| Proposal Number | Subject Category | PI Name | Target Name | Total Chandra Time | Title |
|---|---|---|---|---|---|
| 12200943 | Cool Stars | Thomas Ayres | FK Comae | 120ks | FK Comae, King of Spin: the Movie |
| 12800944 | Clusters of Galaxies | Harald Ebeling | MACSJ0358.8-2955 | 50ks | An In-Depth Study of Dark Matter in the Massive Cluster Merger MACSJ058.8-2955 |
Subject Category: Cool Stars
Proposal Number: 12200943
Title: FK Comae, King of Spin: the Movie
PI Name: Thomas Ayres
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 O VI and C III, 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 Fe XXI 1354 (bridge to the coordinated Chandra HETGS pointing we also are proposing). 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 O VI. Further, we will test whether the striking 100 km/s redshifts of the FUV lines, and similar shifts seen in Ne X by Chandra HETGS, are caused by a massive coronal outflow (perhaps implicated in magnetic braking). 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., Si III 1206 and Si IV 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 flows. Contemporaneous spot maps from the ground will provide a fundamental magnetic context for the coordinated FUV and X-ray "movies."
Subject Category: Clusters of Galaxies
Proposal Number: 12800944
Title: An In-Depth Study of Dark Matter in the Massive Cluster Merger MACSJ058.8-2955
PI Name: Harald Ebeling
We propose multi-passband observations with HST/ACS (F435W, F606W, and F814W) and moderately deep X-ray observations with Chandra/ACIS-I of the massive cluster merger MACSJ0358.8-2955 at z=0.434. The combination of existing HST/ACS and Chandra/ACIS-I snapshots shows strong evidence for a segregation of luminous and dark matter in a linear cluster merger, making this system a prime candidate for a quantitative study of the properties of dark matter. We have already identified and spectroscopically confirmed two sets of strongly lensed multiple-image systems. The resulting tentative model of the mass distribution shows a highly elongated critical line and further supports a linear, post-collision geometry that may enable a third, independent measurement of the self-interaction cross section of dark matter from cluster mergers. The colour and morphological information provided by the proposed HST/ACS observations will allow us to unambiguously identify additional multiple-image systems to refine our mass model, and to obtain further independent constraints on the mass profile from a weak-lensing analysis. The proposed Chandra/ACIS-I will provide complementary information on the intra-cluster gas, most importantly an accurate mapping of its distribution, but also spatially resolved gas temperatures. In combination with groundbased spectroscopy (underway), these observations will permit us to reconstruct the three-dimensional geometry and dynamics of this merger and to perform an independent test of the results on the existence and properties of dark matter obtained for the Bullet Cluster and MACSJ0025.4-1222.
