Rømer Science Mission Plan

Institute of Physics and Astronomy, University of Aarhus Rømer Science Mission Plan Danish Small Satellite Programme Document No.(issue): MONS/IFA/MAN/PLN/0001(1) Date: 2001-05-29 Prepared by: Jørgen Christensen-Dalsgaard, IFA/TAC Authorized by: Hans Kjeldsen Classification: Public

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Rømer Science Mission Plan MONS/IFA/MAN/PLN/0001(1) 29 May 2001 Contents 1 Scope 4 2 Applicable and Reference Documents 5 2.1 Applicable documents............................ 5 3 Introduction 6 3.1 MONS Telescope............................... 6 3.2 The MONS Field Monitor.......................... 6 3.3 Star Trackers................................. 6 3.4 Operational constraints........................... 6 4 Baseline Mission 7 4.1 Overview................................... 7 4.2 Main Telescope Observations........................ 7 4.2.1 Solar-like oscillators......................... 7 4.2.2 Other Main Telescope targets.................... 9 4.3 Parallel science................................ 9 5 Ground-based support observations 10 3

4 sc mis plan 1.tex 1 Scope The present document has been prepared by the Institute of Physics and Astronomy as a contribution to the Systems Definition Phase of the Rømer satellite project within the Danish Small Satellite Programme. The document defines the observations to be carried out with the MONS (for Measuring Oscillations in Nearby Stars) instrument and the other instrumentation on the Rømer satellite.

MONS/IFA/MAN/PLN/0001(1) 5 2 Applicable and Reference Documents 2.1 Applicable documents Note: The numbering adopted here corresponds to the numbering in Rømer Documents List at the End of System Definition Phase, provided by Flemming Hansen, 15 May 2001. AD104 The Rømer Science Mission Specification MONS/IFA/MIS/RS/0001(1) AD119 Rømer Science Data Centre Requirements Specification MONS/IFA/GS/RS/0001(1) AD129 MONS Telescope Optical and Mechanical Sub-system System Definition Phase Design Report MONS/AUS/PL/RP/0001(3) AD130 Rømer Star Tracker Requirement Specifications roemer/teb/sus/rs/0012(1 Draft A) AD131 The MONS Payload Requirements Specification MONS/IFA/PL/RS/0001(2) AD145 MONS Field Monitor Requirements Specification and Parallel Science MONS/IFA/PL/RS/0002(1) AD166 MONS Field Monitor System Definition Phase Design Report MONS/AUS/PL/RP/0002(1) AD167 Requirements for the Star Tracker Parallel Science Programme MONS/IFA/PL/RS/0003(1)

6 sc mis plan 1.tex 3 Introduction A detailed description of the science goals of the missions, with an overview of the instrumentation, is provided in [AD104]. Further information about the instrumentation and its capabilities, as well as on the primary data analysis is given in [AD131], with additional information about the Field Monitor and the parallel science to be carried out with it in [AD145], and about the star tracker parallel science in [AD167]. In this introduction we summarize some key aspects, as a background for the discussion of the mission plan. 3.1 MONS Telescope The telescope is a single-reflection telescope with a mirror-diameter of 32 cm and a f-ratio of 0.9. Light is detected by a CCD camera placed in the middle of the telescope tube, behind the focus, such that the image is strongly defocussed. To suppress scattered light, light passes through a narrow field stop with a diameter of 11 arc minutes. Measurements in two colour bands (red and blue) are obtained on the CCD by placing a partitioned colour filter in the light path. The CCD is cooled to a temperature of around 90 C and the temperature is stabilized to a precision of 0.1 K. A detailed description of the telescope is provided in [AD129]. The primary observable is the ratio between blue and red intensity, as a function of time, although the individual red and blue intensities will also be available for analysis. 3.2 The MONS Field Monitor Owing to the strong defocussing of the Main Telescope, the light detected may contain contributions from faint neighbouring stars. This would be problematic if these stars were variable with substantial amplitudes, since this contribution to the signal could be mistaken for oscillations of the target star. To correct for this potential problem, the Field Monitor will survey the field near the main target. The Field Monitor consists of a small telescope, with an aperture of 50 mm, with a CCD camera a the focus. The field of view is 5 5, with the central part possibly masked to reduce the light from the very bright main target. Details on the Field Monitor can be found in [AD166]. 3.3 Star Trackers The main purpose of the Star Trackers (see [AD130]) is to determine the orientation of the satellite. Rømer will use the standard Terma Star Trackers, with a 24 mm aperture and a field of view of 22 22. However, the Star Trackers will, to the extent that the capabilities of the onboard computing allow it, also be used as science instruments. 3.4 Operational constraints The targets for MONS are bright stars, for which the most precise measurements can be made. This has the disadvantage the selection of stars is somewhat limited; at the same time it is desirable to observe as wide a range of stellar properties as possible. Thus, the mission should be organized such that stars in all part of the sky can be observed at some time during the mission. Also, to achieve adequate noise level and frequency resolution the observations should be nearly continuous and of a typical duration of

MONS/IFA/MAN/PLN/0001(1) 7 30 days for each target. This must be accomplished while minimizing effects of scattered light from the Sun, Earth and Moon. The orbit selected as baseline (the Molniya orbit) generally satisfies these constraints. However, as discussed in some detail in [AD104] light from the Earth restricts observation of some of the key objects to certain orientations of the orbit. These will be achieved during the planned two-year mission; however, as a result, detailed scheduling of the observations depends on the time of launch. 4 Baseline Mission 4.1 Overview In planning the observations, the minimum time block is 1 orbit; with the baseline Molniya orbit this provides around 10.5 hr useful observations out of each 12 hours. Thus repointing of the satellite will take place very infrequently. The baseline operations schedule of the mission is very simple: for most targets the demands of frequency resolution and signal-to-noise ratio require extended and nearly continuous observations of each target. Thus the baseline is to observe each target for at least 30 days, or 60 orbits. Selected particularly important targets may be observed for 100 orbits. The science observations will start after commissioning, which is planned to last 30 days. The total duration of the baseline mission is two years. Thus for science operations we have approximately 1440 orbits. Of these, the majority will be assigned to solar-like stars. However, around 240 orbits will be assigned to observations with the main telescope of other types of stars. The detailed allocation of time to the individual targets will depend on the time of launch, as discussed in Section 3.4; the programme will be established such that, as far as possible, the highest-priority targets are observed early in the mission (see below). 4.2 Main Telescope Observations The Main Telescope CCD will be read out with 2-second exposure time and varying degree of pixel binning, depending on the brightness of the target. The data will be analyzed in blocks of five exposures, to remove cosmic-ray hits through median filtering. The resulting data (red and blue intensities, background and information about the position of the image) correspond to a total data rate of 0.71 Mbyte/day. 4.2.1 Solar-like oscillators The target list was discussed in a workshop in Aarhus in August 2000. The outcome was a selection of high-priority targets, from which the final selection will likely be made. When assigning priorities to solar-like targets for the MONS Main Telescope, consideration must be given to the expected signal-to-noise, as well as to the desire to cover an interesting range in stellar parameters. The relevant stellar properties include mass, evolutionary stage, content of heavy elements (conventionally known as metals), and rotation rate. In some cases identification of stars with the desired property, while still observable with MONS, presents some difficulties; in particular, metal-poor stars are fairly rare and therefore tend to be rather distant and hence difficult to observe. Also, it

8 sc mis plan 1.tex Table 1: Solar-like targets for the MONS Telescope Solar- Low- Higher- Metal-poor Metal- Fast Hotter Total mass mass mass rich rotation 1. α Cen A α Cen B η Boo 4 β Hyi α CMi 2. µ Her υ And ν Ind δ Pav θ Boo 7 β Vir +1 from Group B 2b. ε Eri HD 140283 2 3. δ Eri ζ Her γ Pav ψ Cap α Tri 11 η Cas α For +1 from +3 from Group A Group B 3b. τ Cet κ Cet γ Dor 9 70 Oph χ 1 Ori +3 from Group C 36 Oph Group A: λ Ser, ι Per Group B: π 3 Ori, γ Ser, γ Lep, χ Dra, θ Per, ι Peg Group C: µ Vir, ι Leo, ξ Gem, γ Tuc, HR 4102, α Oct Note that α Cen A and B will be observed simultaneously. may happen that the star is potentially very interesting, but of uncertain observability. In such cases further studies, either from the ground or with brief observations with the MONS Main Telescope, may be required for the final selection. The selection of targets was based on an extensive list of stars of sufficient brightness to be observable with MONS and of the appropriate spectral type. These were then analysed in terms of their properties, as well as for the expected amplitude and observational noise. The result, after discussion within the PI Team, was the list of targets presented in Table 1, with stars grouped according to properties and divided according to the following priorities: 1. Very high priority. Should definitely be observed. 2. High priority. Should be observed. 2b. High priority but may not be feasible. Should be observed if feasible. 3. Excellent target. A small subset of this group will be observed 3b. Excellent target but may not be feasible. Should be added to Priority 3 list if feasible. The 2b and 3b groups contain stars which are very interesting, but which may not show solar-like oscillations because of S/N limitations, or because they are too hot. These stars could be checked out for one or two orbits to determine feasibility. The present target list is based on current information about the stars, as well as on our present understanding of the excitation of the oscillations. The stars of priority 1

MONS/IFA/MAN/PLN/0001(1) 9 will certainly be observed; apart from their intrinsic scientific interest observations from the ground and from the WIRE satellite have demonstrated that they show solar-like oscillations at an amplitude sufficiently high to enable very detailed studies. The final selection of the remaining targets will depend on further studies, including ground-based observations to evaluate the fields and the properties of the stars, as well as on modelling to investigate the extent to which the observations may be expected to provide the required information about the stellar properties. Although a definite programme for the start of the mission will evidently be established well before launch, adjustments of the later programme will be possible, in the light of the information obtained from the first observations. Evidently, information about the general properties of solarlike oscillations obtained with the MOST mission, expected to be launched well before Rømer, will also be taken into account. 4.2.2 Other Main Telescope targets To broaden the scientific results of the project, in terms of the stellar parameters covered, observing time will be allocated to a few other pulsating stars, generally of higher mass than those showing solar-like oscillations. These will include δ Scuti stars, rapidly oscillating Ap stars, and the more massive B stars. Responsibility for proposing targets within these classes of stars resides with the relevant science working groups, organized in the Mons Science Consortium (see [AD104]). For particular targets considerably shorter observing periods (although always at least one orbit) may be used. These include eclipsing binaries and planet transits; here the precise time of the relevant event can be predicted well in advance, and the duration of the event is typically less than one orbit. Such observations will be scheduled, as far as possible, between the main long-duration pointings. However, in exceptional cases it may be possible to interrupt the long pointings for a single orbit, without substantial effect on the data quality. 4.3 Parallel science The goal of the parallel science programme is to study variability in a broad range of astrophysical objects, through observations with the Field Monitor and the Star Trackers. The capabilities of the programme, in terms of sensitivity and continuity, far exceed realistic ground-based programmes for many types of targets, and hence the results will be of major interest to a broad community. Details are provided in [AD104], [AD145] and [AD167]. The objects to be observed include classes of pulsating stars showing oscillations at somewhat larger amplitudes than for the solar-like oscillators. This will allow the project to cover a broad range of stellar types, from highly evolved stars to a selection of massive stars, and hence will provide a broad base for testing the theory of stellar structure and evolution. Furthermore, we expect to be able to detect extra-solar planets, of size as small as Uranus, through the reduction in stellar luminosity when a planet transits the stellar disk; in addition we may find previously unknown objects in our own solar system, such as asteroids or comets. Also, the observations will be sensitive to supernovae in other galaxies out to quite substantial distances, thus potentially allowing rapid follow-up observations with ground-based facilities. In general the parallel-science programme will not be allowed to impact the target selection or the scheduling of the mission. An exception may be in cases where the

10 sc mis plan 1.tex presence of a particularly interesting field for parallel science may affect the choice between two otherwise similarly prioritized targets for the Main Telescope. A variety of read-out procedures will be used for the Star Trackers and Field Monitor. Pulsating stars generally require a relatively rapid cadence; these will be observed through windowed read-out, including, of course, the stars near the Main Telescope target to correct for their possible variations. To study slower variations in a large number of stars, including effects of planet transits and supernovae explosions, complete images, suitably binned, will be accumulated on board and transmitted at regular intervals, at least once per orbit. These images will then be analysed on ground (see [AD119]). The precise scheduling of these observations depends critically on the available on-board computing and memory capacity and will be established later. 5 Ground-based support observations Optimal use of the data from MONS requires as complete additional data on the target stars as possible. This includes determination of the stellar surface temperature and luminosity and, if possible, stellar mass, as well as detailed information on the stellar surface composition. Only if such data are available can the frequencies observed with MONS be used to provide detailed constraints on the properties of the stellar interior. To obtain this information about the target stars, classical observations with ground-based telescopes (photometry, spectroscopy), combined with detailed analyses of the results, are required. Even though the target stars are amongst the brightest in the sky, existing data are not adequate, and hence further observations must be carried out. Similar studies may be performed for selected targets for the parallel science programme. Ground-based observations are also required to investigate the fields of the main-telescope targets, e.g. to test for nearby stars that might disturb the MONS measurements. Results of these observations are evidently required before the launch of Rømer, and hence a programme to carry them out has already been initiated, organized by Chris Sterken, Brussels, who heads the MONS Ground-Support Observations Working Group. For larger-amplitude pulsators, particularly those that will be studied in the parallelscience programme, ground-based data can be used to supplement the observations from Rømer. For example, contemporaneous radial-velocity measurements may help identifying the geometry of the modes observed, while the frequency resolution can be improved by extending the Rømer data with ground-based observations before and after the space observations. Finally, procedures must be established to ensure rapid follow-up on detections with Rømer of supernovae explosions; similarly, there is no doubt that detection from Rømer of extra-solar planets or solar-system objects will be followed by extensive ground-based observing programmes.