arXiv subjects

D. Bohlender

Publications and source records attributed to D. Bohlender.

35 records · Page 2Linked to original sources

arXiv2013

Regular frequency patterns in the young delta Scuti star HD 261711 observed by the CoRoT and MOST satellites

We concentrate on an asteroseismological study of HD 261711, a rather hot delta Scuti type pulsating member of the young open cluster NGC 2264 located at the blue border of the instability region. HD 261711 was discovered to be a pre-main sequence delta Scuti star using the time series photometry obtained by the MOST satellite in 2006. High-precision, time-series photometry of HD 261711 was obtained by the MOST and CoRoT satellites in 4 separate new observing runs that are put into context with the star's fundamental atmospheric parameters obtained from spectroscopy. With the new MOST data set from 2011/12 and the two CoRoT light curves from 2008 and 2011/12, the delta Scuti variability was confirmed and regular groups of frequencies were discovered. The two pulsation frequencies identified in the data from the first MOST observing run in 2006 are confirmed and 23 new delta Scuti-type frequencies were discovered using the CoRoT data. Weighted average frequencies for each group are related to l=0 and l=1 p-modes. Evidence for amplitude modulation of the frequencies in two groups is seen. The effective temperature was derived to be 8600$\pm$200 K, log g is 4.1$\pm$0.2, and the projected rotational velocity is 53$\pm$1km/s. Using our Teff value and the radius of 1.8$\pm$0.5 Rsun derived from SED fitting, we get a log L/Lsun of 1.20$\pm$0.14 which agrees well to the seismologically determined values of 1.65 Rsun and, hence, a log L/Lsun of 1.13. The radial velocity of 14$\pm$2 km/s we derived for HD 261711, confirms the star's membership to NGC 2264. Our asteroseismic models suggest that HD 261711 is a delta Scuti-type star close to the zero-age main sequence (ZAMS) with a mass of 1.8 to 1.9Msun. HD 261711 is either a young ZAMS star or a late PMS star just before the onset of hydrogen-core burning.

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arXiv2013

A coordinated optical and X-ray spectroscopic campaign on HD179949: searching for planet-induced chromospheric and coronal activity

HD179949 is an F8V star, orbited by a close-in giant planet with a period of ~3 days. Previous studies suggested that the planet enhances the magnetic activity of the parent star, producing a chromospheric hot spot which rotates in phase with the planet orbit. However, this phenomenon is intermittent since it was observed in several but not all seasons. A long-term monitoring of the magnetic activity of HD179949 is required to study the amplitude and time scales of star-planet interactions. In 2009 we performed a simultaneous optical and X-ray spectroscopic campaign to monitor the magnetic activity of HD179949 during ~5 orbital periods and ~2 stellar rotations. We analyzed the CaII H&K lines as a proxy for chromospheric activity, and we studied the X-ray emission in search of flux modulations and to determine basic properties of the coronal plasma. A detailed analysis of the flux in the cores of the CaII H&K lines and a similar study of the X-ray photometry shows evidence of source variability, including one flare. The analysis of the the time series of chromospheric data indicates a modulation with a ~11 days period, compatible with the stellar rotation period at high latitudes. Instead, the X-ray light curve suggests a signal with a period of ~4 days, consistent with the presence of two active regions on opposite hemispheres. The observed variability can be explained, most likely, as due to rotational modulation and to intrinsic evolution of chromospheric and coronal activity. There is no clear signature related to the orbital motion of the planet, but the possibility that just a fraction of the chromospheric and coronal variability is modulated with the orbital period of the planet, or the stellar-planet beat period, cannot be excluded. We conclude that any effect due to the presence of the planet is difficult to disentangle.

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arXiv2012

Relationship between low and high frequencies in Delta Scuti stars: Photometric Kepler and spectroscopic analyses of the rapid rotator KIC 8054146

Two years of Kepler data of KIC 8054146 (delta Sct/gamma Dor hybrid) revealed 349 statistically significant frequencies between 0.54 and 191.36 c/d (6.3 microHz to 2.21 mHz). The 117 low frequencies cluster in specific frequency bands, but do not show the equidistant period spacings predicted for gravity modes of successive radial order, n, and reported for at least one other hybrid pulsator. The four dominant low frequencies in the 2.8 to 3.0 c/d (32 to 35 microHz) range show strong amplitude variability with timescales of months and years. These four low frequencies also determine the spacing of the higher frequencies in and beyond the delta Sct pressure-mode frequency domain. In fact, most of the higher frequencies belong to one of three families with spacings linked to a specific dominant low frequency. In the Fourier spectrum, these family regularities show up as triplets, high-frequency sequences with absolutely equidistant frequency spacings, side lobes (amplitude modulations) and other regularities in frequency spacings. Furthermore, within two families the amplitude variations between the low and high frequencies are related. We conclude that the low frequencies (gravity modes, rotation) and observed high frequencies (mostly pressure modes) are physically connected. This unusual behavior may be related to the very rapid rotation of the star: from a combination of high and low-resolution spectroscopy we determined that KIC 8054146 is a very fast rotator (v sin i = 300 +/- 20 km/s) with an effective temperature of 7600 +/- 200 K and a surface gravity log g of 3.9 +/- 0.3. Several astrophysical ideas explaining the origin of the relationship between the low and high frequencies are explored.

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arXiv2012

Magnetic field, differential rotation and activity of the hot-Jupiter hosting star HD 179949

HD 179949 is an F8V star, orbited by a giant planet at ~8 R* every 3.092514 days. The system was reported to undergo episodes of stellar activity enhancement modulated by the orbital period, interpreted as caused by Star-Planet Interactions (SPIs). One possible cause of SPIs is the large-scale magnetic field of the host star in which the close-in giant planet orbits. In this paper we present spectropolarimetric observations of HD 179949 during two observing campaigns (2009 September and 2007 June). We detect a weak large-scale magnetic field of a few Gauss at the surface of the star. The field configuration is mainly poloidal at both observing epochs. The star is found to rotate differentially, with a surface rotation shear of dOmega=0.216\pm0.061 rad/d, corresponding to equatorial and polar rotation periods of 7.62\pm0.07 and 10.3\pm0.8 d respectively. The coronal field estimated by extrapolating the surface maps resembles a dipole tilted at ~70 degrees. We also find that the chromospheric activity of HD 179949 is mainly modulated by the rotation of the star, with two clear maxima per rotation period as expected from a highly tilted magnetosphere. In September 2009, we find that the activity of HD 179949 shows hints of low amplitude fluctuations with a period close to the beat period of the system.

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arXiv2012

An Inexpensive Liquid Crystal Spectropolarimeter for the Dominion Astrophysical Observatory Plaskett Telescope

A new, inexpensive polarimetric unit has been constructed for the Dominion Astrophysical Observatory (DAO) 1.8-m Plaskett telescope. It is implemented as a plug-in module for the telescope's existing Cassegrain spectrograph, and enables medium resolution (R~10,000) circular spectropolarimetry of point sources. A dual-beam design together with fast switching of the wave plate at rates up to 100Hz, and synchronized with charge shuffling on the CCD, is used to significantly reduce instrumental effects and achieve high-precision spectropolarimetric measurements for a very low cost. The instrument is optimized to work in the wavelength range 4700 - 5300A to simultaneously detect polarization signals in the H beta line as well as nearby metallic lines. In this paper we describe the technical details of the instrument, our observing strategy and data reduction techniques, and present tests of its scientific performance.

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arXiv2010

A detailed spectropolarimetric analysis of the planet hosting star WASP-12

The knowledge of accurate stellar parameters is paramount in several fields of stellar astrophysics, particularly in the study of extrasolar planets, where often the star is the only visible component and therefore used to infer the planet's fundamental parameters. Another important aspect of the analysis of planetary systems is the stellar activity and the possible star-planet interaction. Here we present a self-consistent abundance analysis of the planet hosting star WASP-12 and a high-precision search for a structured stellar magnetic field on the basis of spectropolarimetric observations obtained with the ESPaDOnS spectropolarimeter. Our results show that the star does not have a structured magnetic field, and that the obtained fundamental parameters are in good agreement with what was previously published. In addition we derive improved constraints on the stellar age (1.0-2.65 Gyr), mass (1.23-1.49 M/M0), and distance (295-465 pc). WASP-12 is an ideal object to look for pollution signatures in the stellar atmosphere. We analyse the WASP-12 abundances as a function of the condensation temperature and compare them with those published by several other authors on planet hosting and non-planet hosting stars. We find hints of atmospheric pollution in WASP-12's photosphere, but are unable to reach firm conclusions with our present data. We conclude that a differential analysis based on WASP-12 twins will probably clarify if an atmospheric pollution is present, the nature of this pollution and its implications in the planet formation and evolution. We attempt also the direct detection of the circumstellar disk through infrared excess, but without success.

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arXiv2010

Searching for Star-Planet interactions within the magnetosphere of HD 189733

HD 189733 is a K2 dwarf, orbited by a giant planet at 8.8 stellar radii. In order to study magnetospheric interactions between the star and the planet, we explore the large-scale magnetic field and activity of the host star. We collected spectra using the ESPaDOnS and the NARVAL spectropolarimeters, installed at the 3.6-m Canada-France-Hawaii telescope and the 2-m Telescope Bernard Lyot at Pic du Midi, during two monitoring campaigns (June 2007 and July 2008). HD 189733 has a mainly toroidal surface magnetic field, having a strength that reaches up to 40 G. The star is differentially rotating, with latitudinal angular velocity shear of domega = 0.146 +- 0.049 rad/d, corresponding to equatorial and polar periods of 11.94 +- 0.16 d and 16.53 +- 2.43 d respectively. The study of the stellar activity shows that it is modulated mainly by the stellar rotation (rather than by the orbital period or the beat period between the stellar rotation and the orbital periods). We report no clear evidence of magnetospheric interactions between the star and the planet. We also extrapolated the field in the stellar corona and calculated the planetary radio emission expected for HD 189733b given the reconstructed field topology. The radio flux we predict in the framework of this model is time variable and potentially detectable with LOFAR.

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arXiv2010

Discovery of a strong magnetic field in the rapidly rotating B2Vn star HR 7355

We report the detection of a strong, organized magnetic field in the helium-variable early B-type star HR 7355 using spectropolarimetric data obtained with ESPaDOnS on the 3.6-m Canada-France-Hawaii Telescope within the context of the Magnetism in Massive Stars (MiMeS) Large Program. HR 7355 is both the most rapidly rotating known main-sequence magnetic star and the most rapidly rotating helium-strong star, with $v \sin i$ = 300 $\pm$ 15 km s$^{-1}$ and a rotational period of 0.5214404 $\pm$ 0.0000006 days. We have modeled our eight longitudinal magnetic field measurements assuming an oblique dipole magnetic field. Constraining the inclination of the rotation axis to be between $38^{\circ}$ and $86^{\circ}$, we find the magnetic obliquity angle to be between $30^{\circ}$ and $85^{\circ}$, and the polar strength of the magnetic field at the stellar surface to be between 13-17 kG. The photometric light curve constructed from HIPPARCOS archival data and new CTIO measurements shows two minima separated by 0.5 in rotational phase and occurring 0.25 cycles before/after the magnetic extrema. This photometric behavior coupled with previously-reported variable emission of the H$α$ line (which we confirm) strongly supports the proposal that HR 7355 harbors a structured magnetosphere similar to that in the prototypical helium-strong star, $σ$ Ori E.

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arXiv2009

Discovery of a magnetic field in the O9 sub-giant star HD 57682 by the MiMeS Collaboration

We report the detection of a strong, organised magnetic field in the O9IV star HD 57682, using spectropolarimetric observations obtained with ESPaDOnS at the 3.6-m Canada-France-Hawaii Telescope within the context of the Magnetism in Massive Stars (MiMeS) Large Program. From the fitting of our spectra using NLTE model atmospheres we determined that HD 57682 is a $17^{+19}_{-9}$ M$_{\odot}$ star with a radius of $7.0^{+2.4}_{-1.8}$ R$_\odot$, and a relatively low mass-loss rate of $1.4^{+3.1}_{-0.95}\times10^{-9}$ M$_{\odot}$ yr$^{-1}$. The photospheric absorption lines are narrow, and we use the Fourier transform technique to infer $v\sin i=15\pm3$ km s$^{-1}$. This $v\sin i$ implies a maximum rotational period of 31.5 d, a value qualitatively consistent with the observed variability of the optical absorption and emission lines, as well as the Stokes $V$ profiles and longitudinal field. Using a Bayesian analysis of the velocity-resolved Stokes $V$ profiles to infer the magnetic field characteristics, we tentatively derive a dipole field strength of $1680^{+134}_{-356}$ G. The derived field strength and wind characteristics imply a wind that is strongly confined by the magnetic field.

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arXiv2009

Magnetic cycles of the planet-hosting star Tau Bootis: II. a second magnetic polarity reversal

In this paper, we present new spectropolarimetric observations of the planet-hosting star Tau Bootis, using ESPaDOnS and Narval spectropolarimeters at Canada-France-Hawaii Telescope (CFHT) and Telescope Bernard Lyot (TBL), respectively. We detected the magnetic field of the star at three epochs in 2008. It is a weak magnetic field of only a few Gauss, oscillating between a predominant toroidal component in January and a dominant poloidal component in June and July. A magnetic polarity reversal was observed relative to the magnetic topology in June 2007. This is the second such reversal observed in two years on this star, suggesting that Tau Boo has a magnetic cycle of about 2 years. This is the first detection of a magnetic cycle for a star other than the Sun. The role of the close-in massive planet in the short activity cycle of the star is questioned. Tau Boo has strong differential rotation, a common trend for stars with shallow convective envelope. At latitude 40 deg., the surface layer of the star rotates in 3.31 d, equal to the orbital period. Synchronization suggests that the tidal effects induced by the planet may be strong enough to force at least the thin convective envelope into corotation. Tau Boo shows variability in the Ca H & K and Halpha throughout the night and on a night to night time scale. We do not detect enhancement in the activity of the star that may be related to the conjunction of the planet. Further data is needed to conclude about the activity enhancement due to the planet.

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arXiv2008

Discovery of magnetic fields in the very young, massive stars W601 (NGC 6611) and OI 201 (NGC 2244)

Context: Recent spectropolarimetric observations of Herbig Ae/Be stars have yielded new arguments in favour of a fossil origin for the magnetic fields of intermediate mass stars. Aims: To study the evolution of these magnetic fields, and their impact on the evolution of the angular momentum of these stars during the pre-main sequence phase, we observed Herbig Ae/Be members of young open clusters of various ages. Methods: We obtained high-resolution spectropolarimetric observations of Herbig Ae/Be stars belonging to the young open clusters NGC 6611 (< 6 Myr), NGC 2244 (~1.9 Myr), and NGC 2264 (~8 Myr), using ESPaDOnS at theCanada-France-Hawaii Telescope. Results: Here we report the discovery of strong magnetic fields in two massive pre-main sequence cluster stars. We detected, for the first time, a magnetic field in a pre-main sequence rapid rotator: the 10.2 Msun Herbig B1.5e star W601 (NGC 6611; v sin i ~ 190 km/s). Our spectropolarimetric observations yield a longitudinal magnetic field larger than 1 kG, and imply a rotational period shorter than 1.7 days. The spectrum of this very young object (age ~ 0.017 Myr) shows strong and variable lines of He and Si. We also detected a magnetic field in the 12.1 Msun B1 star OI 201 (NGC 2244; v sin i = 23.5 km/s). The Stokes V profile of this star does not vary over 5 days, suggesting a long rotational period, a pole-on orientation, or aligned magnetic and rotation axes. OI 201 is situtated near the Zero-Age Main Sequence on the HR diagram, and exhibits normal chemical abundances and no spectrum variability.

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arXiv2008

Magnetic cycles of the planet-hosting star tauBootis

We have obtained new spectropolarimetric observations of the planet-hosting star tauBootis, using the ESPaDOnS and NARVAL spectropolarimeters at the Canada-France-Hawaii Telescope and Telescope Bernard-Lyot. With this data set, we are able to confirm the presence of a magnetic field at the surface of tauBoo and map its large-scale structure over the whole star. The overall polarity of the magnetic field has reversed with respect to our previous observation (obtained a year before), strongly suggesting that tauBoo is undergoing magnetic cycles similar to those of the Sun. This is the first time that a global magnetic polarity switch is observed in a star other than the Sun; we speculate that the magnetic cycle period of tauBoo is much shorter than that of the Sun. Our new data also allow us to confirm the presence of differential rotation from the latitudinal shearing that the magnetic structure is undergoing. The differential rotation surface shear that tauBoo experiences is found to be 6 to 10 times larger than that of the Sun. We propose that the short magnetic cycle period is due to the strong level of differential rotation. With a rotation period of 3.0 and 3.9 d at the equator and pole respectively, tauBoo appears as the first planet-hosting star whose rotation (at intermediate latitudes) is synchronised with the orbital motion of its giant planet (period 3.3 d). Assuming that this synchronisation is not coincidental, it suggests that the tidal effects induced by the giant planet can be strong enough to force the thin convective enveloppe (though not the whole star) into corotation and thus to play a role in the activity cycle of tauBoo.

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arXiv2007

Vertical stratification of iron abundance in atmospheres of blue horizontal-branch stars

The observed slow rotation and abundance peculiarities of certain blue horizontal branch (BHB) stars suggests that atomic diffusion can be important in their stellar atmospheres and can lead to vertical abundance stratification of chemical species in the atmosphere. To verify this hypothesis, we have undertaken an abundance stratification analysis in the atmospheres of six BHB stars, based on recently acquired McDonald-CE spectra. Our numerical simulations show that the iron abundance is vertically stratified in the atmospheres of two stars in M15: B267 and B279. One star WF2-2541 in M13 also appears to have vertically stratified iron abundance, while for WF4-3085 the signatures of iron stratification are less convincing. In all cases the iron abundances increase towards the lower atmosphere. The other two stars in our sample, WF4-3485 and B84, do not show any significant variation of iron with atmospheric depth. Our results support the idea that atomic diffusion dominates other hydrodynamic processes in the atmospheres of BHB stars.

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arXiv2007

Search for vertical stratification of metals in atmospheres of blue horizontal-branch stars

The observed abundance peculiarities of many chemical species relative to the expected cluster metallicity in blue horizontal-branch (BHB) stars presumably appear as a result of atomic diffusion in the photosphere. The slow rotation (typically $v\sin{i}<$ 10 km s$^{-1}$) of BHB stars with effective temperatures $T_{\rm eff}>$ 11,500 K supports this idea since the diffusion mechanism is only effective in a stable stellar atmosphere. In this work we search for observational evidence of vertical chemical stratification in the atmospheres of six hot BHB stars: B84, B267 and B279 in M15 and WF2-2541, WF4-3085 and WF4-3485 in M13. We undertake an abundance stratification analysis of the stellar atmospheres of the aforementioned stars, based on acquired Keck HIRES spectra. We have found from our numerical simulations that three stars (B267, B279 and WF2-2541) show clear signatures of the vertical stratification of iron whose abundance increases toward the lower atmosphere, while the other two stars (B84 and WF4-3485) do not. For WF4-3085 the iron stratification results are inconclusive. B267 also shows a signature of titanium stratification. Our estimates for radial velocity, $v\sin{i}$ and overall iron, titanium and phosphorus abundances agree with previously published data for these stars after taking the measurement errors into account. The results support the hypothesis regarding the efficiency of atomic diffusion in the stellar atmospheres of BHB stars with $T_{\rm eff}>$ 11,500 K.

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arXiv2007

Towards an understanding of the Of?p star HD 191612: optical spectroscopy

We present extensive optical spectroscopy of the early-type magnetic star HD 191612 (O6.5f?pe-O8fp). The Balmer and HeI lines show strongly variable emission which is highly reproducible on a well-determined 538-d period. Metal lines and HeII absorptions (including many selective emission lines but excluding He II 4686A emission) are essentially constant in line strength, but are variable in velocity, establishing a double-lined binary orbit with P(orb) = 1542d, e=0.45. We conduct a model-atmosphere analysis of the primary, and find that the system is consistent with a O8: giant with a B1: main-sequence secondary. Since the periodic 538-d changes are unrelated to orbital motion, rotational modulation of a magnetically constrained plasma is strongly favoured as the most likely underlying `clock'. An upper limit on the equatorial rotation is consistent with this hypothesis, but is too weak to provide a strong constraint.

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arXiv2007

Vertical abundance stratification in the blue horizontal branch star HD135485

It is commonly believed that the observed overabundances of many chemical species relative to the expected cluster metallicity in blue horizontal branch (BHB) stars appear as a result of atomic diffusion in the photosphere. The slow rotation of BHB stars (with T_eff > 11,500K), typically v sin{i} < 10 km/s, is consistent with this idea. In this work we search for observational evidence of vertical chemical stratification in the atmosphere of HD135485. If this evidence exists, it will demonstrate the importance of atomic diffusion processes in the atmospheres of BHB stars. We undertake an extensive abundance stratification analysis of the atmosphere of HD135485, based on recently acquired high resolution and S/N CFHT ESPaDOnS spectra and a McDonald-CE spectrum. Our numerical simulations show that nitrogen and sulfur reveal signatures of vertical abundance stratification in the stellar atmosphere. It appears that the abundances of these elements increase toward the upper atmosphere. This fact cannot be explained by the influence of microturbulent velocity, because oxygen, carbon, neon, argon, titanium and chromium do not show similar behavior and their abundances remain constant throughout the atmosphere. It seems that the iron abundance may increase marginally toward the lower atmosphere. This is the first demonstration of vertical abundance stratification of metals in a BHB star.

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arXiv2006

The magnetic field of the planet-hosting star $τ$ Bootis

We have obtained high resolution spectropolarimetric data for the planet-hosting star $τ$ Bootis, using the ESPaDOnS spectropolarimeter at CFHT. A weak but clear Stokes $V$ signature is detected on three of the four nights of June 2006 during which we have recorded data. This polarimetric signature indicates with no ambiguity the presence of a magnetic field at the star's surface, with intensity of just a few Gauss. The analysis of the photospheric lines of $τ$ Boo at ultra-high signal-to-noise reveals the presence of an 18% relative differential rotation. Tentative Zeeman-Doppler imaging, using our spectropolarimetric observations covering only a fraction of the star's rotational phase, indicates a magnetic field with a dominant potential field component. The data are best fitted when a 3.1d period of modulation and an intermediate inclination are assumed. Considering the level of differential rotation of $τ$ Boo, this implies a rotation period of 3.0d at the equator and of 3.7d at the pole, and a topology of the magnetic field where its main non-axisymmetric part is located at low latitudes. The planet is probably synchronised with the star's rotation at intermediate latitudes, while the non-axisymmetric part of the magnetic field seems located at lower latitudes. Our limited data do not provide sufficient constraints on the magnetic field to study a possible interaction of the planet with the star's magnetosphere. Investigating this issue will require data with much better phase coverage. Similar studies should also be performed for other stars hosting close-in giant planets.

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