S. V. Jeffers

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 neighbour and one of the best-studied low-mass stars. It has an effective temperature of only around 3,050 kelvin, a luminosity of 0.15 per cent of that of the Sun, a measured radius of 14 per cent of the radius of the Sun and a mass of about 12 per cent of the mass of the Sun. Although Proxima is considered a moderately active star, its rotation period is about 83 days (ref. 3) and its quiescent activity levels and X-ray luminosity are comparable to those of the Sun. Here we report observations that reveal the presence of a small planet with a minimum mass of about 1.3 Earth masses orbiting Proxima with a period of approximately 11.2 days at a semi-major-axis distance of around 0.05 astronomical units. Its equilibrium temperature is within the range where water could be liquid on its surface.

Weak magnetic fields in Ap/Bp stars: Evidence for a dipole field lower limit and a tentative interpretation of the magnetic dichotomy

Aims. We investigated a sample of 28 well-known spectroscopically-identified magnetic Ap/Bp stars, with weak, poorly-determined or previously undetected magnetic fields. The aim of this study is to explore the weak part of the magnetic field distribution of Ap/Bp stars. Methods. Using the MuSiCoS and NARVAL spectropolarimeters at Telescope Bernard Lyot (Observatoire du Pic du Midi, France) and the cross-correlation technique Least Squares Deconvolution (LSD), we obtained 282 LSD Stokes V signatures of our 28 sample stars, in order to detect the magnetic field and to infer its longitudinal component with high precision (median σ = 40 G). Results. For the 28 studied stars, we obtained 27 detections of StokesV Zeeman signatures from the MuSiCoS observations. Detection of the Stokes V signature of the 28th star (HD 32650) was obtained during science demonstration time of the new NARVAL spectropolarimeter at Pic du Midi. This result clearly shows that when observed with sufficient precision, all firmly classified Ap/Bp stars show detectable surface magnetic fields. Furthermore, all detected magnetic fields correspond to longitudinal fields which are significantly greater than some tens of G. To better characterise the surface magnetic field intensities and geometries of the sample, we phased the longitudinal field measurements of each star using new and previously-published rotational periods, and modeled them to infer the dipolar field intensity (Bd, measured at the magnetic pole) and the magnetic obliquity (β). The distribution of derived dipole strengths for these stars exhibits a plateau at about 1 kG, falling off to larger and smaller field strengths. Remarkably, in this sample of stars selected for their presumably weak magnetic fields, we find only 2 stars for which the derived dipole strength is weaker than 300 G. We interpret this “magnetic threshold” as a critical value necessary for the stability of large-scale magnetic fields, and develop a simple quantitative model that is able to approximately reproduce the observed threshold characteristics. This scenario leads to a natural explanation of the small fraction of intermediate-mass magnetic stars. It may also explain the near-absence of magnetic fields in more massive B and O-type stars.

Stellar magnetism: empirical trends with age and rotation

We investigate how the observed large-scale surface magnetic fields of low-mass stars (∼0.1– 2M� ), reconstructed through Zeeman–Doppler imaging, vary with age t, rotation and Xray emission. Our sample consists of 104 magnetic maps of 73 stars, from accreting premain sequence to main-sequence objects (1Myr t 10 Gyr). For non-accreting dwarfs we empirically find that the unsigned average large-scale surface field is related to age as t −0.655 ± 0.045 . This relation has a similar dependence to that identified by Skumanich, used as the basis for gyrochronology. Likewise, our relation could be used as an age-dating method (‘magnetochronology’). The trends with rotation we find for the large-scale stellar magnetism are consistent with the trends found from Zeeman broadening measurements (sensitive to large- and small-scale fields). These similarities indicate that the fields recovered from both techniques are coupled to each other, suggesting that small- and large-scale fields could share the same dynamo field generation processes. For the accreting objects, fewer statistically significant relations are found, with one being a correlation between the unsigned magnetic flux and rotation period. We attribute this to a signature of star–disc interaction, rather than being driven by the dynamo.

A BCool magnetic snapshot survey of solar-type stars

We present the results of a major high-resolution spectropolarimetric BCool project magnetic survey of 170 solar-type stars. Surface magnetic fields were detected on 67 stars, with 21 classified as mature solar-type stars, a result that increases by a factor of 4 the number of mature solar-type stars on which magnetic fields have been observed. In addition, a magnetic fieldwasdetectedfor3outof18ofthesubgiantstarssurveyed.ForthepopulationofK-dwarfs, the mean value of |Bl| (|Bl|mean) was also found to be higher (5.7 G) than |Bl|mean measured for the G-dwarfs (3.2 G) and the F-dwarfs (3.3 G). For the sample as a whole, |Bl|mean increases with rotation rate and decreases with age, and the upper envelope for |Bl| correlates well with the observed chromospheric emission. Stars with a chromospheric S-index greater than about 0.2 show a high magnetic field detection rate and so offer optimal targets for future studies. This survey constitutes the most extensive spectropolarimetric survey of cool stars undertaken to date, and suggests that it is feasible to pursue magnetic mapping of a wide range of moderately active solar-type stars to improve our understanding of their surface fields and

The evolution of surface magnetic fields in young solar-type stars I: the first 250 Myr

The surface rotation rates of young solar-type stars vary rapidly with age from the end of the pre-main sequence through the early main sequence. Important changes in the dynamos operating in these stars may result from this evolution, which should be observable in their surface magnetic fields. Here we present a study aimed at observing the evolution of these magnetic fields through this critical time period. We observed stars in open clusters and stellar associations of known ages, and used Zeeman Doppler imaging to characterize their complex magnetic large-scale fields. Presented here are results for 15 stars, from five associations, with ages from 20 to 250 Myr, masses from 0.7 to 1.2 M⊙, and rotation periods from 0.4 to 6 d. We find complex large-scale magnetic field geometries, with global average strengths from 14 to 140 G. There is a clear trend towards decreasing average large-scale magnetic field strength with age, and a tight correlation between magnetic field strength and Rossby number. Comparing the magnetic properties of our zero-age main-sequence sample to those of both younger and older stars, it appears that the magnetic evolution of solar-type stars during the pre-main sequence is primarily driven by structural changes, while it closely follows the stars’ rotational evolution on the main sequence.

The effect of M dwarf starspot activity on low-mass planet detection thresholds

In light of the growing interest in searching for low mass, rocky planets, we investigate the impact of starspots on radial velocity searches for earth-mass planets in orbit about M dwarf stars. Since new surveys targeting M dwarfs will likely be carried out at infrared wavelengths, a comparison between V and Y band starspot-induced jitter is made, indicating a reduction of up to an order of magnitude when observing in the Y band. The exact reduction in jitter is dependent on the photosphere to spot contrast ratio, with greater improvements at smaller contrasts. We extrapolate a model used to describe solar spot distributions to simulate the spot patterns that we expect to find on M dwarfs. Under the assumption that M dwarfs are near or fully convective, we randomly place starspots on the stellar surface, simulating different levels of spot coverage. Line profiles distorted by spots are derived and are used to investigate the starspot-induced jitter. By making assumptions about the degree of spot activity, detection limits for earth-mass planets in habitable zones are simulated for between 10 and 500 observation epochs. We find that ≤50 epochs are required to detect 1–2M⊕ planets (with

CARMENES input catalogue of M dwarfs - I. Low-resolution spectroscopy with CAFOS

Context. CARMENES is a stabilised, high-resolution, double-channel spectrograph at the 3.5 m Calar Alto telescope. It is optimally designed for radial-velocity surveys of M dwarfs with potentially habitable Earth-mass planets. Aims. We prepare a list of the brightest, single M dwarfs in each spectral subtype observable from the northern hemisphere, from which we will select the best planet-hunting targets for CARMENES. Methods. In this first paper on the preparation of our input catalogue, we compiled a large amount of public data and collected low-resolution optical spectroscopy with CAFOS at the 2.2 m Calar Alto telescope for 753 stars. We derived accurate spectral types using a dense grid of standard stars, a double least-squares minimisation technique, and 31 spectral indices previously defined by other authors. Additionally, we quantified surface gravity, metallicity, and chromospheric activity for all the stars in our sample. Results. We calculated spectral types for all 753 stars, of which 305 are new and 448 are revised. We measured pseudo-equivalent widths of Halpha for all the stars in our sample, concluded that chromospheric activity does not affect spectral typing from our indices, and tabulated 49 stars that had been reported to be young stars in open clusters, moving groups, and stellar associations. Of the 753 stars, two are new subdwarf candidates, three are T Tauri stars, 25 are giants, 44 are K dwarfs, and 679 are M dwarfs. Many of the 261 investigated dwarfs in the range M4.0-8.0 V are among the brightest stars known in their spectral subtype. Conclusions. This collection of low-resolution spectroscopic data serves as a candidate target list for the CARMENES survey and can be highly valuable for other radial-velocity surveys of M dwarfs and for studies of cool dwarfs in the solar neighbourhood.

Data-reduction techniques for high-contrast imaging polarimetry Applications to ExPo

Context. Imaging polarimetry is a powerful tool for detecting and characterizing exoplanets and circumstellar environments. Polarimetry allows a separation of the light coming from an unpolarized source such as a star and the polarized source such as a planet or a protoplanetary disk. Future facilities like SPHERE at the VLT or EPICS at the E-ELT will incorporate imaging polarimetry to detect exoplanets. The Extreme Polarimeter (ExPo) is a dual-beam imaging polarimeter that can currently reach contrast ratios of 10 5 , enough to characterize circumstellar environments. Aims. We present the data-reduction steps for a dual-beam imaging polarimeter that can reach contrast ratios of 10 5 . Methods. The data obtained with ExPo at the William Herschel Telescope (WHT) are analyzed. Instrumental artifacts and noise sources are discussed for an unpolarized star and for a protoplanetary disk (AB Aurigae). Results. The combination of fast modulation and dual-beam techniques allows us to minimize instrumental artifacts. A proper data processing and alignment of the images is fundamental when dealing with high contrasts. Imaging polarimetry proves to be a powerful method to resolve circumstellar environments even without a coronagraph mask or an adaptive optics system.

The search for magnetic fields in mercury-manganese stars

Mercury-manganese (HgMn) stars belong to the class of chemically peculiar (CP) stars. It was recently discovered that some HgMn stars have spots of chemical elements on their surfaces. According to conventional picture of CP stars, magnetic field facilitates the formation and long term stability of chemical spots by controlling stratification of elements in stellar atmosphere. However, previous attempts to find magnetic field in HgMn stars set an upper limit on its strength at the level of about 20-100 Gauss. Observational evidence suggested that even weaker magnetic fields can be responsible for the formation of chemical spots. The main goal of our work was to verify this possibility.The search for weak magnetic fields requires the use of least-squares deconvolution (LSD) technique. This method combines information from many spectral lines providing a mean line profile with increased signal-to-noise ratio. Up to now there was no extensive comparison of the LSD profile with real spectral lines. We showed that the LSD profile of the intensity spectrum does not behave like a real spectral line as a function of chemical composition. However, for circular polarization, LSD profile resembles the profile of a spectral line with mean atomic parameters.We performed a comprehensive search for magnetic field in 47 HgMn stars and their companions, based on high-quality spectropolarimetric data obtained with the HARPSpol polarimeter at the ESO 3.6-m telescope. With the help of LSD technique, an upper limit on the mean longitudinal magnetic field was brought down to 2-10 G for most stars. We concluded that magnetic field is not responsible for the spot formation in HgMn stars.We obtained full rotational phase coverage for the HgMn stars φ Phe and 66 Eri. This enabled us to investigate line profile variability, reconstruct surface maps of chemical elements, and perform a search for magnetic field with very high sensitivity. For φ Phe we derived surface maps of Y, Sr, Ti, Cr, and obtained an upper limit of 4 G on the field strength. We also found marginal indication of vertical stratification of Y and Ti. No magnetic field was detected in both components of 66 Eri, with an upper limit of 10-24 G. We discovered chemical spots of Y, Sr, Ba, and Ti, in the primary star. We demonstrated a relation between the binary orbit and the morphology of these spots.

Two planets around Kapteyn's star : a cold and a temperate super-Earth orbiting the nearest halo red-dwarf

Exoplanets of a few Earth masses can be now detected around nearby low-mass stars using Doppler spectroscopy. In this paper, we investigate the radial velocity variations of Kapteyns star, which is both a sub-dwarf M-star and the nearest halo object to the Sun. The observations comprise archival and new HARPS, HIRES and PFS Doppler measurements. Two Doppler signals are detected at periods of 48 and 120 days using likelihood periodograms and a Bayesian analysis of the data. Using the same techniques, the activity indicies and archival ASAS-3 photometry show evidence for low-level activity periodicities of the order of several hundred days. However, there are no significant correlations with the radial velocity variations on the same time-scales. The inclusion of planetary Keplerian signals in the model results in levels of correlated and excess white noise that are remarkably low compared to younger G, K and M dwarfs. We conclude that Kapteyns star is most probably orbited by two super-Earth mass planets, one of which is orbiting in its circumstellar habitable zone, becoming the oldest potentially habitable planet known to date. The presence and long-term survival of a planetary system seems a remarkable feat given the peculiar origin and kinematic history of Kapteyns star. The detection of super-Earth mass planets around halo stars provides important insights into planet-formation processes in the early days of the Milky Way.