 63, No. 1

41--52

EN

We present our final report on the search for pulsating M dwarfs. We used moderate (<1 m) ground-based telescopes. Our detection was limited to 1 ppt, which is roughly 0.1% of flux variation. We employed both the Fourier technique and Phase Dispersion Minimization (PDM) method. Our data analyses revealed no detection of pulsations in M dwarfs down to the above amplitude level. The results of our survey cannot reject or confirm the theoretical calculations suggesting the M dwarfs can pulsate, however, if the pulsations exist in these stars their amplitudes must be lower than 1 ppt. As a by-product of our search we found several new variable stars, including pulsating stars, binary systems and flare stars.

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Mt. Suhora Survey - Searching for Pulsating M Dwarfs. II

Krzesinski J., Baran A. S., Winiarski M., Fox-Machado L., Dróżdż M., Siwak M., Kozieł-Wierzbowska D.

Acta Astronomica

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2012

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Vol. 62, No. 2

201--212

EN

We present our report on the second part of our ongoing project which aims at finding stellar pulsations in early type M dwarfs. Our search is based on time-series photometric data acquired mainly at Mt. Suhora Observatory, though, several stars have been observed from other sites. Data of 49 M dwarfs are included in this paper. We analyzed the data by means of the Fourier technique as well as Phase Dispersion Minimization method. We still cannot confirm any pulsating M dwarf at the model-predicted period of 23-40 min. In addition to our main goal we searched for flare activities, planetary transits and other types of brightness changes. We did find several new variable stars, including pulsating stars, binary systems and flare stars, however, only flaring M dwarfs are presented in this paper.

5

Evolution of Low Mass Contact Binaries

Stępień K., Gazeas K.

Acta Astronomica

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2012

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Vol. 62, No. 2

153--177

EN

We present a study on low-mass contact binaries (LMCB) with orbital periods shorter than 0.3 days and total mass lower than about 1.4 MSun. We show that such systems have a long pre-contact phase, which lasts for 8-9 Gyrs, while the contact phase takes only about 0.8 Gyr, which is rather a short fraction of the total life. With low mass transfer rate during contact, moderate mass ratios prevail in LMCBs since they do not have enough time to reach extreme mass ratios often observed in higher mass binaries. During the whole evolution both components of LMCBs remain within the MS band. The evolution of cool contact binaries toward merging is controlled by the interplay between the evolutionary expansion of the less massive component resulting in the mass transfer to the more massive component and the angular momentum loss from the system by the magnetized wind. In LMCB the angular momentum loss prevails. As a result, the orbital period systematically decreases until the binary overflows the outer critical Roche surface and the components merge into a single fast rotating star of a solar type surrounded by a remnant disk carrying excess angular momentum. The disk can be a place of planet formation with the age substantially lower than the age of a host star. The calculated theoretical tracks show good agreement with the physical properties of LMCB from the available observations. Estimates of the frequency of occurrence of LMCB and the merger formation rate indicate that about 40 LMCBs and about 100 low mass merger products is expected to exist within 100 pc from the Sun.

6

Mt. Suhora Survey - Searching for Pulsating M Dwarfs in Kepler Public Dataset

Baran A. S., Fox-Machado L., Lykke J., Nielsen M., Telting J. H.

Acta Astronomica

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2011

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Vol. 61, No. 4

325--343

EN

We present our analysis of Kepler short cadence public data to search for stellar pulsations in M dwarfs. Theoretical calculations predict that these stars may suffer from a fundamental radial mode driven by the nuclear ε mechanism. First results published thus far show no significant signal around the expected frequency of 36 c/d down to 1 ppt (parts per thousand). In this paper we include new results obtained from a sample of 86 stars, although a real number of M dwarfs after spectral classification turned out to be small. Unluckily, we have not detected any significant signal typically down to 1-10 ppm (parts per million) in M dwarfs and since the sample of these stars consists of 6 objects only, we cannot make any strong conclusion if the ε mechanism drives perturbation in radius to propagate to a detectable amplitude at the surface. As a by-product of our work we provide a spectral classification of 86 Kepler objects, along with their variability and thus far unknown artifacts residing in Kepler photometry.

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Mt. Suhora Survey - Searching for Pulsating M Dwarfs. I

Baran A. S., Winiarski M., Krzesiński J., Fox-Machado L., Kawaler S. D., Dróżdż M., Faltenbacher A., Thompson M. A., Reed M. D.

Acta Astronomica

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2011

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Vol. 61, No. 1

37--58

EN

We present the first results of our M dwarf survey in search for stellar pulsation in low mass main sequence stars. Theoretical calculations predict that ε mechanism might drive a fundamental radial mode in these stars and therefore pulsations could be observed photometrically. Although M dwarfs are known for their flare and spot activity they have not yet been the subject of dedicated time-series surveys for pulsation. In this work we include the light curves and amplitude spectra of 46 M dwarfs, which have been observed during the first two years of our survey. We did not detect any pulsations yet. As a by-product of our search, we describe the light curves of some flare M dwarfs. The survey will last for two more years and during that period more than a hundred of M0-M4 type main sequence stars will be observed.

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The Optical Gravitational Lensing Experiment. Planetary and Low-Luminosity Object Transits in the Fields of Galactic Disk. Results of the 2003 OGLE Observing Campaigns

Udalski A., Szymański M. K., Kubiak M., Pietrzyński G., Soszyński I., Żebruń K., Szewczyk O., Wyrzykowski Ł.

Acta Astronomica

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2004

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Vol. 54, No. 4

313--345

EN

We present results of two observing campaigns conducted by the OGLE-III survey in the 2003 observing season aiming at the detection of new objects with planetary transiting companions. Six fields of 35'×35' each located in the Galactic disk were monitored with high frequency for several weeks in February-July 2003. Additional observations of three of these fields were also collected in the 2004 season. Altogether about 800 and 1500 epochs were collected for the fields of both campaigns, respectively. The search for low depth transits was conducted on about 230 000 stars with photometry better than 15mmag. It was focused on detection of planetary companions, thus clear non-planetary cases were not included in the final list of selected objects. Altogether we discovered 40 stars with shallow (≤0.05 mag) flat-bottomed transits. In each case several individual transits were observed allowing determination of photometric elements. Additionally, the lower limits on radii of the primary and companion were calculated. From the photometric point of view the new OGLE sample contains many very good candidates for extrasolar transiting planets. However, only the future spectroscopic follow-up observations of the OGLE sample - determination of the amplitude of radial velocity and exclusion of blending possibilities - may allow to confirm their planetary status. In general, the transiting objects may be extrasolar planets, brown dwarfs, M-type dwarfs or fake transits caused by blending. All photometric data of objects with transiting companions discovered during the 2003 campaigns are available to the astronomical community from the OGLE Internet archive.

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The Optical Gravitational Lensing Experiment. Additional Planetary and Low-Luminosity Object Transits from the OGLE 2001 and 2002 Observational Campaigns

Udalski A., Pietrzyński G., Szymański M., Kubiak M., Żebruń K., Soszyński I., Szewczyk O., Wyrzykowski Ł.

Acta Astronomica

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2003

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Vol. 56, No. 2

133--149

EN

The photometric data collected by OGLE-III during the 2001 and 2002 observational campaigns aiming at detection of planetary or low-luminosity object transits were corrected for small scale systematic effects using the data pipeline by Kruszewski and Semeniuk and searched again for low amplitude transits. Sixteen new objects with small transiting companions, additional to previously found samples, were discovered. Most of them are small amplitude cases which remained undetected in the original data. Several new objects seem to be very promising candidates for systems containing substellar objects: extrasolar planets or brown dwarfs. Those include OGLE-TR-122, OGLE-TR-125, OGLE-TR-130, OGLE-TR-131 and a few others. Those objects are particularly worth spectroscopic follow-up observations for radial velocity measurements and mass determination. With well known photometric orbit only a few RV measurements should allow to confirm their actual status. All photometric data of presented objects are available to the astronomical community from the OGLE Internet archive.

10

The Optical Gravitational Lensing Experiment. Search for Planetary and Low-Luminosity Object Transits in the Galactic Disk. Results of 2001 Campaign

Udalski A., Paczyński B., Żebruń K., Szymański M., Kubiak M., Soszyński I., Szewczyk O., Wyrzykowski Ł., Pietrzyński G.

Acta Astronomica

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2002

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Vol. 52, No. 1

1--37

EN

We present results of an extensive photometric search for planetary and low-luminosity object transits in the Galactic disk stars commencing the third phase of the Optical Gravitational Lensing Experiment - OGLE-III. Photometric observations of three fields in the direction of the Galactic center (800 epochs per field) were collected on 32 nights during time interval of 45 days. Out of the total of 5 million stars monitored, about 52 000 Galactic disk stars with photometry better than 1.5% were analyzed for flat-bottomed eclipses with the depth smaller than 0.08 mag. Altogether 46 stars with transiting low-luminosity objects were detected. For 42 of them multiple transits were observed, a total of 185, allowing orbital period determination. Transits in two objects: OGLE-TR-40 and OGLE-TR-10, with the radii ratio of about 0.14 and estimate of the radius of the companion 1.0-1.5 RJup, resemble the well known planetary transit in HD 209458. The sample was selected by the presence of apparent transits only, with no knowledge on any other properties. Hence, it is very well suited for general study of low-luminosity objects. The transiting objects may be Jupiters, brown dwarfs, or M dwarfs. Future determination of the amplitude of radial velocity changes will establish their masses, and will confirm or refute the reality of the so called "brown dwarf desert". The low-mass stellar companions will provide new data needed for the poorly known mass-radius relation for the lower main sequence. All photometric data are available to the astronomical community from the OGLE Internet archive.

11

The Optical Gravitational Lensing Experiment. Search for Planetary and Low-Luminosity Object Transits in the Galactic Disk. Results of 2001 Campaign - Supplement

Udalski A., Żebruń K., Szymański M., Kubiak M., Soszyński I., Szewczyk O., Wyrzykowski Ł., Pietrzyński G.

Acta Astronomica

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2002

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Vol. 52, No. 2

115--128

EN

The photometric data collected during 2001 season OGLE-III planetary/low luminosity object transit campaign were reanalyzed with the new transit search technique - the BLS method by Kovacs, Zucker and Mazeh. In addition to all presented in our original paper transits, additional 13 objects with transiting low-luminosity companions were discovered. We present here a supplement to our original catalog - the photometric data, light curves and finding charts of all 13 new objects. The model fits to the transit light curves indicate that a few new objects may be Jupiter-sized (R<1.6 RJup). OGLE-TR-56 is a particularly interesting case. Its transit has only 13 mmag depth, short duration and a period of 1.21190 days. Model fit indicates that the companion may be Saturn-sized if the passage were central. Spectroscopic follow-up observations are encouraged for final classification of the transiting objects as planets, brown dwarfs or late M-type dwarf stars. We also provide the most recent ephemerides of other most promising planetary transits: OGLE-TR-10 and OGLE-TR-40 based on observations collected in June 2002. All photometric data are available to the astronomical community from the OGLE Internet archive.

12

The Optical Gravitational Lensing Experiment. Planetary and Low-Luminosity Object Transits in the Carina Fields of the Galactic Disk

Udalski A., Szewczyk O., Żebruń K., Pietrzyński G., Szymański M., Kubiak M., Soszyński I., Wyrzykowski Ł.

Acta Astronomica

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2002

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Vol. 52, No. 4

317--359

EN

We present results of the second "planetary and low-luminosity object transit" campaign conducted by the OGLE-III survey. Three fields (35'×35' each) located in the Carina regions of the Galactic disk (l≈290°) were monitored continuously in February-May 2002. About 1150 epochs were collected for each field. The search for low depth transits was conducted on about 103 000 stars with photometry better than 15 mmag. In total, we discovered 62 objects with shallow depth (≤0.08 mag) flat-bottomed transits. For each of these objects several individual transits were detected and photometric elements were determined. Also lower limits on radii of the primary and companion were calculated. The 2002 OGLE sample of stars with transiting companions contains considerably more objects that may be Jupiter-sized (R<1.6 RJup) compared to our 2001 sample. There is a group of planetary candidates with the orbital periods close to or shorter than one day. If confirmed as planets, they would be the shortest period extrasolar planetary systems. In general, the transiting objects may be extrasolar planets, brown dwarfs, or M-type dwarfs. One should be, however, aware that in some cases unresolved blends of regular eclipsing stars can mimic transits. Future spectral analysis and eventual determination of the amplitude of radial velocity should allow final classification. High resolution spectroscopic follow-up observations are, therefore, strongly encouraged. All photometric data are available to the astronomical community from the OGLE Internet archive.