Description of work
In order to undertake a comprehensive modelling and achieve maximum understanding of each exoplanet's atmospheric structure, it is essential to have a good understanding both of the general properties of the exoplanet's host star/s and of the magnetically-driven stellar activity properties. Hence, we will assemble a coherent and uniform database/catalogue of the properties of the host stars, utilising existing multi-waveband data and derived parameters from the ESA, and other, public on-line archives. This information will be presented in a uniform manner and a form suitable for this project (e.g. WPs 5 and 6) and the general community to utilise in exoplanetary modelling. The input information will derive from several ESA missions, principally XMM-Newton, HST and Gaia, and others, e.g. Chandra X-ray Observatory, Kepler/K2, TESS and Spitzer. Currently, the information is not available in a uniform manner and one that can readily be utilised for exoplanetary modelling. Where gaps in the knowledge-base are identified, we will seek to obtain the necessary additional observations (e.g. with XMM-Newton).
Task 4.1. Host-star photosphere, interior structure and galactic context.
Lead: Inta; Participant: Cea.
As we have entered the age of massive astrophysical data, new tools are being developed to gather and analyse this wealth of information. In particular, large amounts of optical and infrared data are being collected with space and ground-based observatories; these are essential to characterize the host star and, therefore, the general properties of the detected planets. Our aim is to collect all this information for the optical-infrared range from multiple sites (journal papers and public databases) and to provide a homogeneous and user-friendly tool. Specifically, the basis and essential parameters effective temperature, luminosity, gravity and age will be derived in a homogeneous way. This will be carried out within the Virtual Observatory initiative using its protocols in order to deliver optimal interoperability (see WP6). Several distinct steps are needed:
1) Search of multi-wavelength photometry from optical to infrared. Our previously developed tool Virtual Observatory Sed Analyzer (VOSA; http://svo2.cab.inta-csic.es/svo/theory/vosa50/) will be used to search in large databases using VO protocol (2MASS, UKIDSS, WISE, VISTA, SDSS, Pan-STARRS, Gaia, or IUE). These data will be used to provide the general characteristics of the host: effective temperature, luminosity, gravity (also as an age estimate) and metallicity.
2) Search for time-resolved photometry. In particular, searching for rotational periods, stellar variability and asteroseismology. Kepler/K2 space mission, Doppler velocity instruments (SONG, http://song.phys.au.dk/) and ground-based photometry (EROS, http://eros.in2p3.fr/; or ASAS, http://www.astrouw.edu.pl/asas/?page=gallery) will be the initial stepping stones, but eventually the TESS mission, which will cover nearly the whole sky, monitoring half a million stars. Rotation –this can also be used as an age estimate (young stars show large ranges of rotation periods while older stars converge to nearly unique rotation rates for a given stellar mass) – and stellar variability are key to understand the space weather and the interaction with the planetary atmosphere. It is also very important to control that both the transit signal extraction and the radial velocity analysis are done properly. Asteroseismology is the best tool to penetrate the stellar atmosphere and to accurately determine global and internal stellar parameters (including its Mass, radius, age and internal dynamics) with a precision of about 7 per cent on mass and 3 per cent on radius (see Fig.13 in Lebreton, Goupil & Montalbàn 2014 for more details
3) Spectroscopy. Stellar spectra will be extracted from different repositories, mainly from observatory archives (Keck, ESO, Calar Alto, etc). In particular, high signal-to-noise (SNR) spectra will be used to carry out a detailed spectral synthesis and/or spectral fit in order to derive the host star properties, specifically metallicity. We will also collect from different sites high spectral resolution data, mainly low SNR spectra, in order to produce a homogeneous and complete database and the measured radial velocities (as an example, the Carnegie Institute database at home.dtm.ciw.edu/ebps/data). Moreover, once the data are released, we will collect the low-resolution spectra from Gaia and all the available information supplied directly by this European mission (metallicity, temperature and gravity). All these datasets will be ingested and analysed.
4) Kinematics. Accurate positions, proper motions and radial velocities will be collected, mainly from Gaia DR2 (expected in April 2018). Galactic trajectories will be derived and possible membership to nearby, young moving groups will be searched for. This information provides an independent age estimate. Moreover, by using the luminosity obtained from Gaia (for these stars, distances will be 500 more accurate compared with Hipparcos) it will be possible to measure the stellar radius with a precision only limited by the precision obtained in the effective temperature. Hence, a more precise value of the planet radius will be derived. When these Gaia luminosities are combined with asteroseismic observables, the correlation between the mass of the star and the initial He abundance is greatly reduced, improving the precision and the accuracy of the computed stellar models.
5) Multiplicity and the local environment. Each target will be studied to derive its multiplicity, mainly through high angular resolution imaging observations from space and ground: HST archival observations will be used for that purpose and we will be ready for new data coming from JWST. Gaia will also provide valuable data to search for astrometric binary systems. We will complement the space missions with Adaptive Optics archival data from different ground-based observatories (e.g. NACO/VLT or Altair/Gemini), or speckle imaging data (Astralux/CAHA). In a second step, we will exploit long-term radial velocity archival data to look for closer companions. Information regarding debris disk will be included by searching information in mid-IR and submm telescopes, special the European Herschel Space Observatory. The final goal is to derive and characterize binaries or multiple systems among the host stars.
6) Stellar and planetary theoretical models. State-of-the-art theoretical models for stars of different spectral types and planets, including atmospheres with different chemical composition, will be collected and collated into a homogeneous format, specially binned to proper spectral resolution for easy comparison with the data. They will be ingested into our tools and added to a public repository under VO standards (see WP6).
7) Our data gathering tools will be ready to ingest new data releases, specifically from European missions, such as subsequent Gaia DR3+, Cheops (2018), Euclid (launch in 2022), PLATO (after 2024).
Task 4.2. Host-star chromosphere, transition region, and corona from ultraviolet to X-rays.
Lead: Uleic; Participants: Univie.
Stellar radiation in the short-wavelength range comprising the ultraviolet (900-3500 A), the extreme ultraviolet (100-900 A) and soft X-ray (1-100 A) ranges is strongly influenced or determined by magnetic field dissipation in magnetically active regions on the star and higher layers of its atmosphere such as the corona. All cool stars (spectral classes F-M) of all ages on the main-sequence emit such types of radiation, but the luminosities in each range depend on the stellar rotation rate and therefore also indirectly the stellar age. On top of steady short-wavelength radiation, flares may temporarily increase the luminosity by up to factors of several. All of this radiation is crucially important as it ionizes, heats and chemically processes higher layers of planetary atmospheres (e.g., the ozone layer formed by absorption of ultraviolet light, or the thermosphere heated by EUV and X-ray photons). These mechanisms can lead to efficient mass loss from planetary upper atmospheres, determining the past and present evolution of the planet. In particular, such radiation is decisive in removing (or otherwise) massive initial hydrogen envelopes accumulated around rocky planets from the initial protoplanetary disk reservoir, and in eroding massive hydrogen atmospheres of Neptunes and Jupiters especially close to the host stars. Associated stellar winds may contribute to such erosion through non-thermal processes as well. Atmospheres of more complex composition are processed chemically through UV-to-X-ray irradiation, ionisation and heating and can be subject to losses into space as well. Of special interest is a closer understanding of water loss from planets driven by such mechanisms.
The short-wavelength radiation is dominated by optically-thin line emission plus continuum, and the spectral shape across the entire range depends on the relative importance of various magnetic structures on the star and the temperature of the hot plasma. Observationally, the X-ray and the ultraviolet ranges are well accessible with modern satellites (XMM-Newton and Chandra and, respectively, the Hubble Space Telescope), while the extreme ultraviolet is barely observable due to strong interstellar absorption of these photons (there is no satellite available at present covering a significant part of this spectral range). To characterize the full short wavelength radiation spectrum, spectral synthesis needs to be performed by fitting low-resolution spectra in X-rays or even only a few "photometric" measurements, while in the EUV range, spectral interpolation is required.
This Task will consist primarily in collecting observational spectroscopic data from the HST, Chandra, and XMM-Newton archives, analysing the data coherently, and verifying if they follow trends and correlations derived from previous samples. The XMM-Newton and Chandra archives contain large numbers of wide-field CCD observations that contain sufficient spectral information to derive characteristic plasma temperatures and therefore reconstruct the underlying synthetic line+continuum spectra for each detected source between 1 and ~100 A. The XMM-Newton mission has undertaken great efforts to automatically extract all detected archival sources and produce a source catalog with rough information on X-ray fluxes in different bands. Chandra equally offers a searchable archive and provides ready-to-use data products. As a first step, this catalog information should be collected for all X-ray-detected host stars. We will then extract archival source spectra from the CCD data of XMM-Newton and Chandra for each point source and perform spectral fits based on thermal, optically-thin coronal emission models using the publicly available XSPEC software. We will thus synthesize the complete high-resolution spectra in the 1-100 A range. In an alternative approach, we will fit template spectra to the multi-channel data published in the XMM catalog to investigate if this approach leads to satisfactory results in a much faster way.
If no XMM-Newton or Chandra data are available, we will seek information in older data archives of the ROSAT, Exosat and Einstein observatories. In addition, we will seek to obtain the necessary additional observing time (with XMM-Newton or Chandra). Equivalent procedures will then be followed as above if any new sources will be detected.
As a separate, but related, exercise outside of the Exoplanet A project, we intend eventually also to propose for simultaneous JWST/XMM or JWST/Chandra observations for a high-priority subset of the systems in order to provide direct comparison of stellar activity and exoplanet atmospheric behaviour. This will have the potential for studying exoplanet atmospheric effects due to time variations in the 'space weather' caused by stellar output (e.g., due to flares).
We will equally use the HST archive to extract ultraviolet spectra from any of the available instruments. The HST archive covers a much smaller sky area but nevertheless HST has already specifically investigated some important transiting exoplanet hosts.
No useful spectral data are available for the intermediate range of the extreme ultraviolet at 100-900 A. Task 4.3 will address this issue.
Task 4.3. Linking the host-star properties to the exoplanet environment and atmosphere.
Lead: Univie; Participants: Inta, Uleic
Output from the host star in the form of radiation, magnetic fields, and ionized winds interacts with planetary atmospheres and leads to chemical alteration, ionisation, and atmospheric erosion. While this modelling is the subject of WP5, the requisite inputs need to be computed or modelled and presented in appropriate form that can be handed over to WP5 as inputs to the subsequent modelling effort, specifically concerning radiation and winds (space weather). In particular, Task 4.3 is responsible for the following work:
1) Sections of short-wavelength spectra from the ultraviolet to the X-ray range are collected from diverse observations in Task 4.1 but may also partly be unavailable. Complete short-wavelength spectra are needed as inputs to thermo-chemical modelling of middle and upper atmospheres as well as to hydrodynamic modelling of thermospheres and exospheres for loss calculations performed in WP5. Task 4.3 is responsible to model the complete spectra coherently as far as feasible. For spectral regions missing in the data archives or inaccessible spectral ranges, computed template spectra or spectra from other observations will be used that are appropriate for the host star and that are normalized to the expected flux levels. This may involve identifying other stars of similar spectral type and activity level for which the required spectra are available. For inaccessible spectral ranges, model spectra will be developed, for example based on theoretical emission models of magnetically active regions on the Sun or other stars (e.g., Fontenla et al. 2009).
2) Stellar winds are also important for atmospheric escape/loss processes (such as charge exchange and ion escape at the wind-exosphere boundary, and possibly at the magnetospheric boundary). Such winds can presently not be observed. Some indirect methods (e.g., based on Lyα line profiles, Wood et al. 2005) exist to indirectly infer wind mass loss rates of the host star, but these are available only for a few exceptionally bright nearby stars. On the other hand, models relating rotation rate, age and therefore spin-down to the mass-loss history of the star are now available in the literature. We will therefore fit the observed rotation parameters of the host stars to available "rotation evolutionary tracks" (e.g., Johnstone et al. 2015), to constrain the range of wind models that should be applied to exoplanetary magnetospheres and atmospheres in WP5. That is, from the observational parameters collected in WP4 (coronal properties, rotation periods, etc), we will constrain wind base temperatures, wind mass loss rates and wind velocities and their allowed ranges based on available models, while the actual application of the wind models and simulations will be performed in WP5. The WP5 goal then is to not only delimit the wind properties at present but to use evolutionary models to constrain the history of the winds for total-loss estimates.
3) This Task will also set constraints on the stellar magnetic fields, their strength and possibly topology based on activity proxies such as X-ray luminosity, rotation period, spottedness (from photometric variations), or other indicators such as Ca II as far as available. Such information is important to model realistic wind expansion in WP5.
Fontenla, J.M., Curdt, W., Haberreiter, M., Harder, J., Tian, H. 2009, ApJ, 707, 842
Johnstone, C., Güdel, M., Brott, I., Lüftinger, T. 2015, A&A, 577, A28
Wood, B. E., Müller, H.-R., Zank, G. P., Linsky, J. L., Redfield, S. 2005, ApJ, 628, L143
Staff plan. Coordinator/leader of the WP will be Uleic (J.Pye), with Inta (D.Barrado) and Univie (M.Güdel) as co-coordinators/deputy-leaders, recognising the wide range of science topics and techniques covered by this WP. They will provide expert leadership of the WP's component Tasks. The required level of effort will involve staff to be employed on the project as follows: (i) a software engineer, 12 person-months, sited at Inta, to work in Task 4.1 and WP6, to upgrade existing tools and develop the needed new ones; (ii) a post-doc, 24 person-months, sited at Uleic, to work in Task 4.2, including cross-linking with activities in Tasks 4.1 and 4.3 and WP5, to perform the required data extractions, calibrations, verification of quality and uniformity, assist in identifying new observations needed, generating observing proposals, performing data analysis, and linking to the modelling efforts; (iii) a post-doc, 12 staff-months, sited at Univie, to work in Task 4.3, to perform the required modelling and generate spectral tables, liaising with Tasks 4.1 and 4.2 and WPs 5 and 6 with regard to Task-4.3 inputs and outputs. Cea (R.Garcia) will provide particular expertise in asteroseismology and internal and photospheric stellar dynamics (in Task 4.1), in conjunction with Inta. J.Nichols (Uleic) will provide expertise in analysis and understanding of HST host-star data. All staff in WP4 will liaise frequently with colleagues in WPs 5 and 6 to ensure delivery of appropriate data products for the exoplanet modelling and the dissemination efforts.
CEA is a French government-funded technological research organisation in four main areas: low-carbon energies, defense and security, information technologies and health technologies. A prominent player in the European Research Area, it is involved in setting up collaborative projects with many partners around the world.