Title Evolution of Magnetic Activity in Main Sequence Stars Pi M. Guedel Time 30 hrs 13.2.1: Single DRSP Evolution of Magnetic Activity in Main Sequence Stars ===================================================== M. Guedel et al. 1. Name of program and authors Chromospheres and transition regions of magnetically active stars emit optically thick thermal bremsstrahlung that is intrinsically weak but is of major importance to estimate the magnetic area. The coronal contribution is rather unimportant, reaching a brightness temperature of perhaps 1000 K at 80-90 GHz but less than 100 K outside active regions (White & Kundu 1992, Sol. Phys. 141, 347). The optically thick layer is reached at temperatures of ~7000 K at 3 mm. If density enhancements are present, then the optically thick layer may shift to larger heights, involving higher T up to several 10^4 K. Enhanced temperatures are found in magnetic active regions (W&S 1992). At 1 mm wavelength, the temperatures at the optically thick level may be significantly smaller, down to below 6000 K for solar chromospheric models. Although stellar surfaces will not be resolved, the integrated brightness will offer important information about the surface coverage with dense (active) chromospheric regions. The proposed observations will investigate trends for three stellar samples: a) Solar analogs back in time to the zero-age main sequence. As the magnetic activity (spot coverage, X-ray emission) largely increases toward younger stars, we expect a fundamental change in the magnetic structure of the chromosphere, providing information on the base magnetic fields and their distribution. A sample of about 5-10 nearby stars of ~1 solar mass with well known characteristics (rotation period, activity indicators, fundamental properties such as Teff and radius) will be observed at one or two millimetric wavelengths. Such observations complement information available from chromospheric/transition region ultraviolet studies. A long-term extension (~10 yrs) could look for magnetic cycles in the chromospheric emission by getting a snapshot every half year. b) M dwarfs. A similar question applies to very cool stars. Their dynamo may be fundamentally different. Polar active regions may be involved. A distributed (alpha^2) dynamo may be at work, leading to chromospheres at variance with the solar example. It is suggested to compare a series of very close M dwarfs, cutting through the parameter space both in mass (from M0 to possibly nearby brown dwarfs) and activity (comparing a couple of active dMe stars and inactive dM stars). c) L, T Dwarfs: Similar study, to investigate chromospheric structure of the lowest-mass stars. Should observe two classes: old, nearby BDs, and a few young examples in star-forming regions (eg, TMC, rho Oph). 3. Number of sources: ~ 10 nearby G dwarfs (solar analogs) ~ 5-10 nearby M dwarfs ~ 5 old BDs, 5 young BDs 4. Coordinates: G stars: 02 05 +77 16 (47 Cas) 14 39 +64 17 (EK Dra) 05 54 +20 16 (chi1 Ori) 08 39 +65 01 (pi1 UMa) 03 19 +03 22 (kappa1 Cet) 13 11 +27 52 (beta Com) 20 04 +17 04 (15 Sge) 14 39 -60 50 (alpha Cen A+B) 00 25 -77 15 (beta Hyi) M dwarfs 20 45 -31 20 (AU Mic) 07 34 +31 52 (YY Gem) 10 19 +19 52 (AD Leo) 01 39 -17 57 (UV Cet) inactive dM TBD BDs: various coordinates (e.g., eps Ind, and other nearby ones) 4.2. Moving target: yes/no (e.g. comet, planet, ...) no 4.3. Time critical: yes/no (e.g. SN, GRB, ...) no 5. Spatial scales: 5.1. Angular resolution (arcsec): point 5.2. Range of spatial scales/FOV (arcsec): (optional: indicate whether single-field, small mosaic, wide-field mosaic...) N/A 5.3. Single dish total power data: yes/no no 5.4. ACA: yes/no no 5.5. Subarrays: yes/no no 6. Frequencies: 6.1. Receiver band: Band 3, 6, 7, or 9 <250 GHz 6.2. Lines and Frequencies (GHz): (continuum) 6.3. Spectral resolution (km/s): N/A 6.4. Bandwidth or spectral coverage (km/s or GHz): N/A 7. Continuum flux density: 7.1. Typical value (Jy): (take average value of set of objects) (optional: provide range of fluxes for set of objects) 30-100 uJy for G and M dwarfs (5-10 pc) at 100 GHz 300-1000 uJy for G and M dwarfs (5-10 pc) at 300 GHz 7.2. Required continuum rms (Jy or K): 5-20 uJy 7.3. Dynamic range within image: (from 7.1 and 7.2, but also indicate whether e.g. weak objects next to bright objects) N/A; point sources, no nearby weaker objects expected 8. Line intensity: 8.1. Typical value (K or Jy): (take average value of set of objects) (optional: provide range of values for set of objects) 8.2. Required rms per channel (K or Jy): 8.3. Spectral dynamic range: 9. Polarization: yes/no (optional) 9.1. Required Stokes I, V 9.2. Total polarized flux density (Jy) unknown but probably very weak 9.3. Required polarization rms and/or dynamic range 5% 9.4. Polarization fidelity 10. Integration time for each observing mode/receiver setting (hr): 3 hrs per target 11. Total integration time for program (hr): 30 hr 12. Comments on observing strategy (e.g. line surveys, Target of Opportunity, Sun, ...): (optional) ************************************************************************** Review Leonardo Testi: It is hard to figure out something here, as all the technical part is TBD. I could not figure out whether line emission is interesting or not. In the most optimistic view this is a full polarization Band 3 continuum project. Assuming a 10 uJy rms for all targets, then the total time required is 1hr/object or 30 hrs total. Comment Ewine: assumed 30 hr for program Reply Guedel: The integration times changed a little bit but are better justified. The emission is all continuum. -------------------------------------------------- Review v2.0: 3.2.1: Single DRSP Evolution of Magnetic Activity in Main Sequence Stars ===================================================== M. Guedel et al. DRSP 2.0 Review Leonardo Testi: I am still unsure whether 2 frequencies (e.g. bands 3/6 or 3/4 or 4/6) are required to determine spectral indices and, in this case, whether measurements should be close (how close) in time to be useful. Answer: Two frequencies should be the minimum requirement. A spectral index can thus be obtained to check for consistency with the optically thick model. Measurements at two (or more) freqencies should be obtained within max. 1 day for the rapid rotators and up to a few days for slow rotators, to ensure that the same active regions are in view. Best would we observations closer in time.