Comparison Stars Research Articles

Helium Enhancement in the Metal-rich Red Giants of ω Centauri

Abstract The helium-enriched (He-enriched) metal-rich red giants of ω Centauri, discovered by Hema & Pandey using the low-resolution spectra from the Vainu Bappu Telescope, and confirmed by the analyses of the high-resolution spectra obtained from the HRS-South African Large Telescope for LEID 34225 and LEID 39048, are reanalyzed here to determine their degree of He enhancement/hydrogen deficiency (H deficiency). The observed MgH band combined with model atmospheres with differing He/H ratios are used for the analyses. The He/H ratios of these two giants are determined by enforcing the fact that the derived Mg abundances from the Mg i lines and from the subordinate lines of the MgH band must be same for the adopted model atmosphere. The estimated He/H ratios for LEID 34225 and LEID 39048 are 0.15 ± 0.04 and 0.20 ± 0.04, respectively, whereas the normal He/H ratio is 0.10. Following the same criteria for the analyses of the other two comparison stars (LEID 61067 and LEID 32169), a normal He/H ratio of 0.10 is obtained. The He/H ratio of 0.15–0.20 corresponds to a mass fraction of helium (Z(He) = Y) of about 0.375–0.445. The range of helium enhancement and the derived metallicity of the program stars are in line with those determined for ω Cen’s blue main-sequence stars. Hence, our study provides the missing link for the evolutionary track of the metal-rich helium-enhanced population of ω Centuari. This research work is the very first spectroscopic determination of the amount of He enhancement in the metal-rich red giants of ω Centauri using the Mg i and MgH lines.

Detailed chemical compositions of planet-hosting stars – I. Exploration of possible planet signatures

ABSTRACT We present a line-by-line differential analysis of a sample of 16 planet-hosting stars and 68 comparison stars using high-resolution, high signal-to-noise ratio spectra gathered using Keck. We obtained accurate stellar parameters and high-precision relative chemical abundances with average uncertainties in Teff, log g, [Fe/H], and [X/H] of 15 K, 0.034 cm s−2 , 0.012 dex, and 0.025 dex, respectively. For each planet host, we identify a set of comparison stars and examine the abundance differences (corrected for Galactic chemical evolution effect) as a function of the dust condensation temperature, Tcond, of the individual elements. While we confirm that the Sun exhibits a negative trend between abundance and Tcond, we also confirm that the remaining planet hosts exhibit a variety of abundance–Tcond trends with no clear dependence upon age, metallicity, or Teff. The diversity in the chemical compositions of planet-hosting stars relative to their comparison stars could reflect the range of possible planet-induced effects present in these planet hosts, from the sequestration of rocky material (refractory poor) to the possible ingestion of planets (refractory rich). Other possible explanations include differences in the time-scale, efficiency and degree of planet formation, or inhomogeneous chemical evolution. Although we do not find an unambiguous chemical signature of planet formation among our sample, the high-precision chemical abundances of the host stars are essential for constraining the composition and structure of their exoplanets.

Observation of the exoplanetary transit for Wasp-2

We present observations of the exoplanet transit for the WASP-2 star carried out with the MTM-500 telescope at the Crimean Astrophysical Observatory of the Russian Academy of Sciences. A brief history of the discovery of the exoplanet WASP-2b is considered. The main characteristics are outlined. The observations and a processing procedure are described. Analysis of the obtained results was performed and compared with information from the ETD and NASA Exoplanet Archive databases. The photometry data of the WASP-2 star and the comparison star GSC 0052201406 are given in the Appendix to the article.

Наблюдение транзитного явления у звезды с экзопланетой WASP-2

Представлены наблюдения транзита экзопланеты у звезды WASP-2, выполненные на телескопе МТМ-500 Крымской астрофизической обсерватории РАН. Рассмотрена краткая история открытия экзопланеты, представлены ее основные характеристики. Описаны наблюдения и процедура обработки. Выполнен анализ полученных результатов и проведено их сравнение с информацией из баз данных ETD и Архива экзопланет NASA. В приложении к статье приведены данные фотометрии звезды WASP-2 и звезды сравнения GSC 0052201406.

Analysis of putative exoplanetary signatures found in light curves of two sdBV stars observed by Kepler

Context. We investigate the validity of the claim that invokes two extreme exoplanetary system candidates around the pulsating B-type subdwarfs KIC 10001893 and KIC 5807616 from the primary Kepler field. Aims. Our goal was to find characteristics and the source of weak signals that are observed in these subdwarf light curves. Methods. To achieve this, we analyzed short- and long-cadence Kepler data of the two stars by means of a Fourier transform and compared the results to Fourier transforms of simulated light curves to which we added exoplanetary signals. The long-cadence data of KIC 10001893 were extracted from CCD images of a nearby star, KIC 10001898, using a point spread function reduction technique. Results. It appears that the amplitudes of the Fourier transform signals that were found in the low-frequency region depend on the methods that are used to extract and prepare Kepler data. We demonstrate that using a comparison star for space telescope data can significantly reduce artifacts. Our simulations also show that a weak signal of constant amplitude and frequency, added to a stellar light curve, conserves its frequency in Fourier transform amplitude spectra to within 0.03 μHz. Conclusions. Based on our simulations, we conclude that the two low-frequency Fourier transform signals found in KIC 5807616 are likely the combined frequencies of the lower amplitude pulsating modes of the star. In the case of KIC 10001893, the signal amplitudes that are visible in the light curve depend on the data set and reduction methods. The strongest signal decreases significantly in amplitude when KIC 10001898 is used as a comparison star. Finally, we recommend that the signal detection threshold is increased to 5σ (or higher) for a Fourier transform analysis of Kepler data in low-frequency regions.

A revisit to the enigmatic variable star 21 Comae

The magnetic chemically peculiar (Ap/CP2) star 21 Com has been extensively studied in the past, albeit with widely differing and sometimes contradictory results, in particular concerning the occurrence of short term variability between about 5 to 90 minutes. We have performed a new investigation of 21 Com using MOST satellite and high-cadence ground-based photometry, time series spectroscopy, and evolutionary and pulsational modeling. Our analysis confirms that 21 Com is a classical CP2 star showing increased abundances of, in particular, Cr and Sr. From spectroscopic analysis, we have derived Teff = 8900(200) K, log g = 3.9(2), and vsini = 63(2) km/s. Our modeling efforts suggest that 21 Com is a main sequence (MS) star seen equator-on with a mass of 2.29(10) M(Sun) and a radius of R = 2.6(2) R(Sun). Our extensive photometric data confirm the existence of rotational light variability with a period of 2.05219(2) d. However, no significant frequencies with a semi-amplitude exceeding 0.2 mmag were found in the frequency range from 5 to 399 c/d. Our RV data also do not indicate short-term variability. We calculated pulsational models assuming different metallicities and ages, which do not predict the occurrence of unstable modes. The star 18 Com, often employed as comparison star for 21 Com in the past, has been identified as a periodic variable (P = 1.41645 d). While it is impossible to assess whether 21 Com has exhibited short-term variability in the past, the new observational data and several issues/inconsistencies identified in previous studies strongly suggest that 21 Com is neither a delta Scuti nor a roAp pulsator but a "well-behaved" CP2 star exhibiting its trademark rotational variability.

Optical variability of some quasars important to ICRF-Gaia CRF link

The Gaia optical observations started a few years ago. As a result, the Gaia Celestial Reference Frame (Gaia CRF) should replace the International CRF (ICRF). This could be done via extragalactic radio sources (mostly quasars - QSOs) visible in optical domain. During about 2.5 years (for the period July 2016 { April 2019) of our original observations of some QSOs outside ICRF list we collected observations in the V and R bands for five objects and their 30 comparison stars. Photometry stability of these objects is of importance for astrometry and the mentioned link. Because of it we did investigation of brightness variability of objects and their suitable comparison stars, and the F{test was performed. As a result, only the brightness of one object (1556+335) does not show variability. Other four objects were examined to determine the quasiperiods of their light curves using the method of Least Squares: 1535+231 (3.1 years in V, and 1.7 and 5.2 years in R filter), 1607+604 (2.7 years in V, and 1.3 and 2.3 years in R), 1722+119 (1.3 and 2.7 years in V, and 1.3 and 5.3 years in R), and 1741+597 (6.5 years in V, and 1.3 and 4.0 years in R). After a similar analysis of variability of comparison stars, the conclusion is that all of them are useful for differential photometry. Also, we provide our finding charts for these objects with suitable comparison stars.

CoLiTecVS – A new tool for the automated reduction of photometric observations

The capabilities of telescopes allow the plotting of light curves a routine task. This one shifts the main attention of the astronomer from plotting to research. To achieve this goal, we developed a new tool for the automated reduction of photometric observations, which includes the computational method for the brightness assessment of the investigated and comparison stars, brightness equalization of astronomical images using an inverse median filter, as well as light curve plotting and processing using different tools.

A new pipeline for astrometry and photometry of asteroids

AbstractUsing Python 3, astropy and astrometry.net, we have developed a pipeline to obtain photometric light curves of asteroids automatically queried by the SkyBoT database from sequential FITS images. The pipeline provides: pre-reduction of data, astrometry, standard differential photometry and light curves by auto-selecting multiple comparison stars (maximum user-defined) from NOMAD catalog via VizieR. The code is an open source, free and hosted on GitHub with the GNU GPL v3 license.

A critical re-evaluation of the Thorne–Żytkow object candidate HV 2112

It has been argued in the literature that the star HV~2112 in the Small Magellanic Cloud is the first known example of a T\.ZO, a Red Supergiant with a degenerate neutron core. This claim is based on the star having a high luminosity ($\log (L/L_\odot)$~> 5), an extremely cool effective temperature, and a surface enriched in in lithium, calcium and various $irp$-process elements. In this paper we re-examine this evidence, and present new measurements of the stellar properties. By compiling archival photometry from blue to mid-IR for HV~2112 and integrating under it's spectral energy distribution we find a bolometric luminosity in the range of $\log (L/L_\odot)$=4.70-4.91, lower than that found in previous work and comparable to bright asymptotic giant branch (AGB) stars. We compare a VLT+XSHOOTER spectrum of HV~2112 to other late type, luminous SMC stars, finding no evidence for enhancements in Rb, Ca or K, though there does seem to be an enrichment in Li. We therefore conclude that a much more likely explanation for HV~2112 is that it is an intermediate mass($\sim$5M$_\odot$) AGB star. However, from our sample of comparison stars we identify a new T\.ZO candidate, HV~11417, which seems to be enriched in Rb but for which we cannot determine a Li abundance.

The Formation and Evolution of Galactic Disks with APOGEE and the Gaia Survey

Abstract We explore the structure and evolutionary history of Galactic disks with Apache Point Observatory Galactic Evolution Experiment data release 13 (DR13 hereafter) and Gaia Tycho-Gaia Astrometric Solution data. We use the [α/M] ratio to allocate stars into particular Galactic components to elucidate the chemical and dynamical properties of the thin and thick disks. The spatial motions of the sample stars are obtained in Galactic Cartesian and cylindrical coordinates. We analyze the abundance trends and metallicity and [α/M] gradients of the thick and thin disks. We confirm the existence of metal-weak thick-disk stars in Galactic disks. A kinematical method is used to select the thin- and thick-disk stars for comparison. We calculate the scale length and scale height of the kinematically and chemically selected thick and thin disks based on the axisymmetric Jeans equation. We conclude that the scale length of the thick disk is approximately equal to that of the thin disk via a kinematical approach. For the chemical selection, this disparity is about 1 kpc. Finally, we get the stellar orbital parameters and try to unveil the formation scenario of the thick disk. We conclude that the gas-rich merger and radial migration are more reasonable formation scenarios for the thick disk.

HR 8844: A New Transition Object between the Am Stars and the HgMn Stars?

Abstract While monitoring a sample of apparently slowly rotating superficially normal early-A stars, we have discovered that HR 8844 (A0 V) is actually a new chemically peculiar star. We first compared the high-resolution spectrum of HR 8844 with that of four slow rotators near A0V (ν Cap, ν Cnc, Sirius A, and HD 72660) to highlight similarities and differences. The lines of Ti ii, Cr ii, Sr ii, and Ba ii are conspicuous features in the high-resolution high signal-to-noise SOPHIE spectra of HR 8844 and much stronger than in the spectra of the normal star ν Cap. The Hg ii line at 3983.93 Å is also present in a 3.5% blend. Selected unblended lines of 31 chemical elements from He up to Hg have been synthesized using model atmospheres computed with ATLAS9 and the spectrum synthesis code SYNSPEC48 including hyperfine structure of various isotopes when relevant. These synthetic spectra have been adjusted to the mean SOPHIE spectrum of HR 8844, and high-resolution spectra of the comparison stars. Chi-squares were minimized to derive abundances or upper limits to the abundances of these elements for HR 8844 and the comparison stars. HR 8844 is found to have underabundances of He, C, O, Mg, Ca, and Sc, mild enhancements of Ti, V, Cr, Mn, and distinct enhancements of the heavy elements Sr, Y, Zr, Ba, La, Pr, Sm, Eu, and Hg, the overabundances increasing steadily with atomic number. This chemical pattern suggests that HR 8844 may actually be a new transition object between the coolest HgMn stars and the Am stars.

Where Is the Flux Going? The Long-term Photometric Variability of Boyajian’s Star

Abstract We present ∼800 days of photometric monitoring of Boyajian’s Star (KIC 8462852) from the All-Sky Automated Survey for Supernovae (ASAS-SN) and ∼4000 days of monitoring from the All Sky Automated Survey (ASAS). We show that from 2015 to the present the brightness of Boyajian’s Star has steadily decreased at a rate of 6.3 ± 1.4 mmag yr−1, such that the star is now 1.5% fainter than it was in 2015 February. Moreover, the longer time baseline afforded by ASAS suggests that Boyajian’s Star has also undergone two brightening episodes in the past 11 years, rather than only exhibiting a monotonic decline. We analyze a sample of ∼1000 comparison stars of similar brightness located in the same ASAS-SN field and demonstrate that the recent fading is significant at ≳99.4% confidence. The 2015–2017 dimming rate is consistent with that measured with Kepler data for the time period from 2009 to 2013. This long-term variability is difficult to explain with any of the physical models for the star’s behavior proposed to date.

Precision Time-series Photometry in the Thermal Infrared with a “Wall-eyed” Pointing Mode at the Large Binocular Telescope

Time-series photometry taken from ground-based facilities is improved with the use of comparison stars due to the short timescales of atmospheric-induced variability. However, the sky is bright in the thermal infrared (3–5 μm), and the correspondingly small fields of view of available detectors make it highly unusual to have a calibration star in the same field as a science target. Here, we present a new method of obtaining differential photometry by simultaneously imaging a science target and a calibrator star, separated by ≲2 amin, onto a 10 × 10 asec2 field-of-view detector. We do this by taking advantage of the Large Binocular Telescope’s (LBT) unique binocular design to point the two co-mounted telescopes apart and simultaneously obtain both targets in three sets of observations. Results indicate that the achievable scatter in LS-band ( μm) is at the percent level for bright targets and possibly better with heavier sampling and characterization of the systematics.

Can magnetic fields suppress convection in the atmosphere of cool white dwarfs? A case study on WD2105−820

Around 10% of white dwarfs exhibit global magnetic structures with fields ranging from 1 kG to hundreds of MG. Recently, the first radiation magnetohydrodynamics simulations of the atmosphere of white dwarfs showed that convection should be suppressed in their photospheres for magnetic fields with strengths B $\gtrsim$ 50 kG. These predictions are in agreement with our knowledge of stellar physics (e.g. energy transfer in strong magnetic field regions of the solar photosphere), but have yet to be directly confirmed from white dwarf observations. We obtained COS far-UV spectroscopy of the weakly magnetic, hydrogen-atmosphere, white dwarf WD2105-820 and of three additional non-magnetic, convective remnants (all in the $T_{\mathrm{eff}}$ range 9000-11,000 K). We fitted both the COS and the already available optical spectra with convective and radiative atmospheric models. As expected, we find that for two of the non-magnetic comparison stars only convective model fits predict consistent $T_{\mathrm{eff}}$ values from both the optical and the FUV spectra. In contrast, for WD2105-820 only the best fitting radiative model produced consistent results.

Optical polarimetry of KIC 8462852 in 2017 May–August

Abstract We present optical polarimetry in the period of 2017 May–August of the enigmatic ‘dipping’ star KIC 8462852. During that period, three ∼1 per cent photometric dips were reported by other observers. We measured the average absolute polarization of the source, and find no excess or unusual polarization compared to a nearby comparison star. We place tight upper limits on any change in the degree of polarization of the source between epochs in-dip and out-of-dip of <0.1 per cent (8500 Å) and <0.2 per cent (7050 Å and 5300 Å). How our limits are interpreted depends on the specific model being considered. If the whole stellar disc were covered by material with an optical depth of ∼0.01, then the fractional polarization introduced by this material must be less than 10–20 per cent. While our non-detection does not constrain the comet scenario, it predicts that even modest amounts of dust that have properties similar to Solar system comets may be detectable. We note that the sensitivity of our method scales with the depth of the dip. Should a future ∼20 per cent photometric dip be observed (as was previously detected by Kepler), our method would constrain any induced polarization associated with any occulting material to 0.5–1.0 per cent.

A comparison of radiometric calibration techniques for lunar impact flashes

Video observations of lunar impact flashes have been made by a number of researchers since the late 1990's and the problem of determination of the impact energies has been approached in different ways Bellot Rubio et al. (2000a, b), Yanagisawa et al. (2008), Bouley et al. (2012), Suggs et al. (2014), Rembold and Ryan (2015), Ortiz et al. (2015), Madiedo et al. (2015). The wide spectral response of the unfiltered video cameras in use for all published measurements necessitates color correction for the standard filter magnitudes available for the comparison stars but this is not typically considered. In our approach, the published color of the comparison star and an estimate of the color of the impact flash is used to correct it to the chosen filter bandpass. Magnitudes corrected to standard filters are then used to determine the luminous energy in the filter bandpass according to the stellar atmosphere calibrations of Bessell et al. (1998). In this paper we compare the various photometric calibration techniques and calculation of luminous energy (radiometry) of impact flashes. This issue has significant implications for determination of luminous efficiency, predictions of impact crater sizes for observed flashes, and the determination of the flux of meteoroids in the 10s of grams to kilograms mass range.

Rayleigh scattering in the transmission spectrum of HAT-P-18b

We have performed ground-based transmission spectroscopy of the hot Jupiter HAT-P-18b using the ACAM instrument on the William Herschel Telescope (WHT). Differential spectroscopy over an entire night was carried out at a resolution of $R \approx 400$ using a nearby comparison star. We detect a bluewards slope extending across our optical transmission spectrum which runs from 4750 to 9250\AA. The slope is consistent with Rayleigh scattering at the equilibrium temperature of the planet (852K). We do not detect enhanced sodium absorption, which indicates that a high-altitude haze is masking the feature and giving rise to the Rayleigh slope. This is only the second discovery of a Rayleigh scattering slope in a hot Jupiter atmosphere from the ground, and our study illustrates how ground-based observations can provide transmission spectra with precision comparable to the Hubble Space Telescope.

Flexible Variable Star Extractor - new software for variable stars detection using CCD photometry

We developed a computer program for variable stars detection using CCD photometry. It works with "varfind data" that could be exported after processing CCD frames using C-Munipack. The program chooses the comparison stars automatically, processes all time series using multiple comparison stars to get final light curves. We developed few filters and criteria that allow reducing the impact of outlying points, imaging artefacts and low quality CCD frames without careful manual time series reduction. We implemented the calculation of various variable detection indices. The pipeline has a possibility of plotting a two-channel diagram of selected pair of indices or mean brightness of the star for manual check if any outlying point is a variable candidate. The program is available at http://uavso.org.ua/varsearch/.

A Search for Binary Star Companions to the KELT Planet Hosts and a Comparison Sample. I. Results of DSSI Observations

Abstract We are conducting a search for binary companions around 11 hot-Jupiter hosts from the Kilodegree Extremely Little Telescope (KELT) survey and a large comparison sample of stars shown by KELT to not host a transiting hot Jupiter. The primary stars are bright (7.5 < V < 11) and of similar distance from Earth (100 < d < 300 pc). In this paper, we present the results of our observations using the Differential Speckle Survey Instrument. We observed 9 of the 14 KELT planet hosts that are visible from the northern hemisphere and 51 comparison stars, discovering two new potential companions and re-observing two previously known possible binary systems and one confirmed binary system. We provide an estimate of the chance alignment probability for our observed candidate binaries.