arXiv subjects

Stefan Dreizler

Publications and source records attributed to Stefan Dreizler.

At least 55 records · Page 3Linked to original sources

arXiv2018

Ground-based detection of an extended helium atmosphere in the Saturn-mass exoplanet WASP-69b

Hot gas giant exoplanets can lose part of their atmosphere due to strong stellar irradiation, affecting their physical and chemical evolution. Studies of atmospheric escape from exoplanets have mostly relied on space-based observations of the hydrogen Lyman-α line in the far ultraviolet which is strongly affected by interstellar absorption. Using ground-based high-resolution spectroscopy we detect excess absorption in the helium triplet at 1083 nm during the transit of the Saturn-mass exoplanet WASP-69b, at a signal-to-noise ratio of 18. We measure line blue shifts of several km/s and post transit absorption, which we interpret as the escape of part of the atmosphere trailing behind the planet in comet-like form. [Additional notes by authors: Furthermore, we provide upper limits for helium signals in the atmospheres of the exoplanets HD 209458b, KELT-9b, and GJ 436b. We investigate the host stars of all planets with detected helium signals and those of the three planets we derive upper limits for. In each case we calculate the X-ray and extreme ultraviolet flux received by these planets. We find that helium is detected in the atmospheres of planets (orbiting the more active stars and) receiving the larger amount of irradiation from their host stars.]

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arXiv2018

Thorium in solar twins: implications for habitability in rocky planets

We have investigated the thorium (Th) abundance in a sample of 53 thin disc solar twins covering a wide range of ages. These data provide constrains on the mantle energy budget of terrestrial planets that can be formed over the evolution of the Galaxy's thin disc. We have estimated Th abundances with an average precision of 0.025\,dex (in both [Th/H] and [Th/Fe]) through comprehensive spectral synthesis of a Th\,II line present at 4019.1290\,Å, using very high resolution (R\,=\,115,000) high quality HARPS spectra obtained at the ESO La Silla Observatory. We have confirmed that there is a large energy budget from Th decay for maintaining mantle convection inside potential rocky planets around solar twins, from the Galactic thin disc formation until now, because the pristine [Th/H]$_{\rm ZAMS}$ is super-solar on average under a uniform dispersion of 0.056\,dex (varying from +0.037 up to +0.138\,dex based on linear fits against isochrone stellar age). Comparing to neodymium (Nd) and europium (Eu), two others neutron-capture elements, the stellar pristine abundance of Th follows Eu along the Galactic thin disc evolution, but it does not follow Nd, probably because neodymium has a significant contribution from the $s$-process (about 60\,per\,cent).

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arXiv2018

The Chemical Homogeneity of Sun-like Stars in the Solar Neighborhood

The compositions of stars are a critical diagnostic tool for many topics in astronomy such as the evolution of our Galaxy, the formation of planets, and the uniqueness of the Sun. Previous spectroscopic measurements indicate a large intrinsic variation in the elemental abundance patterns of stars with similar overall metal content. However, systematic errors arising from inaccuracies in stellar models are known to be a limiting factor in such studies, and thus it is uncertain to what extent the observed diversity of stellar abundance patterns is real. Here we report the abundances of 30 elements with precisions of 2% for 79 Sun-like stars within 100 parsecs. Systematic errors are minimized in this study by focusing on solar twin stars and performing a line-by-line differential analysis using high-resolution, high-signal-to-noise spectra. We resolve [X/Fe] abundance trends in galactic chemical evolution at precisions of $10^{-3}$ dex Gyr$^{-1}$ and reveal that stars with similar ages and metallicities have nearly identical abundance patterns. Contrary to previous results, we find that the ratios of carbon-to-oxygen and magnesium-to-silicon in solar metallicity stars are homogeneous to within 10% throughout the solar neighborhood, implying that exoplanets may exhibit much less compositional diversity than previously thought. Finally, we demonstrate that the Sun has a subtle deficiency in refractory material relative to >80% of solar twins (at 2$σ$ confidence), suggesting a possible signpost for planetary systems like our own.

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arXiv2018

The atmosphere of WASP-17b: Optical high-resolution transmission spectroscopy

High-resolution transmission spectroscopy is a method for understanding the chemical and physical properties of upper exoplanetary atmospheres. Due to large absorption cross-sections, resonance lines of atomic sodium D-lines (at 5889.95 $Å$ and 5895.92 $Å$) produce large transmission signals. Our aim is to unveil the physical properties of WASP-17b through an accurate measurement of the sodium absorption in the transmission spectrum. We analyze 37 high-resolution spectra observed during a single transit of WASP-17b with the MIKE instrument on the 6.5 meter Magellan Telescopes. We exclude stellar flaring activity during the observations by analyzing the temporal variations of H$_α$ and Ca II infra-red triplet (IRT) lines. Then we obtain the excess absorption light curves in wavelength bands of 0.75, 1, 1.5 and 3 $Å$ around the center of each sodium line (i.e., the light curve approach). We model the effects of differential limb-darkening, and the changing planetary radial velocity on the light curves. We also analyze the sodium absorption directly in the transmission spectrum, which is obtained through dividing in-transit by out-of-transit spectra (i.e., the division approach). We then compare our measurements with a radiative transfer atmospheric model. Our analysis results in a tentative detection of exoplanetary sodium: we measure the width and amplitude of the exoplanetary sodium feature to be $σ_{\mathrm{Na}}$ = (0.128 $\pm$ 0.078) $Å$ and A$_{\mathrm{Na}}$ = (1.7 $\pm$ 0.9)% in the excess light curve approach and $σ_{\mathrm{Na}}$ = (0.850 $\pm$ 0.034) $Å$ and A$_{\mathrm{Na}}$ = (1.3 $\pm$ 0.6)% in the division approach. By comparing our measurements with a simple atmospheric model, we retrieve an atmospheric temperature of 1550 $^{+170} _{-200}$ K and radius (at 0.1 bar) of 1.81 $\pm$ 0.02 R$_{\rm Jup}$ for WASP-17b.

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arXiv2018

Cluster kinematics and stellar rotation in NGC 419 with MUSE and adaptive optics

We present adaptive optics (AO) assisted integral-field spectroscopy of the intermediate-age star cluster NGC 419 in the Small Magellanic Cloud. By investigating the cluster dynamics and the rotation properties of main sequence turn-off stars (MSTO), we demonstrate the power of AO-fed MUSE observations for this class of objects. Based on 1 049 radial velocity measurements, we determine a dynamical cluster mass of 1.4+/-0.2x10^5 M_sun and a dynamical mass-to-light ratio of 0.67+/-0.08, marginally higher than simple stellar population predictions for a Kroupa initial mass function. A stacking analysis of spectra at both sides of the extended MSTO reveals significant rotational broadening. Our analysis further provides tentative evidence that red MSTO stars rotate faster than their blue counterparts. We find average V sin i values of 87+/-16 km/s and 130+/-22 km/s for blue and red MSTO stars, respectively. Potential systematic effects due to the low spectral resolution of MUSE can reach 30 km/s but the difference in V sin i between the populations is unlikely to be affected.

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arXiv2018

The MICADO first light imager for the ELT: overview, operation, simulation

MICADO will enable the ELT to perform diffraction limited near-infrared observations at first light. The instrument's capabilities focus on imaging (including astrometric and high contrast) as well as single object spectroscopy. This contribution looks at how requirements from the observing modes have driven the instrument design and functionality. Using examples from specific science cases, and making use of the data simulation tool, an outline is presented of what we can expect the instrument to achieve.

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arXiv2018

The Solar Twin Planet Search: The age - chromospheric activity relation

It is well known that the magnetic activity of solar type stars decreases with age, but it is widely debated in the literature whether there is a smooth decline or if there is an early sharp drop until 1-2 Gyr followed by a relatively inactive constant phase. We revisited the activity-age relation using time-series observations of a large sample of solar twins whose precise isochronal ages and other important physical parameters have been determined. We measured the Ca II H and K activity indices using 9000 HARPS spectra of 82 solar twins. We measured an average solar activity of $S_{\rm MW}$ = 0.1712 $\pm$ 0.0017 during solar magnetic cycles 23$-$24 covered by HARPS observations and we also inferred an average of $S_{\rm MW}$ = 0.1694 $\pm$ 0.0025 for cycles 10$-$24, anchored on a S index vs. sunspot number correlation. Also, a simple relation between the average and dispersion of the activity levels of solar twins was found. This enabled us to predict the stellar variability effects on the age-activity diagram and, consequently, estimate the chromospheric age uncertainties due to the same phenomena. The age-activity relation is still statistically significant up to ages around 6$-$7 Gyr, in agreement with previous works using open clusters and field stars with precise ages. Our research confirms that Ca II H \& K lines remain a useful chromospheric evolution tracer until stars reach ages of at least 6$-$7 Gyr. We found an evidence that, for the most homogeneous set of old stars, the chromospheric activity indices seem to continue decreasing after the solar age towards the end of the main-sequence. Our results indicate that a significant part of the scatter observed in the age-activity relation of solar twins can be attributed to stellar cycle modulations effects. The Sun seems to have a normal activity level and variability for its age.

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arXiv2018

The CARMENES search for exoplanets around M dwarfs: A low-mass planet in the temperate zone of the nearby K2-18

K2-18 is a nearby M2.5 dwarf, located at 34 pc and hosting a transiting planet which was first discovered by the {\it K2} mission and later confirmed with {\it Spitzer Space Telescope} observations. With a radius of $\sim 2 \, R_{\oplus}$ and an orbital period of $\sim 33$ days, the planet lies in the temperate zone of its host star and receives stellar irradiation similar to Earth. Here we perform radial velocity follow-up observations with the visual channel of CARMENES with the goal of determining the mass and density of the planet. We measure a planetary semi-amplitude of $K_b \sim 3.5$ \mpersec\ and a mass of $M_b \sim 9 \, M_{\oplus}$, yielding a bulk density around $ρ_b \sim 4 \, \mathrm{g \,cm^{-3}}$. This indicates a low-mass planet with a composition consistent with a solid core and a volatile-rich envelope. A signal at 9 days was recently reported using radial velocity measurements taken with the HARPS spectrograph. This was interpreted as being due to a second planet. We see a weaker, time and wavelength dependent signal in the CARMENES data set and thus favor stellar activity for its origin. \ktwo\ joins the growing group of low-mass planets detected in the temperate zone of M dwarfs. The brightness of the host star in the near-infrared makes the system a good target for detailed atmospheric studies with the {\it James Webb Space Telescope}.

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arXiv2018

A detached stellar-mass black hole candidate in the globular cluster NGC 3201

As part of our massive spectroscopic survey of 25 Galactic globular clusters with MUSE, we performed multiple epoch observations of NGC 3201 with the aim of constraining the binary fraction. In this cluster, we found one curious star at the main-sequence turn-off with radial velocity variations of the order of 100 km/s, indicating the membership to a binary system with an unseen component since no other variations appear in the spectra. Using an adapted variant of the generalized Lomb-Scargle periodogram, we could calculate the orbital parameters and found the companion to be a detached stellar-mass black hole with a minimum mass of 4.36 $\pm$ 0.41 solar masses. The result is an important constraint for binary and black hole evolution models in globular clusters as well as in the context of gravitational wave sources.

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arXiv2017

On the Origin of Diffuse Ionized Gas in the Antennae Galaxy

The "Antennae Galaxy" (NGC 4038/39) is the closest major interacting galaxy system and therefore often taken as merger prototype. We present the first comprehensive integral field spectroscopic dataset of this system, observed with the MUSE instrument at the ESO VLT. We cover the two regions in this system which exhibit recent star-formation: the central galaxy interaction and a region near the tip of the southern tidal tail. In these fields, we detect HII regions and diffuse ionized gas to unprecedented depth. About 15% of the ionized gas was undetected by previous observing campaigns. This newly detected faint ionized gas is visible everywhere around the central merger, and shows filamentary structure. We estimate diffuse gas fractions of about 60% in the central field and 10% in the southern region. We are able to show that the southern region contains a significantly different population of HII regions, showing fainter luminosities. By comparing HII region luminosities with the HST catalog of young star clusters in the central field, we estimate that there is enough Lyman-continuum leakage in the merger to explain the amount of diffuse ionized gas that we detect. We compare the Lyman-continuum escape fraction of each HII region against ionization-parameter sensitive emission line ratios. While we find no systematic trend between these properties, the most extreme line ratios seem to be strong indicators of density bounded ionization. Extrapolating the Lyman-continuum escape fractions to the southern region, we conclude that just from the comparison of the young stellar populations to the ionized gas there is no need to invoke other ionization mechanisms than Lyman-continuum leaking HII regions for the diffuse ionized gas in the Antennae.

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arXiv2017

The temporal evolution of neutron-capture elements in the Galactic discs

Important insights into the formation and evolution of the Galactic disc(s) are contained in the chemical compositions of stars. We analysed high-resolution and high signal to noise HARPS spectra of 79 solar twin stars in order to obtain precise determinations of their atmospheric parameters, ages ($σ$$\sim$0.4 Gyr) and chemical abundances ($σ$$<$0.01~dex) of 12 neutron-capture elements (Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, and Dy). This valuable dataset allows us to study the [X/Fe]-age relations over a time interval of $\sim$10 Gyr and among stars belonging to the thin and thick discs. These relations show that i) the $s$-process has been the main channel of nucleosynthesis of $n$-capture elements during the evolution of the thin disc; ii) the thick disc is rich in $r$-process elements which suggests that its formation has been rapid and intensive. %; iii) a chemical continuity between the thin and thick discs is evident in the abundances of Ba. In addition, the heavy (Ba, La, Ce) and light (Sr, Y, Zr) $s$-process elements revealed details on the dependence between the yields of AGB stars and the stellar mass or metallicity. Finally, we confirmed that both [Y/Mg] and [Y/Al] ratios can be employed as stellar clocks, allowing ages of solar twin stars to be estimated with an average precision of $\sim$0.5~Gyr.

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arXiv2017

A stellar census in globular clusters with MUSE: The contribution of rotation to cluster dynamics studied with 200 000 stars

This is the first of a series of papers presenting the results from our survey of 25 Galactic globular clusters with the MUSE integral-field spectrograph. In combination with our dedicated algorithm for source deblending, MUSE provides unique multiplex capabilities in crowded stellar fields and allows us to acquire samples of up to 20 000 stars within the half-light radius of each cluster. The present paper focuses on the analysis of the internal dynamics of 22 out of the 25 clusters, using about 500 000 spectra of 200 000 individual stars. Thanks to the large stellar samples per cluster, we are able to perform a detailed analysis of the central rotation and dispersion fields using both radial profiles and two-dimensional maps. The velocity dispersion profiles we derive show a good general agreement with existing radial velocity studies but typically reach closer to the cluster centres. By comparison with proper motion data we derive or update the dynamical distance estimates to 14 clusters. Compared to previous dynamical distance estimates for 47 Tuc, our value is in much better agreement with other methods. We further find significant (>3sigma) rotation in the majority (13/22) of our clusters. Our analysis seems to confirm earlier findings of a link between rotation and the ellipticities of globular clusters. In addition, we find a correlation between the strengths of internal rotation and the relaxation times of the clusters, suggesting that the central rotation fields are relics of the cluster formation that are gradually dissipated via two-body relaxation.

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arXiv2017

Mapping diffuse interstellar bands in the local ISM on small scales via MUSE 3D spectroscopy

We map the interstellar medium (ISM) including the diffuse interstellar bands (DIBs) in absorption toward the globular cluster NGC 6397 using VLT/MUSE. This pilot study demonstrates the power of MUSE for mapping the local ISM on very small scales which provides a new window for ISM observations. Assuming the absorbers are located at the rim of the Local Bubble we trace small-scale variations in NaI and KI as well as in several DIBs structures on the order of mpc (milliparsec, a few thousand AU). The sightlines defined by binned stellar spectra are separated by only a few arcseconds and we probe the absorption within a physically connected region. This analysis utilized the fitting residuals of individual stellar spectra of NGC 6397 member stars and analyzed lines from neutral species and several DIBs in Voronoi-binned composite spectra with high signal-to-noise ratio (S/N). We verify the suitability of the MUSE 3D spectrograph for such measurements and gain new insights by probing a single physical absorber with multiple sight lines.

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arXiv2017

Spectroscopic binaries in the Solar Twin Planet Search program: from substellar-mass to M dwarf companions

Previous studies on the rotation of Sun-like stars revealed that the rotational rates of young stars converge towards a well-defined evolution that follows a power-law decay. It seems, however, that some binary stars do not obey this relation, often by displaying enhanced rotational rates and activity. In the Solar Twin Planet Search program we observed several solar twin binaries, and found a multiplicity fraction of $42\% \pm 6\%$ in the whole sample; moreover, at least three of these binaries (HIP 19911, HIP 67620 and HIP 103983) clearly exhibit the aforementioned anomalies. We investigated the configuration of the binaries in the program, and discovered new companions for HIP 6407, HIP 54582, HIP 62039 and HIP 30037, of which the latter is orbited by a $0.06$ M$_\odot$ brown dwarf in a 1-month long orbit. We report the orbital parameters of the systems with well-sampled orbits and, in addition, the lower limits of parameters for the companions that only display a curvature in their radial velocities. For the linear trend binaries, we report an estimate of the masses of their companions when their observed separation is available, and a minimum mass otherwise. We conclude that solar twin binaries with low-mass stellar companions at moderate orbital periods do not display signs of a distinct rotational evolution when compared to single stars. We confirm that the three peculiar stars are double-lined binaries, and that their companions are polluting their spectra, which explains the observed anomalies.

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arXiv2017

Exploring plausible formation scenarios for the planet candidate orbiting Proxima Centauri

We present a study of four different formation scenarios that may be able to explain the origin of the recently announced planet (`Proxima b') orbiting Proxima Centauri. The aim is to examine how the formation scenarios differ in their predictions for the multiplicity of the Proxima system, the water/volatile content of Proxima b and its orbital eccentricity, so that these can be tested by future observations. A scenario of in situ formation via giant impacts from a locally enhanced disc of planetary embryos/planetesimals predicts that Proxima b will be in a multiplanet system with a measurably finite eccentricity. Assuming that the local solid enhancement needed to form a Proxima b analogue arises because of the inwards drift of solids in the form of small planetesimals/boulders, this scenario also results in Proxima b analogues being only moderately endowed with water/volatiles. A scenario in which multiple embryos form, migrate and mutually collide within a gas disc results in Proxima b being a member of a multiple system, possibly displaying mean motion resonances, but where the constituent members are Ocean planets due to accretion occurring mainly outside of the snowline. A scenario in which a single accreting embryo forms outside the snowline, and migrates inwards while accreting planetesimals/pebbles results in Proxima b being an isolated Ocean planet on a circular orbit. A scenario in which Proxima b formed via pebble accretion interior to the snowline produces a dry planet on a circular orbit. Future observations that characterise the physical and orbital properties of Proxima b, and any additional planets in the system, will provide valuable insights into the formation history of this neighbouring planetary system.

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arXiv2016

The Solar Twin Planet Search. V. Close-in, low-mass planet candidates and evidence of planet accretion in the solar twin HIP 68468

[Methods]. We obtained high-precision radial velocities with HARPS on the ESO 3.6 m telescope and determined precise stellar elemental abundances (~0.01 dex) using MIKE spectra on the Magellan 6.5m telescope. [Results]. Our data indicate the presence of a planet with a minimum mass of 26 Earth masses around the solar twin HIP 68468. The planet is a super-Neptune, but unlike the distant Neptune in our solar system (30 AU), HIP 68468c is close-in, with a semi-major axis of 0.66 AU, similar to that of Venus. The data also suggest the presence of a super-Earth with a minimum mass of 2.9 Earth masses at 0.03 AU; if the planet is confirmed, it will be the fifth least massive radial velocity planet discovery to date and the first super-Earth around a solar twin. Both isochrones (5.9 Gyr) and the abundance ratio [Y/Mg] (6.4 Gyr) indicate an age of about 6 billion years. The star is enhanced in refractory elements when compared to the Sun, and the refractory enrichment is even stronger after corrections for Galactic chemical evolution. We determined a NLTE Li abundance of 1.52 dex, which is four times higher than what would be expected for the age of HIP 68468. The older age is also supported by the low log(R'HK) (-5.05) and low jitter. Engulfment of a rocky planet of 6 Earth masses can explain the enhancement in both lithium and the refractory elements. [Conclusions]. The super-Neptune planet candidate is too massive for in situ formation, and therefore its current location is most likely the result of planet migration that could also have driven other planets towards its host star, enhancing thus the abundance of lithium and refractory elements in HIP 68468. The intriguing evidence of planet accretion warrants further observations to verify the existence of the planets that are indicated by our data and to better constrain the nature of the planetary system around this unique star.

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arXiv2016

A terrestrial planet candidate in a temperate orbit around Proxima Centauri

At a distance of 1.295 parsecs, the red-dwarf Proxima Centauri ($α$ Centauri C, GL 551, HIP 70890, or simply Proxima) is the Sun's closest stellar neighbor and one of the best studied low-mass stars. It has an effective temperature of only $\sim$ 3050 K, a luminosity of $\sim$0.1 per cent solar, a measured radius of 0.14 R$_\odot$ and a mass of about 12 per cent the mass of the Sun. Although Proxima is considered a moderately active star, its rotation period is $\sim$ 83 days, and its quiescent activity levels and X-ray luminosity are comparable to the Sun's. New observations reveal the presence of a small planet orbiting Proxima with a minimum mass of 1.3~Earth masses and an orbital period of $\sim$11.2 days. Its orbital semi-major axis is $\sim0.05$ AU, with an equilibrium temperature in the range where water could be liquid on its surface.

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arXiv2016

The Solar Twin Planet Search: IV. The Sun as a typical rotator and evidence for a new rotational braking law for Sun-like stars

It is still unclear how common the Sun is when compared to other similar stars in regards to some of its physical properties, such as rotation. Considering that gyrochronology relations are widely used today to estimate ages of stars in the main sequence, and that the Sun is used to calibrate it, it is crucial to assess if these procedures are acceptable. We analyze the rotational velocities -- limited by the unknown rotation axis inclination angle -- of an unprecedented large sample of solar twins in order to study the rotational evolution of Sun-like stars, and assess if the Sun is a typical rotator. We use high-resolution ($R = 115000$) spectra obtained with the HARPS spectrograph and ESO's 3.6 m telescope at La Silla Observatory. The projected rotational velocities for 82 solar twins are estimated by line profile fitting with synthetic spectra. Macroturbulence velocities are inferred from a prescription that accurately reflects their dependence with effective temperature and luminosity of the stars. Our sample of solar twins include some spectroscopic binaries with enhanced rotational velocities, and we do not find any non-spectroscopic binaries with unusually high rotation velocities. We verified that the Sun does not have a peculiar rotation, but the solar twins exhibit rotational velocities that depart from the Skumanich relation. The Sun is a regular rotator when compared to solar twins with a similar age. Additionally, we obtain a rotational braking law that better describes the stars in our sample ($v \propto t^{-0.6}$) in contrast to previous, often-used scalings.

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