Observing Application

Date : Aug, 01 2012 Observing Application Proposal ID : VLA/13A-471 Legacy ID : AH1104 PI : Gregg Hallinan Type : Regular Category : Solar System, Stars, Planetary Systems Total Time : 36.0

The Thermal Atmospheres of Nearby Solar-type Stars

Abstract: We request 36 hours of shared risk observing with the JVLA to observe 5 nearby solar type stars (F8V - K2V) with levels of magnetic activity expected to be similar to that of the Sun. The goal of our survey is the detection of quiescent gyroresonance emission from above active regions at the base of the coronae. Any detection would constitute the first radio detection of a star of similar magnetic activity levels to the Sun, and provide the first insight into coronal field strengths and plasma temperatures for other solar-type stars. The total time request is 36 hours.

Authors: Name Institution Email Status Gregg Hallinan California Institute of [email protected] Graduating: N/A Technology Thesis: false Jackie Villadsen California Institute of [email protected] Graduating: N/A Technology Thesis: false Stephen Bourke California Institute of [email protected] Technology

Principal Investigator: Gregg Hallinan Contact: Gregg Hallinan Telephone: 5104092840 Email: [email protected] Related proposals:

Joint: Not a Joint Proposal

Observing type(s): Continuum, Single Pointing(s)

VLA Resources Name Conf. Frontend & Backend Setup Ku Band Any Ku Band 2 cm 12000 - 18000 MHz Catalog ID: 12542 Shared Risk Observing ResourceName: 12581 Resource ID: Ku band

Name Conf. Frontend & Backend Setup X band Any X Band 3.6 cm 8000 - 12000 MHz Catalog ID: 12542 Shared Risk Observing ResourceName: X band Resource ID: 12543

Testing Resource Images Sources: Name Position Velocity Group Coordinate System Equatorial Convention Radio Equinox J2000 03:32:55.84 epsilon Eridani Right Ascension Ref. Frame LSRK EpsEri 00:00:00.0 -9:27:29.7 Declination Velocity 0.00 00:00:00.0 Coordinate System Equatorial Convention Radio Equinox J2000 01:44:04.8 tau Ceti Right Ascension Ref. Frame LSRK Tau Ceti 00:00:00.0 -15:56:15.0 Declination Velocity 0.00 00:00:00.0 Coordinate System Equatorial Convention Radio Equinox J2000 04:15:16.32 40 Eridani Right Ascension Ref. Frame LSRK 40Eri 00:00:00.0 -7:39:10.3 Declination Velocity 0.00 00:00:00.0 Coordinate System Equatorial Convention Radio Equinox J2000 00:49:06.29 eta Cassiopeiae Right Ascension Ref. Frame LSRK EtaCass 00:00:00.0 +57:48:54.67 Declination Velocity 0.00 00:00:00.0 Coordinate System Equatorial Convention Radio Equinox J2000 03:19:55.6505 82 Eridani Right Ascension Ref. Frame LSRK 82Eri 00:00:00.0 -43:04:11.221 Declination Velocity 0.00 00:00:00.0 Coordinate System Equatorial Convention Radio Equinox J2000 00:00:00.0 OneSource Right Ascension Ref. Frame LSRK FollowUp 00:00:00.0 +00:00:00.0 Declination Velocity 0.00 00:00:00.0

Sessions: Name Session Time Repeat Separation LST minimum LST maximum Elevation (hours) Minimum EpsEri 3.00 1 0 day 00:00:00 24:00:00 0 EpsEri 3.00 1 0 day 00:00:00 24:00:00 0 TauCet 3.00 1 0 day 00:00:00 24:00:00 0 TauCet 3.00 1 0 day 00:00:00 24:00:00 0 40Eri 3.00 1 0 day 00:00:00 24:00:00 0 40Eri 3.00 1 0 day 00:00:00 24:00:00 0 EtaCass 3.00 1 0 day 00:00:00 24:00:00 0 EtaCass 3.00 1 0 day 00:00:00 24:00:00 0 2

Name Session Time Repeat Separation LST minimum LST maximum Elevation (hours) Minimum 82Eri 3.00 1 0 day 00:00:00 24:00:00 0 82Eri 3.00 1 0 day 00:00:00 24:00:00 0 Follow-up 3.00 1 0 day 00:00:00 24:00:00 0 Follow-up 3.00 1 0 day 00:00:00 24:00:00 0

Session Constraints: Name Constraints Comments

Session Source/Resource Pairs: Session Name Source Resource Time Figure of Merit Subarray EpsEri epsilon Eridani Ku Band 3.0 hour 0.003 mJy/bm

EpsEri epsilon Eridani X band 3.0 hour 0.003 mJy/bm

TauCet tau Ceti X band 3.0 hour 0.003 mJy/bm

TauCet epsilon Eridani Ku Band 3.0 hour 0.003 mJy/bm

40Eri 40 Eridani X band 3.0 hour 0.003 mJy/bm

40Eri epsilon Eridani Ku Band 3.0 hour 0.003 mJy/bm

EtaCass eta Cassiopeiae X band 3.0 hour 0.003 mJy/bm

EtaCass eta Cassiopeiae Ku Band 3.0 hour 0.003 mJy/bm

82Eri 82 Eridani X band 3.0 hour 0.003 mJy/bm

Follow-up OneSource Ku Band 3.0 hour 0.003 mJy/bm

Present for observation: yes Staff support: None Plan of Dissertation: no

3 Detecting the Thermal Atmospheres of Nearby Solar-Type Stars

Abstract

We request 36 hours of shared risk observing with the JVLA to observe 5 nearby solar type stars (F8V - K2V) with levels of magnetic activity expected to be similar to that of the Sun. The goal of our survey is the detection of quiescent gyroresonance emission from above active regions at the base of the coronae. Any detection would constitute the first radio detection of a star of similar magnetic activity levels to the Sun, and provide the first insight into coronal field strengths and plasma temperatures for other solar-type stars. The total time request is 36 hours.

Introduction The detection of quiescent radio emission from the coronae of a wide range of main sequence dwarf stars was one of the major breakthroughs with the advent of the VLA and completely revolutionized the ﬁeld of stellar radio astronomy (Gary & Linsky 1981ApJ...250..284G). The implied luminosities and hence brightness temperatures associated with this quiescent emission conﬁrmed that such stars have non-thermal coronae, where large populations of electrons are continuously accelerated to high energies, a totally unexpected result as there is simply no solar counterpart. Quiescent microwave radio emission detected from the sun is typically due to thermal bremsstrahlung emission from the chromosphere (typically at ν < 3 GHz) and gyroresonance (cyclotron) emission above active regions (best observed from ν = 3 − 20 GHz), with a luminosity ∼ 1011 erg s−1 Hz−1. The much brighter quiescent radio emission from nearby active dwarf stars, on the other hand, is thought to be due to gyrosynchrotron emission from the population of continuously accelerated electrons that constitute the non-thermal corona. Invariably such radio detected dwarf stars are also bright x-ray sources and rapid rotators, and thus represent extreme levels of magnetic activity. To date, no star with magnetic activity levels similar to the Sun has ever been detected at radio frequencies.

The EVLA: A New Era in Stellar Radio Astronomy The unprecedented sensitivity and spectral resolution of the EVLA opens entirely new avenues into the study of the radio emission from main sequence stars. Whereas the VLA was only sensitive to the most luminous, and hence active, members of the nearby stellar population, the EVLA can probe entire populations of nearby stars and investigate radio luminosity as a function of parameters such as age, mass and rotation rate as well as enabling correlation with other activity tracers such as X-ray luminosity. In particular, for the first time, nearby solar analogs (spectral type F8V-K2V) of similar age, mass and magnetic activity level to the Sun, can be detected producing quiescent radio emission of the same nature as that detected from the Sun. Figure 1 shows the flux of the Sun at various bands of the EVLA at a distance of 1.3 pc. The thermal blackbody photosphere of the Sun would be detectable at the higher bands of K and Ka out to a distance of 6 pc in a single 4 hour observation. However, although the detection of the photosphere of a solar-type star would be a landmark achievement for the EVLA, a result of higher scientific merit would be the detection of thermal bremmstrahlung (free-free) emission from the stellar chromosphere and gyroresonance (cyclotron) emission above active regions at the base of stellar coronae. In particular, we will focus on the latter mechanism which can potentially provide excellent diagnostic information on the magnetic field strengths, filling factors and plasma temperatures at the base of stellar coronae.

1 Furthermore, monitoring such emission can reveal rotational modulation as compact active regions rotate in and out of view on the stellar disk and longer term monitoring can reveal the presence of magnetic cycles, similar to the well characterized solar cycle.

The Diagnostic Power of Gyroresonance Emission from Stellar Active Regions Gyroresonance emission produced above active regions at the base of the solar corona is the most powerful diagnostic tool used in the study of the radio Sun. Similar diagnostic potential is available for stellar coronae with the advent of the EVLA. Gyroresonance emission is produced through reso- nance between electromagnetic waves and electrons spiraling in magnetic fields at the local electron 6 cyclotron frequency, νc = 2.8 × 10 B Hz, where B is measured in Gauss. The radio emission is produced at low harmonics (s=1,2,3,4) with the opacity in a given electromagnetic mode of the plasma decreasing by two orders of magnitude for each increase in harmonic number. For solar coronal conditions, the x mode radiation is typically optically thick for the gyroresonant layer corresponding to s=3, whereas the o mode is typically optically thick for the gyroresonant layer s=2. Therefore, radio observations of active regions in the solar corona at a given frequency effectively reveal a thin isosurface of uniform magnetic field strength such that s=3 for emission with circular polarization corresponding to the x mode and s=2 for emission with circular polarization corresponding to the o mode. Multi-frequency observations can thus produce true 3-D magnetograms near the base of the corona above active regions. Furthermore, optically thick gyroresonance emission is detected with a brightness temperature corresponding to the actual temperature of the thermal population of electrons responsible for the emission. Together with constraints on source size, such emission reveals the temperature of plasma in the lower corona. With sufficient sensitivity similar diagnostics can be achieved for stellar coronae, although we forgo the spatial resolution available for the Sun. Indeed, the detection of such radio emission is a much more useful diagnostic than the detection of non-thermal gyrosynchrotron radiation, the dominant source of radio emission in observations of cool stars thus far. Figure 1 shows a recent EVLA observation of the flare star UV Ceti which highlights the new potential of the EVLA to probe stellar coronae. The detection of gyroresonance emission at higher frequencies confirms magnetic fields in the lower corona of strength ∼ 4.5 kilogauss for UV Ceti, assuming emission detected at the gyresonant layer s=3. We note that emission at higher harmonics may be possible for UV Ceti due to higher plasma temperatures. The brightness temperature of this thermal emission is 5 − 7.5 × 107K, even assuming a source the size of the stellar disk. Consequently, we can infer a super-heated 75MK plasma trapped in kilogauss magnetic fields with a source size equivalent to the entire stellar disk.

Proposed Observations We propose to carry out a JVLA observations consisting of a volume limited survey of the nearest evolved solar type stars (F8V - K2V) with levels of magnetic activity expected to be similar to that of the Sun. The goal of our survey is the detection of quiescent gyroresonance emission from active regions at the base of the coronae. We limit our survey to 5 objects (Table 1) out to a distance of 6 pc, the maximum distance at which we can expect to detect emissions of a similar nature to those detected from Sun. For each object, we request 3 hours at each of C band and Ku band. Assuming cyclotron emission produced at harmonics of the electron gyrofrequency, s ≤ 4, this will allow us to probe magnetic ﬁeld strengths of ∼ 350 − 3200 Gauss with our initial sample. We also request a further 3 hours at each of K and Ka band following data reduction of the C and Ku band data for follow up observations of the most promising candidate detected in these bands. This will

2 Table 1. Selected Sample of Solar Type Stars

Name Spectral Type Distance (pc) V MAG) B-V Mag

Epsilon Eridani K2 V 3.2 6.18 0.881 Tau Ceti G8.5V 3.65 5.68 0.727 40 Eridani K0.5V 5 5.92 0.820 Eta Cassiopeiae F9V 6 4.59 0.574 82 Eridani G8V 6 5.35 0.708 allow us to fully characterize the spectral behavior of the detected gyroresonance emission, as well as detect the photosphere of the star. The later will be particularly useful in determining the true ﬂux levels of the detected gyroresonance emission.

Technical Justification We request 3 hours at X band and 3 hours at Ku band on each of our 5 targets. We also request follow-up observations of 3 hours at each of K and Ka band for the most promising candidate detection, following reduction of our X band and Ku band observations. Our total time request is 36 hours. Considering our sources are unresolved for all bands and configurations, we can be scheduled in any configuration or reconfiguration time. We expect a maximum data rate of 51.76 GB/h with a total data set size of < 2TB. For our 3 hour block observations, we anticipate overheads of 15% at X band, 25% at Ku band and 30% at each of K and Ka band, taking into account gain calibration, flux density bootstrapping and reference pointng for the higher frequency bands. The resulting RMS noise for our 3 hour block observations are as follows - X band: 2.5 µJy, Ku Band: 2.5 µJy, K band: 5 µJy, Ka band: 5 µJy. The maximum data rate for our observations is 51.76 GB/h with a total data set size < 2.0 TB.

Shared Risk Observing The use of the 3-bit samplers at X and Ku band requires Shard Risk status for our project. Our one hour of test time will be spent in characterizing the RFI environment across the entirety of each band, with particular emphasis on the optimum placement of spectral windows to minimize impact on clean parts of the spectrum. Hallinan and Bourke will be available to visit Socorro in the event of technical issues with our proposal. No support is required in this event, bar accommodation in the NRAO Guest House.

3 Figure 1 The quiet Sun as a star at 1.3 pc, the distance of Alpha Centauri. Luminosities for solar maximum and minimum are shown.Also shown is the RMS sensitivity of the JVLA for the requested 3 hour observations. Below 3 GHz, the emission is predominantly chromospheric bremmstrahlung. From 3-20GHz, the emission is dominated by thermal gyroresonance, while the highest frequencies correspond to the solar photosphere. Adapted from White (2002AN....323..265W)