arXiv subjects

F. Grundahl

Publications and source records attributed to F. Grundahl.

At least 73 records · Page 4Linked to original sources

arXiv2013

A Giant Planet beyond the Snow Line in Microlensing Event OGLE-2011-BLG-0251

We present the analysis of the gravitational microlensing event OGLE-2011-BLG-0251. This anomalous event was observed by several survey and follow-up collaborations conducting microlensing observations towards the Galactic Bulge. Based on detailed modelling of the observed light curve, we find that the lens is composed of two masses with a mass ratio q=1.9 x 10^-3. Thanks to our detection of higher-order effects on the light curve due to the Earth's orbital motion and the finite size of source, we are able to measure the mass and distance to the lens unambiguously. We find that the lens is made up of a planet of mass 0.53 +- 0.21,M_Jup orbiting an M dwarf host star with a mass of 0.26 +- 0.11 M_Sun. The planetary system is located at a distance of 2.57 +- 0.61 kpc towards the Galactic Centre. The projected separation of the planet from its host star is d=1.408 +- 0.019, in units of the Einstein radius, which corresponds to 2.72 +- 0.75 AU in physical units. We also identified a competitive model with similar planet and host star masses, but with a smaller orbital radius of 1.50 +- 0.50 AU. The planet is therefore located beyond the snow line of its host star, which we estimate to be around 1-1.5 AU.

astro-ph.EP↗

arXiv2013

The Asymptotic Giant Branches of GCs: Selective Entry Only

The handful of available observations of AGB stars in Galactic Globular Clusters suggest that the GC AGB populations are dominated by cyanogen-weak stars. This contrasts strongly with the distributions in the RGB (and other) populations, which generally show a 50:50 bimodality in CN band strength. If it is true that the AGB populations show very different distributions then it presents a serious problem for low mass stellar evolution theory, since such a surface abundance change going from the RGB to AGB is not predicted by stellar models. However this is only a tentative conclusion, since it is based on very small AGB sample sizes. To test whether this problem really exists we have carried out an observational campaign specifically targeting AGB stars in GCs. We have obtained medium resolution spectra for about 250 AGB stars across 9 Galactic GCs using the multi-object spectrograph on the AAT (2df/AAOmega). We present some of the preliminary findings of the study for the second parameter trio of GCs: NGC 288, NGC 362 and NGC 1851. The results indeed show that there is a deficiency of stars with strong CN bands on the AGB. To confirm that this phenomenon is robust and not just confined to CN band strengths and their vagaries, we have made observations using FLAMES/VLT to measure elemental abundances for NGC 6752.We present some initial results from this study also. Our sodium abundance results show conclusively that only a subset of stars in GCs experience the AGB phase of evolution. This is the first direct, concrete confirmation of the phenomenon.

astro-ph.SR↗

arXiv2013

A WFC3/HST view of the three stellar populations in the Globular Cluster NGC6752

Multi-band Hubble Space Telescope photometry reveals that the main sequence, sub-giant, and the red giant branch of the globular cluster NGC6752 splits into three main components in close analogy with the three distinct segments along its horizontal branch stars. These triple sequences are consistent with three stellar groups: a stellar population with a chemical composition similar to field halo stars (population a), a population (c) with enhanced sodium and nitrogen, depleted carbon and oxygen and enhanced helium abundance (Delta Y ~0.03), and a population (b) with an intermediate (between population a and c) chemical composition and slightly helium enhanced (Delta Y ~0.01). These components contain ~25% (population a), ~45% (population b), and ~30% (population c) of the stars. No radial gradient for the relative numbers of the three populations has been identified out to about 2.5 half mass radii.

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arXiv2013

Multiple populations in Galactic globular clusters: a survey in the Strömgren system

We are coming to believe that stellar populations in globular clusters are not as simple as they were once thought to be. A growing amount of photometric and spectroscopic evidence shows that globular clusters host at least two different stellar populations. In our contribution to these proceedings we present the first results of a survey we are conducting to look for the presence of multiple populations in a significant number of Galactic globular clusters, using the Strömgren system. We intend to photometrically separate these populations and characterize their radial distributions and extensions.

astro-ph.GA↗

arXiv2012

Disentangling multiple stellar populations in globular clusters using the Strömgren system

An increasing amount of spectroscopic and photometric evidence is showing that the stellar populations of globular clusters are not as simple as they have been considered for many years. The presence of at least two different populations of stars is being discovered in a growing number of globular clusters, both in our Galaxy and in others. We have started a series of observations of Galactic globular clusters using the Strömgren photometric system in order to find the signatures of these multiple populations and establish their presence in a more complete sample of globular clusters in the Milky Way, and to study their radial distributions and extensions. We present here the first results of our survey.

astro-ph.GA↗

arXiv2012

Cyanogen in NGC 1851 RGB and AGB Stars: Quadrimodal Distributions

The Galactic globular cluster NGC 1851 has raised much interest since HST photometry revealed that it hosts a double subgiant branch. Here we report on our homogeneous study into the cyanogen (CN) bandstrengths in the RGB population (17 stars) and AGB population (21 stars) using AAOmega/2dF spectra with R $\sim 3000$. We discover that NGC 1851 hosts a quadrimodal distribution of CN bandstrengths in its RGB and AGB populations. This result supports the merger formation scenario proposed for this cluster, such that the CN quadrimodality could be explained by the superposition of two `normal' bimodal populations. A small sample overlap with an abundance catalogue allowed us to tentatively explore the relationship between our CN populations and a range of elemental abundances. We found a striking correlation between CN and [O/Na]. We also found that the four CN peaks may be paired -- the two CN-weaker populations being associated with low Ba and the two CN-stronger populations with high Ba. If true then s-process abundances would be a good diagnostic for disentangling the two original clusters in the merger scenario. More observations are needed to confirm the quadrimodality, and also the relationship between the subpopulations. We also report CN results for NGC 288 as a comparison. Our relatively large samples of AGB stars show that both clusters have a bias towards CN-weak AGB populations.

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arXiv2012

Microlensig Binaries with Candidate Brown Dwarf Companions

Brown dwarfs are important objects because they may provide a missing link between stars and planets, two populations that have dramatically different formation history. In this paper, we present the candidate binaries with brown dwarf companions that are found by analyzing binary microlensing events discovered during 2004 - 2011 observation seasons. Based on the low mass ratio criterion of q < 0.2, we found 7 candidate events, including OGLE-2004-BLG-035, OGLE-2004-BLG-039, OGLE-2007-BLG-006, OGLE-2007-BLG-399/MOA-2007-BLG-334, MOA-2011-BLG-104/OGLE-2011-BLG-0172, MOA-2011-BLG-149, and MOA-201-BLG-278/OGLE-2011-BLG-012N. Among them, we are able to confirm that the companions of the lenses of MOA-2011-BLG-104/OGLE-2011-BLG-0172 and MOA-2011-BLG-149 are brown dwarfs by determining the mass of the lens based on the simultaneous measurement of the Einstein radius and the lens parallax. The measured mass of the brown dwarf companions are (0.02 +/- 0.01) M_Sun and (0.019 +/- 0.002) M_Sun for MOA-2011-BLG-104/OGLE-2011-BLG-0172 and MOA-2011-BLG-149, respectively, and both companions are orbiting low mass M dwarf host stars. More microlensing brown dwarfs are expected to be detected as the number of lensing events with well covered light curves increases with new generation searches.

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arXiv2012

Characterizing Low-Mass Binaries From Observation of Long Time-scale Caustic-crossing Gravitational Microlensing Events

Despite astrophysical importance of binary star systems, detections are limited to those located in small ranges of separations, distances, and masses and thus it is necessary to use a variety of observational techniques for a complete view of stellar multiplicity across a broad range of physical parameters. In this paper, we report the detections and measurements of 2 binaries discovered from observations of microlensing events MOA-2011-BLG-090 and OGLE-2011-BLG-0417. Determinations of the binary masses are possible by simultaneously measuring the Einstein radius and the lens parallax. The measured masses of the binary components are 0.43 $M_{\odot}$ and 0.39 $M_{\odot}$ for MOA-2011-BLG-090 and 0.57 $M_{\odot}$ and 0.17 $M_{\odot}$ for OGLE-2011-BLG-0417 and thus both lens components of MOA-2011-BLG-090 and one component of OGLE-2011-BLG-0417 are M dwarfs, demonstrating the usefulness of microlensing in detecting binaries composed of low-mass components. From modeling of the light curves considering full Keplerian motion of the lens, we also measure the orbital parameters of the binaries. The blended light of OGLE-2011-BLG-0417 comes very likely from the lens itself, making it possible to check the microlensing orbital solution by follow-up radial-velocity observation. For both events, the caustic-crossing parts of the light curves, which are critical for determining the physical lens parameters, were resolved by high-cadence survey observations and thus it is expected that the number of microlensing binaries with measured physical parameters will increase in the future.

astro-ph.SR↗

arXiv2012

Age and helium content of the open cluster NGC 6791 from multiple eclipsing binary members. II. age dependencies and new insights

Models of stellar structure and evolution can be constrained by measuring accurate parameters of detached eclipsing binaries in open clusters. Multiple binary stars provide the means to determine helium abundances in these old stellar systems, and in turn, to improve estimates of their age. In the first paper of this series, we demonstrated how measurements of multiple eclipsing binaries in the old open cluster NGC6791 sets tighter constraints on the properties of stellar models than has previously been possible, thereby potentially improving both the accuracy and precision of the cluster age. Here we add additional constraints and perform an extensive model comparison to determine the best estimates of the cluster age and helium content, employing as many observational constraints as possible. We improve our photometry and correct empirically for differential reddening effects. We then perform an extensive comparison of the CMDs and eclipsing binary measurements to Victoria and DSEP isochrones to estimate cluster parameters. We also reanalyse a spectrum of the star 2-17 to improve [Fe/H] constraints. We find a best estimate of the age of ~8.3 Gyr while demonstrating that remaining age uncertainty is dominated by uncertainties in the CNO abundances. The helium mass fraction is well constrained at Y = 0.30 \pm 0.01 resulting in dY/dZ ~ 1.4 assuming that such a relation exists. During the analysis we firmly identify blue straggler stars, including the star 2-17, and find indications for the presence of their evolved counterparts. Our analysis supports the RGB mass-loss found from asteroseismology and we determine precisely the absolute mass of stars on the lower RGB, 1.15\pm0.02Msun. This will be an important consistency check for the detailed asteroseismology of cluster stars.

astro-ph.SR↗

arXiv2012

A New Type of Ambiguity in the Planet and Binary Interpretations of Central Perturbations of High-Magnification Gravitational Microlensing Events

High-magnification microlensing events provide an important channel to detect planets. Perturbations near the peak of a high-magnification event can be produced either by a planet or a binary companion. It is known that central perturbations induced by both types of companions can be generally distinguished due to the basically different magnification pattern around caustics. In this paper, we present a case of central perturbations for which it is difficult to distinguish the planetary and binary interpretations. The peak of a lensing light curve affected by this perturbation appears to be blunt and flat. For a planetary case, this perturbation occurs when the source trajectory passes the negative perturbation region behind the back end of an arrowhead-shaped central caustic. For a binary case, a similar perturbation occurs for a source trajectory passing through the negative perturbation region between two cusps of an astroid-shaped caustic. We demonstrate the degeneracy for 2 high-magnification events of OGLE-2011-BLG-0526 and OGLE-2011-BLG-0950/MOA-2011-BLG-336. For OGLE-2011-BLG-0526, the $χ^2$ difference between the planetary and binary model is $\sim$ 3, implying that the degeneracy is very severe. For OGLE-2011-BLG-0950/MOA-2011-BLG-336, the stellar binary model is formally excluded with $Δχ^2 \sim$ 105 and the planetary model is preferred. However, it is difficult to claim a planet discovery because systematic residuals of data from the planetary model are larger than the difference between the planetary and binary models. Considering that 2 events observed during a single season suffer from such a degeneracy, it is expected that central perturbations experiencing this type of degeneracy is common.

astro-ph.EP↗

arXiv2011

The Observatorio del Teide welcomes SONG: The Stellar Observations Network Group

The Stellar Observations Network Group (SONG) is an international network project aiming to place eight 1-m robotic telescopes around the globe, with the primary objectives of studying stellar oscillations and planets using ultra-precision radial velocity measurements. The prototype of SONG is scheduled to be installed and running at the Observatorio del Teide by Summer 2011. In these proceedings we present the project, primary scientific objectives, and instrument, and discuss the observing possibilities for the Spanish community.

astro-ph.SR↗

arXiv2011

The SONG project and the prototype node at Tenerife

SONG (Stellar Observations Network Group) is a global network of 1-m class robotic telescopes that is under development. The SONG prototype will shortly be operational at Observatorio del Teide, Tenerife, and first light is expected by December 2011. The main scientific goals of the SONG project are asteroseismology of bright stars and follow-up and characterization of exo-planets by means of precise measurements of stellar surface motions and brightness variations. We present the Tenerife SONG node and its instruments.

astro-ph.IM↗

arXiv2011

Stroemgren photometry of the Bulge: the Baade's Window and the globular cluster NGC 6522

We present Ca-uvby Stroemgren photometry of the Baade's Window, including the Galactic globular cluster NGC 6522. We separate field and cluster stars by adopting color-color planes and proper motions. We then estimate the global metallicity of red-giants (RGs) in NGC 6522 by using a new theoretical metallicity calibration of the Stroemgren index hk presented in Calamida et al. (2011). We find that metallicities estimated by adopting the hk index and the (Ca-y) color are systematically more metal-rich than metallicities estimated with hk and the (u-y), (v-y) and (b-y) colors. Current evidence indicate that the hk metallicity index is affected not only by the Calcium abundance, but also by another source of opacity.

astro-ph.SR↗

arXiv2011

On a new theoretical calibration of the Stroemgren hk metallicity index: NGC6522 as a first test case

We present a new theoretical calibration of the Stroemgren metallicity index hk by using alpha-enhanced evolutionary models transformed into the observational plane by using atmosphere models with the same chemical mixture. We apply the new Metallicity--Index--Color (MIC) relations to a sample of 85 field red giants (RGs) and find that the difference between photometric estimates and spectroscopic measurements is on average smaller than 0.1 dex with a dispersion of sigma = 0.19 dex. The outcome is the same if we apply the MIC relations to a sample of eight RGs in the bulge globular cluster NGC6522, but the standard deviation ranges from 0.26 (hk, v-y) to 0.49 (hk, u-y). The difference is mainly caused by a difference in photometric accuracy. The new MIC relations based on the (Ca-y) color provide metallicities systematically more metal-rich than the spectroscopic ones. We found that the Ca-band is affected by Ca abundance and possibly by chromospheric activity.

astro-ph.SR↗

arXiv2011

Solar-like oscillations in the G9.5 subgiant beta Aquilae

An interesting asteroseismic target is the G9.5 IV solar-like star beta Aql. This is an ideal target for asteroseismic investigations, because precise astrometric measurements are available from Hipparcos that greatly help in constraining the theoretical interpretation of the results. The star was observed during six nights in August 2009 by means of the high-resolution échelle spectrograph SARG operating with the TNG 3.58 m Italian telescope on the Canary Islands, exploiting the iodine cell technique. We present the result and the detailed analysis of high-precision radial velocity measurements, where the possibility of detecting time individual p-mode frequencies for the first and deriving their corresponding asymptotic values will be discussed. The time-series analysis carried out from \sim 800 collected spectra shows the typical p-mode frequency pattern with a maximum centered at 416 \muHz. In the frequency range 300 - 600 \muHz we identified for the first time six high S/N (\gtrsim 3.5) modes with l = 0,2 and 11 < n < 16 and three possible candidates for mixed modes (l = 1), although the p-mode identification for this type of star appears to be quite difficult owing to a substantial presence of avoided crossings. The large frequency separation and the surface term from the set of identified modes by means of the asymptotic relation were derived for the first time. Their values are Δν= 29.56 \pm 0.10 \muHz and ε= 1.29 \pm 0.04, consistent with expectations. The most likely value for the small separation is δν_{02} = 2.55 \pm 0.71 \muHz.

astro-ph.SR↗

arXiv2011

Asteroseismology of old open clusters with Kepler: direct estimate of the integrated RGB mass loss in NGC6791 and NGC6819

Mass loss of red giant branch (RGB) stars is still poorly determined, despite its crucial role in the chemical enrichment of galaxies. Thanks to the recent detection of solar-like oscillations in G-K giants in open clusters with Kepler, we can now directly determine stellar masses for a statistically significant sample of stars in the old open clusters NGC6791 and NGC6819. The aim of this work is to constrain the integrated RGB mass loss by comparing the average mass of stars in the red clump (RC) with that of stars in the low-luminosity portion of the RGB (i.e. stars with L <~ L(RC)). Stellar masses were determined by combining the available seismic parameters numax and Dnu with additional photometric constraints and with independent distance estimates. We measured the masses of 40 stars on the RGB and 19 in the RC of the old metal-rich cluster NGC6791. We find that the difference between the average mass of RGB and RC stars is small, but significant (Delta M=0.09 +- 0.03 (random) +- 0.04 (systematic) Msun). Interestingly, such a small DeltaM does not support scenarios of an extreme mass loss for this metal-rich cluster. If we describe the mass-loss rate with Reimers' prescription, a first comparison with isochrones suggests that the observed DeltaM is compatible with a mass-loss efficiency parameter in the range 0.1 <~ eta <~ 0.3. Less stringent constraints on the RGB mass-loss rate are set by the analysis of the ~ 2 Gyr-old NGC6819, largely due to the lower mass loss expected for this cluster, and to the lack of an independent and accurate distance determination. In the near future, additional constraints from frequencies of individual pulsation modes and spectroscopic effective temperatures, will allow further stringent tests of the Dnu and numax scaling relations, which provide a novel, and potentially very accurate, means of determining stellar radii and masses.

astro-ph.SR↗

arXiv2011

An asteroseismic membership study of the red giants in three open clusters observed by Kepler: NGC6791, NGC6819, and NGC6811

Studying star clusters offers significant advances in stellar astrophysics due to the combined power of having many stars with essentially the same distance, age, and initial composition. This makes clusters excellent test benches for verification of stellar evolution theory. To fully exploit this potential, it is vital that the star sample is uncontaminated by stars that are not members of the cluster. Techniques for determining cluster membership therefore play a key role in the investigation of clusters. We present results on three clusters in the Kepler field of view based on a newly established technique that uses asteroseismology to identify fore- or background stars in the field, which demonstrates advantages over classical methods such as kinematic and photometry measurements. Four previously identified seismic non-members in NGC6819 are confirmed in this study, and three additional non-members are found -- two in NGC6819 and one in NGC6791. We further highlight which stars are, or might be, affected by blending, which needs to be taken into account when analysing these Kepler data.

astro-ph.SR↗

arXiv2011

Amplitudes of solar-like oscillations: constraints from red giants in open clusters observed by Kepler

Scaling relations that link asteroseismic quantities to global stellar properties are important for gaining understanding of the intricate physics that underpins stellar pulsation. The common notion that all stars in an open cluster have essentially the same distance, age, and initial composition, implies that the stellar parameters can be measured to much higher precision than what is usually achievable for single stars. This makes clusters ideal for exploring the relation between the mode amplitude of solar-like oscillations and the global stellar properties. We have analyzed data obtained with NASA's Kepler space telescope to study solar-like oscillations in 100 red giant stars located in either of the three open clusters, NGC 6791, NGC 6819, and NGC 6811. By fitting the measured amplitudes to predictions from simple scaling relations that depend on luminosity, mass, and effective temperature, we find that the data cannot be described by any power of the luminosity-to-mass ratio as previously assumed. As a result we provide a new improved empirical relation which treats luminosity and mass separately. This relation turns out to also work remarkably well for main-sequence and subgiant stars. In addition, the measured amplitudes reveal the potential presence of a number of previously unknown unresolved binaries in the red clump in NGC 6791 and NGC 6819, pointing to an interesting new application for asteroseismology as a probe into the formation history of open clusters.

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