Christoph Bergmann

Precise radial velocities of giant stars - VII. Occurrence rate of giant extrasolar planets as a function of mass and metallicity

(abridged) We have obtained precise radial velocities for a sample of 373 G and K type giants at Lick Observatory regularly over more than 12 years. Planets have been identified around 15 giant stars; an additional 20 giant stars host planet candidates. We investigate the occurrence rate of substellar companions around giant stars as a function of stellar mass and metallicity. We probe the stellar mass range from about 1 to beyond 3 M_Sun, which is not being explored by main-sequence samples. We fit the giant planet occurrence rate as a function of stellar mass and metallicity with a Gaussian and an exponential distribution, respectively. We find strong evidence for a planet-metallicity correlation among the secure planet hosts of our giant star sample, in agreement with the one for main-sequence stars. However, the planet-metallicity correlation is absent for our sample of planet candidates, raising the suspicion that a good fraction of them might indeed not be planets. Consistent with the results obtained by Johnson for subgiants, the giant planet occurrence rate increases in the stellar mass interval from 1 to 1.9 M_Sun. However, there is a maximum at a stellar mass of 1.9 +0.1/-0.5 M_Sun, and the occurrence rate drops rapidly for masses larger than 2.5-3.0 M_Sun. We do not find any planets around stars more massive than 2.7 M_Sun, although there are 113 stars with masses between 2.7 and 5 M_Sun in our sample (corresponding to a giant planet occurrence rate < 1.6% at 68.3% confidence in that stellar mass bin). We also show that this result is not a selection effect related to the planet detectability being a function of the stellar mass. We conclude that giant planet formation or inward migration is suppressed around higher mass stars, possibly because of faster disk depletion coupled with a longer migration timescale.

The Pan-Pacific Planet Search III: five companions orbiting giant stars

We report a new giant planet orbiting the K giant HD 155233, as well as four stellar-mass companions from the Pan–Pacific Planet Search, a Southern hemisphere radial velocity survey for planets orbiting nearby giants and sub-giants. We also present updated velocities and a refined orbit for HD 47205b (7 CMa b), the first planet discovered by this survey. HD 155233b has a period of 885 ± 63 d, eccentricity e = 0.03 ± 0.20, and m sin i = 2.0 ± 0.5 MJup. The stellar-mass companions range in m sin i from 0.066 to 0.33 M�. Whilst HD 104358B falls slightly below the traditional 0.08 M� hydrogen-burning mass limit, and is hence a brown dwarf candidate, we estimate only a 50 per cent a priori probability of a truly sub-stellar mass.

ON THE TRANSIT POTENTIAL OF THE PLANET ORBITING IOTA DRACONIS

Most of the known transiting exoplanets are in short-period orbits, largely due to the bias inherent in detecting planets through the transit technique. However, the eccentricity distribution of the known radial velocity planets results in many of those planets having a non-negligible transit probability. One such case is the massive planet orbiting the giant star iota Draconis, a situation where both the orientation of the planets eccentric orbit and the size of the host star inflate the transit probability to a much higher value than for a typical hot Jupiter. Here we present a revised fit of the radial velocity data with new measurements and a photometric analysis of the stellar variability. We provide a revised transit probability, an improved transit ephemeris, and discuss the prospects for observing a transit of this planet from both ground and space.

Searching for Earth-mass planets around α Centauri: precise radial velocities from contaminated spectra

This work is part of an ongoing project which aims to detect terrestrial planets in our neighbouring star system α Centauri using the Doppler method. Owing to the small angular separation between the two components of the α Cen AB binary system, the observations will to some extent be contaminated with light coming from the other star. We are accurately determining the amount of contamination for every observation by measuring the relative strengths of the H-α and NaD lines. Furthermore, we have developed a modified version of a well-established Doppler code that is modelling the observations using two stellar templates simultaneously. With this method we can significantly reduce the scatter of the radial velocity (RV) measurements due to spectral cross-contamination and hence increase our chances of detecting the tiny signature caused by potential Earth-mass planets. After correcting for the contamination we achieve RV precision of ∼2.5 ms-1 for a given night of observations. We have also applied this new Doppler code to four southern double-lined spectroscopic binary systems (HR159, HR913, HR7578 and HD181958) and have successfully recovered radial velocities for both components simultaneously.

Planets around Giant Stars

We present results from a radial‐velocity survey of 373 giant stars at Lick Observatory, which started in 1999. The previously announced planets ι Dra b and Pollux b are confirmed by continued monitoring. The frequency of detected planetary companions appears to increase with metallicity. The star ν Oph is orbited by two brown dwarf companions with masses of 22.3 MJup and 24.5 MJup in orbits with a period ratio close to 6:1. It is likely that the two companions to ν Oph formed in a disk around the star.

The conjectured S-type retrograde planet in ν Octantis: more evidence including four years of iodine-cell radial velocities

We report 1212 radial-velocity (RV) measurements obtained in the years 2009–2013 using an iodine cell for the spectroscopic binary ν Octantis (K1 III/IV). This system (abin ∼ 2.6 au, P ∼ 1050 d) is conjectured to have a Jovian planet with a semimajor axis half that of the binary host. The extreme geometry only permits long-term stability if the planet is in a retrograde orbit. Whilst the reality of the planet (P ∼ 415 d) remains uncertain, other scenarios (stellar variability or apsidal motion caused by a yet unobserved third star) continue to appear substantially less credible based on cross-correlation function bisectors, line-depth ratios and many other independent details. If this evidence is validated but the planet is disproved, the claims of other planets using RVs will be seriously challenged. We also describe a significant revision to the previously published RVs and the full set of 1437 RVs now encompasses nearly 13 yr. The sensitive orbital dynamics allow us to constrain the 3D architecture with a broad prior probability distribution on the mutual inclination, which with posterior samples obtained from an N-body Markov chain Monte Carlo is found to be 152. ◦ 5±0.7 0.6. None of these samples are dynamically stable beyond 106 yr. However, a grid search around the best-fitting solution finds a region that has many models stable for 107 yr, and includes one model within 1σ that is stable for at least 108 yr. The planet’s exceptional nature demands robust independent verification and makes the theoretical understanding of its formation a worthy challenge.

State Policy and Local Performance: Pasture Use and Pastoral Practices in the Kumaon Himalaya

In the Kumaon Himalaya, British colonial administrators as well as agents of the independent Indian Union intervened heavily in pasture use by adopting rationally governed and scientifically sanctioned development schemes. These measures mostly originated from outside and largely ignored local cultural logics through which a pastoral life also takes its form. We use the case of the Bhotiyas of the Kumaon Himalaya to explicate this interaction of state policy and local performance. On the one hand, we analyse recent development trends that occurred after India started to liberalise its market in the early 1990s. On the other hand, we describe a ritual practice through which the Bhotiyas channel emerging power relations and conflicts towards the outside of their migratory cycle. We conclude by suggesting an interdisciplinary perspective on pastoral practices in the Himalayan region.

Veloce Rosso: Australia's new precision radial velocity spectrograph

Veloce is an ultra-stable fibre-fed R4 echelle spectrograph for the 3.9 m Anglo-Australian Telescope. The first channel to be commissioned, Veloce ‘Rosso’, utilises multiple low-cost design innovations to obtain Doppler velocities for sun-like and M-dwarf stars at <1 ms -1 precision. The spectrograph has an asymmetric white-pupil format with a 100-mm beam diameter, delivering R>75,000 spectra over a 580-930 nm range for the Rosso channel. Simultaneous calibration is provided by a single-mode pulsed laser frequency comb in tandem with a traditional arc lamp. A bundle of 19 object fibres ensures full sampling of stellar targets from the AAT site. Veloce is housed in dual environmental enclosures that maintain positive air pressure at a stability of ±0.3 mbar, with a thermal stability of ±0.01 K on the optical bench. We present a technical overview and early performance data from Australias next major spectroscopic machine.

Trans-Himalayan Trade in an Imperial Environment

This chapter explores how the Bhotiyas dealt with imperial forms of power and authority in order to realize their trade under British colonial rule in nineteenth-century Kumaon. Based on a combined analysis of oral and written historical sources I show that British imperial sovereignty, similar to that of older regimes in the area, remained malleable and contested within the wider relational field of the Bhotiyas’ trans-Himalayan trade. When the British East India Company annexed Kumaon in 1815 it was recognized as a so-called Non-regulation Province, which meant that government officials could flexibly interpret executive orders to suit the realities on the ground. Procedural simplicity and discretionary decisions created scope for both shaping and contesting British hegemony, leading to an adaptive transformation of imperial rule. Through a close examination of British interactions with Kumaon’s traders, the chapter will reveal the frictions that arose from this exceptional legal status. This focus serves to address the broader question of how sovereign claims work through multiple and shifting articulations, from frontier narratives to cartographic representations and from fluid relationships of allegiance to fixed state boundaries. The analysis considers a previous call to conceive High Asia as a continuous zone and an agentive site of political action by arguing that confluent territories and overlapping sovereignties are key to understanding trans-Himalayan trade in an imperial environment.

Tribal Identities and Scalar Politics in Postcolonial India

In this chapter I analyze how the Bhotiyas have leveraged the performativity of scale through various narratives and social practices to effectively voice their demands for tribal status in postcolonial India. The argument is divided into four empirical parts: As an entry point into the discussion, the first part introduces the narrative of Old Lady Jasuli, a widow who lived in the Darma valley during late colonial rule. The activities of the Kumaon Bhotiya Peoples’ Federation (KBPF), which was founded shortly before India’s independence in 1947, are analyzed in the second part. Its’ members re-assembled representations that the British had coined about Kumaon’s trans-Himalayan traders, especially in terms of gender relations. In so doing they articulated the Bhotiyas’ tribalness by producing Bhot (or the region inhabited by the Bhotiyas) as a scale effect in independent India. This effect of scale materialized as a ‘real’ frame of social action when state officers notified the Bhotiyas as a Scheduled Tribe (ST) in 1967. Only a few years later, as explored in the third part, a new generation of politically organized Bhotiyas started to challenge and transform these inherited scalar categorizations and arrangements, which were then perceived to reinforce rather unequal power relations within the ST community. They deployed the character of Old Lady Jasuli to convey an alternative sense of their tribal identity whilst reproducing independent India as a scale effect in Bhot. In the fourth part I then analyze how caste Hindus challenged the Bhotiyas’ tribal status when Uttarakhand was carved out as an independent state of the Indian Union in 2000. These new competitors staked the claim that the Bhotiyas’ ST-designation is a scalar expression that does not pertain to a specific people but to a region, including all its ‘native’ residents. The research contributes to emergent scholarship focusing on the ways in which identity and scale interact as categories of practice, particularly at times of macro-institutional transformations: the shift from British imperial rule to nation-state sovereignty or the creation of new states within a country’s federal system of government.