arXiv subjects

C. Karoff

Publications and source records attributed to C. Karoff.

At least 73 records · Page 4Linked to original sources

arXiv2010

The solar-like CoRoT target HD 170987: spectroscopic and seismic observations

The CoRoT mission is in its third year of observation and the data from the second long run in the galactic centre direction are being analysed. The solar-like oscillating stars that have been observed up to now have given some interesting results, specially concerning the amplitudes that are lower than predicted. We present here the results from the analysis of the star HD 170987.The goal of this research work is to characterise the global parameters of HD 170987. We look for global seismic parameters such as the mean large separation, maximum amplitude of the modes, and surface rotation because the signal-to-noise ratio in the observations do not allow us to measure individual modes. We also want to retrieve the stellar parameters of the star and its chemical composition.We have studied the chemical composition of the star using ground-based observations performed with the NARVAL spectrograph. We have used several methods to calculate the global parameters from the acoustic oscillations based on CoRoT data. The light curve of the star has been interpolated using inpainting algorithms to reduce the effect of data gaps. We find power excess related to p modes in the range [400 - 1200]muHz with a mean large separation of 55.2+-0.8muHz with a probability above 95% that increases to 55.9 +-0.2muHz in a higher frequency range [500 - 1250] muHz and a rejection level of 1%. A hint of the variation of this quantity with frequency is also found. The rotation period of the star is estimated to be around 4.3 days with an inclination axis of i=50 deg +20/-13. We measure a bolometric amplitude per radial mode in a range [2.4 - 2.9] ppm around 1000 muHz. Finally, using a grid of models, we estimate the stellar mass, M=1.43+-0.05 Msun, the radius, R=1.96+-0.046 Rsun, and the age ~2.4 Gyr.

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arXiv2010

Ground-based observations of Kepler asteroseismic targets

We present the ground-based activities within the different working groups of the Kepler Asteroseismic Science Consortium (KASC). The activities aim at the systematic characterization of the 5000+ KASC targets, and at the collection of ground-based follow-up time-series data of selected promising Kepler pulsators. So far, 36 different instruments at 31 telescopes on 23 different observatories in 12 countries are in use, and a total of more than 530 observing nights has been awarded. (Based on observations made with the Isaac Newton Telescope, William Herschel Telescope, Nordic Optical Telescope, Telescopio Nazionale Galileo, Mercator Telescope (La Palma, Spain), and IAC-80 (Tenerife, Spain). Also based on observations taken at the observatories of Sierra Nevada, San Pedro Martir, Vienna, Xinglong, Apache Point, Lulin, Tautenburg, Loiano, Serra la Nave, Asiago, McDonald, Skinakas, Pic du Midi, Mauna Kea, Steward Observatory, Mt Wilson, Bialkow Observatory of the Wroclaw University, Piszkesteto Mountain Station, Observatoire de Haute Provence, and Centro Astronomico Hispano Aleman at Calar Alto. Based on data from the AAVSO International Database.)

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arXiv2010

How did the Sun affect the climate when life evolved on the Earth?

Using kappa Ceti as a proxy for the young Sun we show that not only was the young Sun much more effective in protecting the Earth environment from galactic cosmic rays than the present day Sun; it also had flare and corona mass ejection rates up to three orders of magnitude larger than the present day Sun. The reduction in the galactic cosmic ray influx caused by the young Sun's enhanced shielding capability has been suggested as a solution to what is known as the faint young Sun paradox, i.e. the fact that the luminosity of the young Sun was only around 75% of its present value when life started to evolve on our planet around four billion years ago. This suggestion relies on the hypothesis that the changing solar activity results in a changing influx of galactic cosmic rays to the Earth, which results in a changing low-altitude cloud coverage and thus a changing climate. Here we show how the larger corona mass ejection rates of the young Sun would have had an effect on the climate with a magnitude similar to the enhanced shielding capability of the young Sun.

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arXiv2010

Automated Asteroseismic Analysis of Solar-type Stars

The rapidly increasing volume of asteroseismic observations on solar-type stars has revealed a need for automated analysis tools. The reason for this is not only that individual analyses of single stars are rather time consuming, but more importantly that these large volumes of observations open the possibility to do population studies on large samples of stars and such population studies demand a consistent analysis. By consistent analysis we understand an analysis that can be performed without the need to make any subjective choices on e.g. mode identification and an analysis where the uncertainties are calculated in a consistent way. Here we present a set of automated asterosesimic analysis tools. The main engine of these set of tools is an algorithm for modelling the autocovariance spectra of the stellar acoustic spectra allowing us to measure not only the frequency of maximum power and the large frequency separation, but also the small frequency separation and potentially the mean rotational rate and the inclination. The measured large and small frequency separations and the frequency of maximum power are used as input to an algorithm that estimates fundamental stellar parameters such as mass, radius, luminosity, effective temperature, surface gravity and age based on grid modeling. All the tools take into account the window function of the observations which means that they work equally well for space-based photometry observations from e.g. the NASA Kepler satellite and ground-based velocity observations from e.g. the ESO HARPS spectrograph.

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arXiv2010

A multi-site campaign to measure solar-like oscillations in Procyon. II. Mode frequencies

We have analyzed data from a multi-site campaign to observe oscillations in the F5 star Procyon. The data consist of high-precision velocities that we obtained over more than three weeks with eleven telescopes. A new method for adjusting the data weights allows us to suppress the sidelobes in the power spectrum. Stacking the power spectrum in a so-called echelle diagram reveals two clear ridges that we identify with even and odd values of the angular degree (l=0 and 2, and l=1 and 3, respectively). We interpret a strong, narrow peak at 446 muHz that lies close to the l=1 ridge as a mode with mixed character. We show that the frequencies of the ridge centroids and their separations are useful diagnostics for asteroseismology. In particular, variations in the large separation appear to indicate a glitch in the sound-speed profile at an acoustic depth of about 1000 s. We list frequencies for 55 modes extracted from the data spanning 20 radial orders, a range comparable to the best solar data, which will provide valuable constraints for theoretical models. A preliminary comparison with published models shows that the offset between observed and calculated frequencies for the radial modes is very different for Procyon than for the Sun and other cool stars. We find the mean lifetime of the modes in Procyon to be 1.29 +0.55/-0.49 days, which is significantly shorter than the 2-4 days seen in the Sun.

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arXiv2010

Solar-like oscillations in low-luminosity red giants: first results from Kepler

We have measured solar-like oscillations in red giants using time-series photometry from the first 34 days of science operations of the Kepler Mission. The light curves, obtained with 30-minute sampling, reveal clear oscillations in a large sample of G and K giants, extending in luminosity from the red clump down to the bottom of the giant branch. We confirm a strong correlation between the large separation of the oscillations (Delta nu) and the frequency of maximum power (nu_max). We focus on a sample of 50 low-luminosity stars (nu_max > 100 muHz, L <~ 30 L_sun) having high signal-to-noise ratios and showing the unambiguous signature of solar-like oscillations. These are H-shell-burning stars, whose oscillations should be valuable for testing models of stellar evolution and for constraining the star-formation rate in the local disk. We use a new technique to compare stars on a single echelle diagram by scaling their frequencies and find well-defined ridges corresponding to radial and non-radial oscillations, including clear evidence for modes with angular degree l=3. Measuring the small separation between l=0 and l=2 allows us to plot the so-called C-D diagram of delta nu_02 versus Delta nu. The small separation delta nu_01 of l=1 from the midpoint of adjacent l=0 modes is negative, contrary to the Sun and solar-type stars. The ridge for l=1 is notably broadened, which we attribute to mixed modes, confirming theoretical predictions for low-luminosity giants. Overall, the results demonstrate the tremendous potential of Kepler data for asteroseismology of red giants.

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arXiv2010

The asteroseismic potential of Kepler: first results for solar-type stars

We present preliminary asteroseismic results from Kepler on three G-type stars. The observations, made at one-minute cadence during the first 33.5d of science operations, reveal high signal-to-noise solar-like oscillation spectra in all three stars: About 20 modes of oscillation may be clearly distinguished in each star. We discuss the appearance of the oscillation spectra, use the frequencies and frequency separations to provide first results on the radii, masses and ages of the stars, and comment in the light of these results on prospects for inference on other solar-type stars that Kepler will observe.

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arXiv2009

Sounding stellar cycles with Kepler - preliminary results from ground-based chromospheric activity measurements

Due to its unique long-term coverage and high photometric precision, observations from the Kepler asteroseismic investigation will provide us with the possibility to sound stellar cycles in a number of solar-type stars with asteroseismology. By comparing these measurements with conventional ground-based chromospheric activity measurements we might be able to increase our understanding of the relation between the chromospheric changes and the changes in the eigenmodes. In parallel with the Kepler observations we have therefore started a programme at the Nordic Optical Telescope to observe and monitor chromospheric activity in the stars that are most likely to be selected for observations for the whole satellite mission. The ground-based observations presented here can be used both to guide the selection of the special Kepler targets and as the first step in a monitoring programme for stellar cycles. Also, the chromospheric activity measurements obtained from the ground-based observations can be compared with stellar parameters such as ages and rotation in order to improve stellar evolution models.

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arXiv2009

Characterization of CoRoT Target Fields with the Berlin Exoplanet Search Telescope: Identification of Periodic Variable Stars in the LRa1 Field

In this paper, we report on observations of the CoRoT LRa1 field with the Berlin Exoplanet Search Telescope (BEST). The current paper is part of a series of papers describing the results of our stellar variability survey. The BEST is a small aperture telescope with a wide field of view (FOV). It is dedicated to searching for stellar variability within the target fields of the CoRoT space mission to aid in minimizing false-alarm rates and identify potential targets for additional science. The LRa1 field is CoRoT's second long run field located in the galactic anticenter direction. We observed the LRa1 stellar field on 23 nights between November and March 2005/2006. From 6099 stars marked as variable, 39 were classified as periodic variable stars and 27 of them are within the CoRoT FOV. We also confirmed the variability for four stars listed in the General Catalogue of Variable Stars (GCVS).

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arXiv2009

Identification of Variable Stars in COROT's First Main Observing Field (LRc1)

The COROT space mission will monitor several target fields for up to 150 days to perform asteroseismology and to search for extrasolar planets by photometric transits. Variable stars in the target fields are important objects for additional scientific studies but can also disturb the search for planetary transits. A variability characterization of the target fields prior to COROT observations is therefore important for two reasons: to find interesting variable stars to monitor further and to make an analysis of the impact of the variable stars on detecting extrasolar planet transits with COROT. The Berlin Exoplanet Search Telescope (BEST) is a small wide-angle telescope dedicated to high-precision photometry. It has observed a 9 square degree field of view centered at (alpha, delta)=(19h00m00.0s, +00deg01'55.2") (J2000.0) over 98 nights to search for variable stars in the surroundings of the first long-run target field (LRc1) of the COROT space mission. In this data set we identified 92 periodic variable stars, 86 of which are new discoveries and 6 of which are known from the General Catalogue of Variable Stars (GCVS). For five of the GCVS stars, variability could not be confirmed. Forty-three of the 92 detected periodic variable stars are identified as eclipsing binaries. We have evaluated the completeness of our survey for eclipsing binaries by comparing it to the expected fraction of eclipsing binaries based on Hipparcos observations. From this evaluation we show that the BEST data set presented here has a completeness of 20%-30% for periods longer than 1 day and is complete relative to Hipparcos for short-period binaries.

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arXiv2009

Sounding stellar cycles with Kepler - I. Strategy for selecting targets

The long-term monitoring and high photometric precision of the Kepler satellite will provide a unique opportunity to sound the stellar cycles of many solar-type stars using asteroseismology. This can be achieved by studying periodic changes in the amplitudes and frequencies of the oscillation modes observed in these stars. By comparing these measurements with conventional ground-based chromospheric activity indices, we can improve our understanding of the relationship between chromospheric changes and those taking place deep in the interior throughout the stellar activity cycle. In addition, asteroseismic measurements of the convection zone depth and differential rotation may help us determine whether stellar cycles are driven at the top or at the base of the convection zone. In this paper, we analyze the precision that will be possible using Kepler to measure stellar cycles, convection zone depths, and differential rotation. Based on this analysis, we describe a strategy for selecting specific targets to be observed by the Kepler Asteroseismic Investigation for the full length of the mission, to optimize their suitability for probing stellar cycles in a wide variety of solar-type stars.

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arXiv2009

Mode lifetimes of stellar oscillations - Implications for asteroseismology

Successful inference from asteroseismology relies on at least two things: that the oscillations in the stars have amplitudes large enough to be clearly observable; and that the oscillations themselves be stable enough to enable precise measurements of mode frequencies and other parameters. Solar-like p modes are damped by convection, and hence the stability of the modes depends on the lifetime. We seek a simple scaling relation between the mean lifetime of the most prominent solar-like p modes in stars, and the fundamental stellar parameters. We base our search for a relation on use of stellar equilibrium and pulsation computations of a grid of stellar models, and the first asteroseismic results on lifetimes of main-sequence, sub-giant and red-giant stars. We find that the mean lifetimes of all three classes of solar-like stars scale like $T_{\rm eff}^{-4}$ (where $T_{\rm eff}$ is the effective temperature). When this relation is combined with the well-known scaling relation of Kjeldsen & Bedding (1995) for mode amplitudes observed in narrow-band intensity observations, we obtain the unexpected result that the height (the maximum power spectral density) of mode peaks in the frequency power spectrum scales as $g^{-2}$ (where $g$ is the surface gravity). As it is the mode height (and not the amplitude) that fixes the S/N at which the modes can be measured, and as $g$ changes only slowly along the main sequence, this suggests that stars cooler than the Sun might be as good targets for asteroseismology as their hotter counterparts. When observations are instead made in Doppler velocity, our results imply that mode height then does increase with increasing effective temperature.

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arXiv2008

Solar-like oscillations in the G8 V star tau Ceti

We used HARPS to measure oscillations in the low-mass star tau Cet. Although the data were compromised by instrumental noise, we have been able to extract the main features of the oscillations. We found tau Cet to oscillate with an amplitude that is about half that of the Sun, and with a mode lifetime that is slightly shorter than solar. The large frequency separation is 169 muHz, and we have identified modes with degrees 0, 1, 2, and 3. We used the frequencies to estimate the mean density of the star to an accuracy of 0.45% which, combined with the interferometric radius, gives a mass of 0.783 +/- 0.012 M_sun (1.6%).

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arXiv2008

On Flare Driven Global Waves

We recently presented evidence of a strong correlation between the energy in the high-frequency part of the acoustic spectrum of the Sun and the solar X-ray flux Karoff & Kjeldsen (2008). The discovery indicates that flares drive global oscillations in the Sun in the same way that the entire Earth is set ringing for several weeks after a major earthquake, such as the 2004 December Sumatra-Andaman one. If this indication turns out to be true we might be able to use the relation between flares and the energy in the high-frequency part of the acoustic spectrum to detect e.g. flares on the far side of the Sun and flares on other solar-like stars. But, the discovery also opens many new questions such as why is it only the high-frequency part of the acoustic spectrum that is correlated with the X-ray flux? And, are there energy enough in solar flares do drive global oscillations?

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arXiv2008

Massive B-type pulsators in low-metallicity environments

Massive B type pulsators such as beta Cep and slowly pulsating B (SPB) stars pulsate due to layers of increased opacity caused by partial ionization. The increased opacity blocks the energy flux to the surface of the stars which causes the layers to rise and the opacity to drop. This cyclical behavior makes the star act as a heat engine and the star will thus pulsate. For beta Cep and SPB stars the increased opacity is believed to be caused by partial ionization of iron and these stars should therefore contain non-insignificant quantities of the metal. A good test of this theory is to search for beta Cep and SPB stars in low-metallicity environments. If no stars are found the theory is supported, but on the other hand if a substantial number of beta Cep and SPB stars are found in these environments then the theory is not supported and a solutions needed. With a growing number of identified beta Cep and SPB stars in the low-metallicity Magellanic Clouds we seem to be left with the second case. We will in this context discuss recent findings of beta Cep and SPB stars in the Magellanic Clouds and some possible solutions to the discrepancy between these observations and the theory. We also describe an ambitious project that we have initiated on the Small Magellanic Cloud open cluster NGC 371 which will help to evaluate these solutions.

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arXiv2008

Evidence that solar flares drive global oscillations in the Sun

Solar flares are large explosions on the Sun's surface caused by a sudden release of magnetic energy. They are known to cause local short-lived oscillations travelling away from the explosion like water rings. Here we show that the energy in the solar acoustic spectrum is correlated with flares. This means that the flares drive global oscillations in the Sun in the same way that the entire Earth is set ringing for several weeks after a major earthquake like the December 2004 Sumatra-Andaman Earthquake. The correlation between flares and energy in the acoustic spectrum of disk-integrated sunlight is stronger for high-frequency waves than for ordinary p-modes which are excited by the turbulence in the near surface convection zone immediately beneath the photosphere.

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arXiv2008

SPB stars in the open SMC cluster NGC 371

Pulsation in beta Cep and SPB stars are driven by the kappa mechanism which depends critically on the metallicity. It has therefore been suggested that beta Cep and SPB stars should be rare in the Magellanic Clouds which have lower metallicities than the solar neighborhood. To test this prediction we have observed the open SMC cluster NGC 371 for 12 nights in order to search for beta Cep and SPB stars. Surprisingly, we find 29 short-period B-type variables in the upper part of the main sequence, many of which are probably SPB stars. This result indicates that pulsation is still driven by the kappa mechanism even in low metallicity environments. All the identified variables have periods longer than the fundamental radial period which means that they cannot be beta Cep stars. Within an amplitude detection limit of 5 mmag no stars in the top of the HR-diagram show variability with periods shorter than the fundamental radial period. So if beta Cep stars are present in the cluster they oscillate with amplitudes below 5 mmag, which is significantly lower than the mean amplitude of beta Cep stars in the Galaxy. We see evidence that multimode pulsation is more common in the upper part of the main sequence than in the lower. We have also identified 5 eclipsing binaries and 3 periodic pulsating Be stars in the cluster field.

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arXiv2007

Oscillations in Procyon A: First results from a multi-site campaign

Procyon A is a bright F5IV star in a binary system. Although the distance, mass and angular diameter of this star are all known with high precision, the exact evolutionary state is still unclear. Evolutionary tracks with different ages and different mass fractions of hydrogen in the core pass, within the errors, through the observed position of Procyon A in the Hertzsprung-Russell diagram. For more than 15 years several different groups have studied the solar-like oscillations in Procyon A to determine its evolutionary state. Although several studies independently detected power excess in the periodogram, there is no agreement on the actual oscillation frequencies yet. This is probably due to either insufficient high-quality data (i.e., aliasing) or due to intrinsic properties of the star (i.e., short mode lifetimes). Now a spectroscopic multi-site campaign using 10 telescopes world-wide (minimizing aliasing effects) with a total time span of nearly 4 weeks (increase the frequency resolution) is performed to identify frequencies in this star and finally determine its properties and evolutionary state.

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