FK Comae is an ultra-fast rotating, single yellow giant, product of a recent W UMa merger. Extraordinary levels of FUV and X-ray emission rate FK Comae a coronal powerhouse on par with the most extreme of the better known activity heavyweights: short-period RS CVn binaries. As a single star, FK Comae has clear advantages as a laboratory for exploring the outer limits of magnetospheric activity among the coronal cool stars. FK Comae has a long history of attention at optical and X-ray wavelengths, thanks to its generously spotted surface, and proclivity to flare regularly at high energies. FUSE discovered ultra-broad, redshifted profiles of OVI and CIII, but unfortunately the singular observation could not be repeated, thanks to the satellite's flaky attitude system. The remarkable FUV spectrum was taken just a few months before STIS failed in 2004, so there was no opportunity to turn the more powerful gaze of Hubble to the task. Now, finally, the amazing sensitivity of Cosmic Origins Spectrograph can be brought to bear: a single orbit can capture an FUV spectrum of FK Comae with S/N at instrumental limits for bright lines, and digging down to faint FeXXI 1354 (bridge to the coordinated Chandra HETGS pointing we are carrying out). We will trace how the bright FUV regions relate spatially to the photospheric dark spots, to inform ideas of coronal structure and heating in these advanced objects. We will probe whether a global magnetosphere exists, and whether the field lines are loaded with hot coronal gas (>10 MK), as well as the cooler 0.3 MK material already suggested by highly broadened FUSE OVI. Further, we will test whether the striking 100 km/s redshifts of the FUV lines, and similar shifts seen in NeX by Chandra HETGS, are caused by persistent coronal flows (outflows, perhaps implicated in magnetic braking; or inflows, like "coronal rain" on the Sun). Our method is to exploit, on the one hand, emission-line "Doppler imaging," whereby bright surface regions are mapped onto specific locations in the global profile, according to the line-of-sight rotational velocity. On the other hand, we compare features of different opacity and excitation (e.g., SiIII 1206, SiIII 1892, and SiIV 1393) to deduce whether, say, a red asymmetry is caused by blueshifted absorption, or alternatively by infall of the entire feature. Multiple epochs spaced over two rotation periods break the degeneracy between profile distortions caused by disk passage of hot patches (Doppler imaging part), and those caused by large-scale gas kinematics. Contemporaneous starspot maps from the ground will provide a fundamental magnetic context for the coordinated FUV and X-ray campaigns.

Overview ------------- This the HST part of a joint Chandra-HST program approved by the Chandra peer review to obtain simultaneous FUV and X-ray spectra of FK Comae, member of an eponymous class of unusual yellow giants. A second piece of the project was earlier approved by the HST TAC (GO-12279) and also has a joint Chandra component. Following the HST peer review, and with knowledge of the outcome, the subsequent Chandra TAC added 240 ks of HETGS time and 11 more COS orbits to the 120 ks and 11 orbits awarded initially by HST. The combination of the two joint programs will allow a more comprehensive and powerful observational approach than was anticipated in the original HST-Chandra "King of Spin" project (GO-12279), as described in more detail below. The observational objective of the combined campaign is to carry out time-domain spectroscopy of the fast-rotating, hyperactive, heavily "spotted" yellow giant FK Comae. We focus on small segments of the NUV and FUV regions where nevertheless numerous important diagnostic emission lines fall. The GO-12279 program calls for eleven COS single-orbit observations spaced roughly evenly over two spin periods (about 5 days total), the minimum necessary to separate persistent surface features, rotationally mapped onto the very broad line profiles, from transient flares, a common occurrence on this magnetically challenged object. The eleven HST pointings will be coordinated with Chandra transmission grating spectroscopy and Zeeman Doppler Imaging from groundbased telescopes. The GO-12279 part of the program obtains wide but shallow coverage of the rotational phases. The primary objective of the companion (this) program, GO-12376, is to supplement the "main sequence" with fewer, but much deeper, FUV observations, achieving broader spectral coverage and designed to reach down to the key FeXXI 1354 feature, whose brightness is predicted to be at the limit of sensitivity of the single-orbit pointings of the GO-12279 part. A secondary goal is to use one orbit with STIS to check the intensity of the stellar Lyman-alpha emission to assess whether we might safely use the B side of COS G130M. A third goal is to obtain a COS pointing about a month in advance of the main program, to test the acquisition strategy and capture an early-epoch spectrum similar to those of the main program. COS Acquisition & Science Exposures ------------------------------------- The COS observations for GO-12376 will be divided into three 3-orbit visits, which we call the "long program," and consist of an NUV imaging acquisition with the BOA and MIRROR-A; then a short (64 s) exposures of the 2800 A region with G285M-2676, at FP-POS 3 through the PSA, to capture the key MgII h & k features (on stripe "C"); followed by a deeper exposure (685 s) with G185M-1900 to reach the SiIII 1892 and CIII 1909 intersystem lines (B stripe), and a few important chromospheric emissions that happen to fall on stripes A and C. The NUV exposures are followed by a quartet of G160M-1577 observations, for 300 s each, at FP-POS 1- 4, to fill out the first orbit. The second orbit features a quartet of G130M-1291 exposures of 635 s each, but on detector segment "A" only (pending the Ly-alpha safety check described below). The third orbit is identical to the second. All exposures are in T-TAG mode. The SiIV 1393 feature -- important because it is the most isolated of the bright "hot lines" and thus least affected by rotationally-exaggerated blending -- is captured in both the G160M and G130M segments, providing temporal continuity throughout the 3 orbits. Further, the two orbits of exposure with G130M achieve 5 ks of exposure depth at FeXXI, nearly 3X that expected for the single-orbit visits ("short program") of companion GO-12279. The exposures are arranged so that the "reconfiguation" overhead is hidden between orbits one and two. Ideally, one of the long-program visits will be scheduled at the beginning of the "main sequence" (of short programs), one at the end, and one in the middle. The main sequence provides a broad but shallow context, while the long programs probe deeply into the spectral properties of the source, but only at a few phases. 1) Imaging ACQ A UV-VIS spectral energy distribution for FK Com was carefully constructed from the extensive IUE low-dispersion SW and LW material available for this object, for wavelengths below about 3200 A; joined at the longer wavelengths to a Kurucz model selected on the basis of the match to the longward end of the IUE SED. Assuming a stellar radius of 8.6 R_sun and the approximate Hipparcos distance of 230 pc, the best fit SED was for log(g)=3.0 and Teff= 4900 K, very reasonable for a mid-G giant with a large surface coverage of cool spots. The COS ETC then indicated that FK Com is a few times brighter than safety limits for the PSA/MIRROR-B combination (NUV imaging acquisition), so we were forced to use the BOA/MIRROR-A combination. The recommended exposure time for S/N=60 is 42 s. Other possible acquisition approaches were investigated, including several types of dispersed light ACQ/SEARCH followed by XD and D peakups, but the NUV imaging acquisition was faster than the other cases considered, and sufficiently robust for this target whose coordinates and (almost negligible) proper motions are well known. 2) FUV (& NUV) S/N S/N expectations for the cumulative COS exposures were estimated by creating a proxy high-resolution FUV spectrum from STIS observations of the hyperactive RS CVn star HR1099. The flux densities of the latter were scaled to FK Com by determining the ratio of the average IUE SW-LO spectrum of FK to that of HR1099, Gaussian smoothed to the same resolution, in bins of 50 A over the full grasp of G130M-1291 (i.e., including both detector segments). The scaled proxy FK spectrum then was smoothed by an appropriate rotational broadening to account for the suppression of the flux densities compared with the more slowly rotating HR1099. Results are given in COS.A322770 for G130M-1291, which shows that the key FUV lines like CII 1335 and SiIV 1393 will be recorded at peak S/N per resol of around 40 in each single orbit pointing. (The NUV G285M exposure will yield S/N~15 at the peaks of the two MgII features, based on an average IUE LW-HI moderate-resolution spectrum [COS.A322755].) The long program FUV exposures should achieve correspondingly higher S/N, especially with the multiple FP-POSs to mitigate fixed pattern noise. The NUV exposures, all done at FP-POS=3, would not benefit from multiple FP-POSs to the same extent, because the expected S/N does not approach the NUV MAMA limit (S/N~50) beyond which fixed pattern suppression becomes important. The single-orbit pointing Visit 02 will be used to test the acquisition strategy well enough in advance of the main program (~1 month) to ensure plenty of time to modify the Phase 2 for the main part in the (unlikely) event that a more robust ACQ is required. The single orbit utilizes the standard NUV imaging ACQ described earlier, and follows with the normal G285-2676 single exposure, plus double exposures of G160M and G130M at FP-POS=3 and 4, to fill out the orbit. The G130M exposure times are shorter than those of the main "short program," but the addition of the G160M setting allows for a more complete spectral reconaissance in advance of the main sequence part. 3) Over-bright Ly-alpha? There is one important caveat to the FUV simulation: the Lyman-alpha line of the proxy spectrum is much brighter than anticipated from FK Com itself. Because of geocoronal contamination of the IUE spectra at the HI line, the scaling for the HR1099 spectrum is grossly overestimated at those wavelengths. Furthermore, the FK Com Ly-alpha line is expected to be much weaker relative to that of HR1099 simply because of the large interstellar column (~1E21 per cm^2), including some local HI absorption, which will obliterate the central peak of the line, and impact the surrounding broad wings as well. Thus, the true Ly-alpha peak of FK should be well below the predictions, which nevertheless violate safety limits by a factor of 2, or so. We therefore examined IUE SW-LO spectra of FK Com taken through the large aperture (10"x20") compared with those through the small aperture (3" diameter). If the target was well centered in both apertures, the Ly-alpha flux should be mainly stellar in the SMAP but mainly geocoronal in the LAP. Scaling down the LAP geocoronal peak flux by the ratio of SMAP to LAP areas indicates only about a 30% geocoronal correction to the SMAP Ly-alpha flux. The residual peak is at about 2E-13 in flux density, which translates to about 2E-12 in integrated flux, when the ~50% throughput of the SMAP and ~5 A resolution are taken into account. The broad FUV emissions of FK Com observed by FUSE have FWHMs of around 500 km/s, or about 2 A at Ly-alpha. Thus, a very rough flux density for the peak Ly-alpha emission would be around 1E-12. This is a factor of only ~2 below the detector safety threshold. The FUSE observation of FK Com shows a broad feature at Ly-beta, with a peak flux density of 2E-14. The flux density scaling between Ly-alpha and -beta is around 20, or more, based on a cursory examination of active stars like HR1099. This also would predict a Ly-alpha flux density near 1E-12, consistent with the IUE scaling within the large uncertainties. Unfortunately, this is about as good as it gets, prediction-wise. The reluctant conclusion is that the FK Com FUV hydrogen emission could be close enough to safety limits to jeopardize the entire sequence, if an overlight condition is triggered. The implication is that detector side B should be inactivated for the G130M exposures (as was done for the majority of targets in Cycle 11 COS program 11687), pending the results of a safety check we intend to perform with STIS, as mentioned earlier. They key spectral diagnostics, fortunately, mainly fall on the A side of G130M, so there will be minimal loss of information if the safety check dictates that we must keep side B off. Safety Checking ------------------ As far as bright-object safety limits are concerned, FK Com already has been cleared for a Cycle 17 SNAP program (11687), mainly on the basis of an earlier FUSE pointing, through its LSA, which showed no hot star continuum sources in the field of the target. This check will hold for many decades owing to the negligible proper motion of FK Com. In addition, while the hyperactive star is known to flare frequently, it has been observed many times by IUE (and other high-energy experiments) so the flare statistics and amplitude distributions are well known. Typically, the FUV emissions increase in strength by only factors of ~2 during large flares, which is far below the FUV safety limits. It is important to understand, with regard to the flaring behavior, that the FK Comae stars are distinguished by slow, long-duration (~hours) flares of modest amplitude, but large integrated energy; quite unlike the M-dwarf flare stars whose FUV outbursts tend to be very short (~minutes), but larger in amplitude, and very occasionally in the realm of "super flares." The long history of X-ray, UV, and optical observations of FK Com, prototype of a larger class whose members also have been extensively observed, has not captured any behavior analogous to the -- albeit very rare -- dMe super flares, so there is good reason to believe that the flare mechanism on the fast-spinning FK Com yellow giants is quite distinct from that on the tiny, more slowly rotating M dwarfs, although of course strong magnetic fields ultimately are responsible for the transient events on both types of object. As for the Ly-alpha safety check, we intend to obtain a 1-orbit STIS E140M exposure of the target as early in the new Cycle as practical, to measure the HI flux profile. This will provide directly the information needed to evaluate any detector safety issues. (We also will take a short G230MB exposure to obtain the MgII 2800 flux: this will tie the E140M observation of Ly-alpha into the long-term record of IUE MgII measurements, so we can evaluate how the Ly-alpha emission stands in view of the extremes of the well characterized MgII variability of the source.) Scheduling ------------- The initial single-orbit STIS Ly-alpha safety check (Visit 01) was specified for the mid-November 2010 timeframe, the earliest feasible opportuinity, using the "BETWEEN" option. The acquisition test and early-epoch COS sequence (Visit 02) was put in the mid-March 2011 timeframe, the closest scheduable period well enough before the main GO-12279 sequence, which currently is scheduled for late-April/early-May 2011. The visibility period in April/May 2011 is optimum for the main part of the combined campaigns because of lack of constraints on the Chandra side, and good access for the groundbased ZDI component. The "BETWEEN" range was selected to place Visit 03, the first of the three "long programs," within 1 day of the beginning of the 11 single orbit pointings ("short programs") over 5 days in GO-12279; "AFTER Visit 03" BY 2-3 d was specified for Visit 04 to place it in the middle of the 12279 sequence; and "AFTER dd-mmm-yyy" plus "AFTER Visit 03 BY 6-7d" was used for final Visit 05 to put it within 1 d after the end of the 12279 sequence. The goal, as mentioned earlier, is to bracket the GO-12279 sequence with a long program at the beginning, another at the end, and one in the middle. COOL STARS 12 false 120 0.0 false false false false SUPPORTED EXT-STAR G III-I FK Comae Berenices; G5II; probably closer to G5III; R~9Rsun; Prot~2.4d; vsini~160km/s FK-COM J2000 13 30 46.7600 +24 13 57.60 J2000 COS/FUV Spectroscopic No No G130M 1291 The scheduling data was generated by CASM 19.0. CASM was run in Phase II mode. 1290308209000 318.5126086959363 (Tic-Visit :proposal-id 12376 :visit-id "01" :proposal-version -1 :trans-version "20100902" :date "02-Mar-2011 10:32:30.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 249024 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12376 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12376 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2915 :al-overhead-at-end 0 :internal-dur-at-end 377 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3249 :pcs-gap 0 ) true 1 0 The scheduling data was generated by CASM 19.0. CASM was run in Phase II mode. 1299458766000 247.1189941088869 (Tic-Visit :proposal-id 12376 :visit-id "02" :proposal-version -1 :trans-version "20100902" :date "02-Mar-2011 10:32:33.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 600240 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12376 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12376 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2906 :al-overhead-at-end 0 :internal-dur-at-end 409 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3249 :pcs-gap 0 ) true 1 0 COS.A321599: optimum exp time for S/N=60 is 42 s; for model SED based on average IUE spectrum for FUV and NUV up to 3100 A, followed by Kurucz log(g)=3, Teff=5000 K scaled to ~4900 K to fit the longward end of IUE composite spectrum (for Rstar= 8.6 Rsun). The scheduling data was generated by CASM 19.0. 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The scheduling data was generated by CASM 19.0. 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The scheduling data was generated by CASM 19.0. 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The scheduling data was generated by CASM 19.0. CASM was run in Phase II mode. (Tic-Visit :proposal-id 12376 :visit-id "55" :proposal-version -1 :trans-version "20100902" :date "10-May-2011 16:33:54.0" :trans-errors '(:WARNINGS 3 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 1313632 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12376 :visit-id "55" :obset-id "55" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12376 :visit-id "55" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2906 :al-overhead-at-end 0 :internal-dur-at-end 388 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3249 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12376 :visit-id "55" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3027 :al-overhead-at-end 0 :internal-dur-at-end 50 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3249 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12376 :visit-id "55" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3027 :al-overhead-at-end 0 :internal-dur-at-end 362 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3249 :pcs-gap 0 ) true 3 0 COS.A321599: optimum exp time for S/N=60 is 42 s; for model SED based on average IUE spectrum for FUV and NUV up to 3100 A, followed by Kurucz log(g)=3, Teff=5000 K scaled to ~4900 K to fit the longward end of IUE composite spectrum (for Rstar= 8.6 Rsun).