APPLICATION FOR OBSERVING TIME

Transcription

1 Time Allocation Committee for MPG time at the ESO 2.2m-telescope c/o MPI für Astronomie Königstuhl 17 D Heidelberg / Germany Application No. Observing period Apr-Sep 2014 Received APPLICATION FOR OBSERVING TIME from X MPIA MPG institute other 1. Telescope: 2.2-m X 2.1 Applicant Dr Wolfgang Brandner MPIA Name Institute Königstuhl Heidelberg street ZIP code - city brandner@mpia.de ESO User Portal username 2.2 Collaborators M. Janson, C. Bergfors, J. Schlieder Queen s Univ. Belfast, UCL, MPIA name(s) institute(s) M. Bonnefoy, Th. Henning IPAG, MPIA name(s) institute(s) 2.3 Observers Carolina Bergfors, Wolfgang Brandner Markus Janson name name By specifying the names under item 2.3 it is obligatory to also send out these observers to La Silla, if required. Correspondence on the rating of this application will be sent to the applicant (P.I.) as quoted under 2.1 above. 3. Observing programme: Category: E Title Abstract : : Dynamical masses of young M-dwarf binaries identified by AstraLux Precise age dating of stars is very important for many purposes, but also often very difficult. Among the best methods is to identify stars as members of young moving groups (YMGs), i.e., unbound associations of stars dynamically tracing back to a common origin. Here, we propose to observe 32 M-dwarfs, which are members of YMGs, and which have been identified as close, relatively short period binaries by our AstraLux Lucky Imaging surveys. Using FEROS to determine radial velocities at multiple epochs, we will be able to constrain orbital parameters, and hence derive dynamical masses for the stellar components, which in turn can be used for isochronal analysis. The results will be highly useful for precise age dating of YMGs, and for testing for age spreads among their member stars. 4. Instrument: WFI X FEROS GROND 5. Brightness range of objects to be observed: from 8 to 12 i-mag 6. Number of hours: applied for already awarded still needed 27 none 53 no restriction grey dark 7. Optimum date range for the observations: Usable range in local sideral time LST:... 0:00h 24:00h

2 8a. Description of the observing programme Astrophysical context Determining the ages of stars is of fundamental importance to many areas of astronomical research, but it is also a notoriously difficult property to measure accurately. One particular area highlighting this issue in recent years is direct imaging of exoplanets, where the inferred properties of the planet depend strongly on the age of the system, leading to large uncertainties in many cases where the age is poorly known. One promising solution lies in young moving groups (YMGs): Young ( 100 Myr or less) stars often retain a memory of the dynamics of their birth cluster. Thus, groups of young stars can be identified through their clustering in phase space, and a robust age can be inferred from statistical age dating methods applied to the whole group. This has been used to infer the ages of most directly imaged planets or planet-like objects so far [1, 2, 3, 4, 5]. However, uncertainties remain with this method. Different age determinations of YMGs often vary by a factor 2 [6, 7], and it also been debated to which extent stars can truly be inferred to be coeval from their shared dynamics [8, 9]. Immediate aim Here, we propose to address these issues through isochronal analysis of M-dwarf binaries. In contrast to more massive stars, M-dwarfs remain in their pre-main sequence evolutionary phase for more than 100 Myr, and thus continue to evolve in a mass-luminosity diagram throughout the YMG phase. Hence, M-dwarf binaries where both the dynamical mass and the luminosity can be determined to good precision consitute excellent clocks at this stage of evolution. We have identified a sample of 30 M-dwarf binaries that are members of YMGs, and have orbital periods ranging from 1 30 years, and are monitoring them astrometrically with high-resolution imaging. We also wish to monitor them with radial velocity, which is the purpose of this proposal. Radial velocity adds to the orbital/isochronal analysis in several ways: 1) By giving extra information about the motion of the stars outside the plane of the sky, the RV data in combination with high-resolution imaging provide much stronger constraints on the mutual orbital parameters (e.g. period, eccentricity, angle of periapsis) than would be possible with either method in isolation. 2) The RV provides information about the mass ratio in the system, whereas relative astrometry alone is limited to providing the total system mass. 3) RV observations will efficiently detect any additional close third components in the system that are unresolved in the images. This is of critical importance for the isochronal analysis, as an unresolved pair thought to be a single star will lead to an incorrect isochronal age. With the orbital parameters determined to within a few percent, it will be possible to determine relative individual ages by more than an order of magnitude better precision that has been previously possible. This immediately gives information about issues such as coevality within YMGs, and the relative ages of different YMGs, entirely independent of any isochronal models. Furthermore, by calibrating the isochronal models against the YMGs with best determined absolute ages, equally good absolute ages can be determined for the full sample of YMGs as well. Previous work This work builds on our M-star multiplicity survey using Lucky Imaging [100, 101, 102, 103]. Covering 700 M-dwarfs the AstraLux survey is the largest binary survey among low mass stars carried out to date. By focusing on multiplicity of young M-dwarfs in general, it yielded enough multiples in young moving groups for a coherent focused examination. We also have experience in using FEROS for RV studies of M-dwarf multiplicity from an observing campaign in 2011 and 2012 (Bergfors et al., in prep.), hence we have all the necessary expertise in hand for handling the data. Layout of observations We will structure the observations in much the same way as our previous RV survey. The targets in our list are foreseen to be observed 1-2 times per observing period over the next three periods, yielding in total 5 RV points per star with individual measurement precisions of 200 m/s. Two observing runs will be scheduled per observing period with a baseline of several months between them, and the targets out of our list that are in the visibility window for each run will be observed at that time. Strategic importance for MPIA The program reinforces the strong results (more than 80 citiations already to the results from the M-dwarf binary survey, and 60 citations to our companion surveys of exoplanet hosts) that have been and are continually being acquired for stellar multiplicity with the AstraLux cameras, both of which were developped and built at MPIA. Furthermore, the research area is of fundamental importance to direct imaging studies of planets and brown dwarfs, which will greatly expand in scope over the coming years with the advent of Extreme AO facilities (including SPHERE), as well as many other research topics involving stellar age determinations. It can therefore be expected to generate a significant scientific impact in stellar and exo-planetary astronomy. 2

3 8b. Figures and tables Figure 1: AstraLux i -band image of the target binary J , which has been astrometrically monitored over a long time baseline (Bonnefoy et al., in prep.) Figure 2: Example of astrometric data and a corresponding preliminary orbit fit and predictions for the future, in this case for the target binary J Figure 3: Example of FEROS monitoring of the M- dwarf J The RV variations suggest a close spectroscopic binary (uncertainties of the individual RV measurements are less than 0.2 km.s). Overplotted by a dashed curve is only possible orbital solution, though clearly at least two additional RV points are required to constrain the orbit (Bergfors et al., in prep.). 3

4 9. Objects to be observed (Objects to be observed with high priority should be marked in last column) Designation α (2000) δ (2000) magnitude in spectral range to be observed priority 2MASS J h 11 m 25 ṣ J= MASS J h 49 m 02 ṣ J= MASS J h 32 m 35 ṣ J= MASS J h 24 m 42 ṣ J= MASS J h 37 m 37 ṣ J= MASS J h 59 m 58 ṣ J= MASS J h 28 m 44 ṣ J= MASS J h 30 m 18 ṣ J= MASS J h 32 m 04 ṣ J= MASS J h 11 m 30 ṣ J= MASS J h 13 m 45 ṣ J= MASS J h 16 m 10 ṣ J= MASS J A 06 h 43 m 45 ṣ J= MASS J h 28 m 51 ṣ J= MASS J h 03 m 20 ṣ J= MASS J A 08 h 22 m 47 ṣ J= MASS J h 47 m 56 ṣ J= MASS J h 14 m 08 ṣ J= MASS J h 17 m 26 ṣ J= MASS J h 22 m 05 ṣ J= MASS J h 31 m 55 ṣ J= MASS J h 07 m 27 ṣ J= MASS J h 20 m 21 ṣ J= MASS J h 49 m 33 ṣ J= MASS J h 57 m 34 ṣ J= MASS J h 03 m 57 ṣ J= MASS J h 17 m 31 ṣ J= MASS J h 16 m 33 ṣ J= MASS J h 53 m 14 ṣ J= MASS J h 17 m 28 ṣ J= MASS J h 20 m 57 ṣ J= MASS J h 49 m 53 ṣ J=

5 10. Justification of the amount of observing time requested: Our time requirement is based on our past experience of using FEROS for acquiring radial velocities of M-type stars in this brightness range. In the previous program led by Carolina Bergfors (086.A-9007(A) and 089.A-9013(A)), we found that an average of 30 min including overheads is required for acquiring one radial velocity data point with a precision of 200 m/s for an i = 11 mag target. Acquiring 5 radial velocity points for each of the 32 binaries in our sample thus requires a total of 80h, distributed over 3 observing periods. FEROS is particularly useful for this purpose, since the data can be homogenously combined with previous data for those targets that overlap with our previous program. We note here that while there is a small target overlap (10%) with the previous program, the science case for this study is different from the past one, hence we consider this as a stand-alone program rather than a continuation of previous efforts. 11. Constraints for scheduling observations for this application: The observations should be performed in two runs, with one scheduled toward the beginning of the semester, and the other toward the end. 12. Observational experience of observer(s) named under 2.3: (at least one observer must have sufficient experience) We have extensive experience with all aspects of this program, given our previous use of FEROS for radial velocity of M-stars, as well as our experience with M-dwarf multiplicity and stellar evolution in general. All of the listed observers have significant observational experience, with facilities such as the ESO 2.2m, NTT and the Calar Alto 2.2m telescope as well as many others. 13. Observing runs at the ESO 2.2m-telscope (preferably during the last 3 years) and publications resulting from these Telescope instrument date hours success rate publications 2.2m WFI P % all data pre-red., astrometric analysis on-going 5

6 14. References for items 8 and 13: [1] Chauvin, G. et al. 2005, A&A 438, L25 [2] Marois et al. 2010, Nature 468, 1080 [3] Lagrange, A.-M. et al. 2010, Science 329, 57 [4] Rameau, J. et al. 2013, ApJ 776, L17 [5] Carson, J.C. et al. 2013, 736, L32 [6] Close, L. et al. 2005, Nature 433, 286 [7] Luhman, K. et al. 2005, 628, L69 [8] Hinz, P.M. et al. 2010, ApJ 716, 417 [9] Malo, L. et al. 2013, ApJ 762, 88 [100] Hormuth, F. et al. 2007, A&A 463, 707 [101] Janson, M. et al. 2007, A&A 462, 615 [102] Bergfors, C. et al. 2010, A&A 520, 54 [103] Janson, M. et al. 2012, ApJ 754, 44 6

7 Tolerance limits for planned observations: maximum seeing: 2.0 minimum transparency: 50% maximum airmass: 2.5 photometric conditions: no moon: max. phase / : 1/30 min. / max. lag: / nights