Sorry for yesterday's confusion. I circulated a crude draft of the astrometry section of the DRM for comments, and Dennis responded to everybody. I have incorporated his section, deleted mine, and generally tried to tidy things up. As per Michael's desire to get the DRM ball rolling, here is the current version for comments, etc. My own opinion is that it is too long. The DRM is not supposed to demonstrate the scrivener's brilliance (although this is inevitable), but rather to hit the high points with enough detail to convince the vaguely qualified referee that LSST makes all other washday products seem pale by comparison. I think that Michael's suggestion of 2-3 pages per topic is pretty good, and the appended is way over this even without the photometry and/or whatever other sections Michael suggested for inclusion (astro.princeton.edu is asleep this morning so I cannot review msg.131). Fire away. I have been volunteered to be the target, so please make sure that I get copies of the corrections, etc. Michael will have a non-trivial task of merging all of these suggestions together, and my hope is that editorial and stylistic changes can be delayed until that time. -Dave Monet is dgm@nofs.navy.mil ========================================================================= Selected Topics in Stellar Astronomy Rather than present a shopping list of many interesting projects that LSST can do, we choose to present a discussion of three topics in which LSST will make fundamental advances. They are a) structure and composition of the Galactic halo, b) astrometry, and c) <whatever else Michael thinks belongs here>. All of these are enabled by LSST's unique combination of sky coverage, repeated observations, and astrometric and photometric accuracy. A. Structure and Composition of the Galactic Halo A.1 Tracing the Smooth Luminous Halo From measurements of relatively local stars the current understanding appears to be that the galaxy has a stellar halo of steeply falling density (rho proportional to r^{-3} Ivezic et al.) and a possible cutoff at r ~ 50 kpc. Using SDSS data, the halo and its inhomogeneities, are traced with stars that are brighter than M ~ 0 at several tens of kpc (m_r ~ 20 at the suggested cutoff radius). LSST will reach m_r ~ 26, which is fainter than the main sequence turnoff at the suggested cutoff radius (M ~ 6). The turnoff stars then become an excellent tracer of the halo (other types of stars that are that blue (B- V ~ 0.4) and that faint will be vastly outnumbered (factor?) by the main sequence stars and should be available perhaps out to perhaps 100 kpc. Proper motions can be used to further remove contaminants (foreground white dwarfs will be ~100 times closer). The tracing of the halo can be done in a highly statistical manner by matching models with specific properties to the number counts and proper motion distributions. One should keep in mind that unlike the current status, where because we are relatively data starved we need to be fairly confident of each star (particularly in the use of giants in the halo), in the LSST era much more can be done confidently in a statistical manner. Even with SDSS, the structures that are being found are the ones we can point to the overheads (this will change with time once the cream has been skimmed and the data deluge recedes). Goals: 1) Confirm/refute presence of cutoff at 50 kpc (trace the 3-D shape of the cutoff). This is simple for LSST, but see issue about colors and the use of colors to discriminate giants/dwarfs (below). 2) Identify any further cutoffs (shell structure? multiple shells as seen in secondary infall calculations?). Tests of smooth infall vs. hierarchical. 3) Measure radial behavior of stellar halo from 50 kpc to 200+ kpc. 4) Measure velocity ellipsoid vs. r to constrain infall models and halo orbit families (important for formation/evolution modeling (deposition of angular momentum) and for mass profile determinations).This would be done with local halo populations for which the proper motions would be measurable (the 10-20 km/sec proper motions envisioned for the final survey would be ideal). A.2 Tracing the Unseen Halo LSST can play a role in the study of Galactic dark matter in at least two ways: 1) it will provide dynamical information to map the inner halo via proper motions, 2) it will identify RR Lyrae stars at large distances to greatly increase the number of dynamical tracers in the outer halo, 3) it will provide the most comprehensive survey of the luminous Galactic components. A.2.1 One of the most compelling arguments for a large dark matter halo comes from the fastest moving star (one assumes that is is bound and therefore derives a mass). It is a simple argument. By actually measuring the full 3-D velocities of stars in the local neighborhood, we would have not just a single fast moving star but a sample of fast moving stars from which orbits could be constructed. A self-consistent mass model would not only need to explain the velocity distribution function but would need to tie that into the density of objects at larger radii (i.e. if we find that many local stars have apogalacticons of 100 kpc, there better be a corresponding population of stars at 100 kpc). Tracing a population in manner is much more constraining on a model than simply using a satellite galaxy (or two) at large radii. A.2.2 Dynamical tracers at large radii are important because they provide independent measures of the total enclosed mass. Again, the consistency required by the study of these objects with the local velocity distribution function is well beyond anything available to date. Examining such studies as that by Kochanek (1996) or Wilkinson and Evans (1999), where the dynamics of various populations at different radii are combined, the tightening of the constraints by including a range of objects at different radial scales is evident. The current state of the art includes ~ 30 objects, ongoing work reaches ~100 objects over Schmidt plates (Clewley et al. 2002). LSST will dwarf these studies. A.2.3 The interpretation of the microlensing results depends on several factors. The LSST can provide critical information on the radial distribution of stellar lensing populations and perhaps some information on the tangential velocities of lensing sources. The ambiguities in the interpretation that that can be introduced deviating from a standard halo can be seen in the study by Geza, Evans, & Gates (1998). Goals: 1) Precise radial profile for stellar halo components. 2) Precise measurement of the structure within the halo (what are the likely over/under densities along the LMC and SMC lines-of-sight?). 3) Precise measurement of the tangential velocities (perhaps available only for the nearer lenses). At 1/2 the distance to the LMC (where lensing is favored), the proper motions will be good to 25-50 km/sec and so a lensing populations should be separable from the LMC stars, and a dispersion may be measurable if it is comparable to they of the Galactic halo (~ 150 km/sec). C. Tracing the Lumpy Halo To best trace the streamers we need both distances and velocities to halo stars. LSST provides the best avenues for obtaining each of these over large areas of the sky. First, RR Lyrae stars will be identified throughout the halo and provide distances for any overdensity of stars identified as a potential streamer. MSTO stars can also be used (see above). Second, proper motion measurements will provide some information on the kinematic coherence of any overdensity. The First Light observations are not useful for this program, but the first repeat and then the completed mission probe the range of expected halo velocities first to about 50 kpc and eventually to 200 kpc and beyond. Ironically, although radial velocities are generally much easier to obtain than tangential velocities, the all-sky nature of LSST will make proper motions easier to obtain than radial velocities over the same area of sky for the large number of stars involved. With the tremendous number of stars involved, we do not need to identify individual streamers but rather quantify the ``lumpiness" of the velocity distribution to constrain the number of streamers contributing to the halo population. Goals: 1) Completely trace streamers our to several hundred kpc. (do streamers come primarily from one type of orbit family, for example, satellites on radial orbits). 2) Measure coherence of streamers over full length of arcs (a measurement of the roughness of the Galactic potential). 3) Measure width of streamers (a combination of Galactic potential roughness and the initial internal velocity dispersion of satellite - important for understanding initial population). The internal velocity dispersion of the original object that was disrupted is expected to be several tens of km/sec for the most massive objects. It is therefore important to obtain proper motions that are as precise as possible and certainly better than several tens of km/sec to be able to say anything regarding the coldness of the stream and potentially about the original system. 4) Measure proper motions along stream to solve for ``guiding center" orbit (determines potential depth and shape). B. Astrometry Although not explicitly discussed by the Decadal Review, the ability of LSST to provide accurate positions for objects is a necessity. Astrometric accuracy at the level of a few tenths of the size of the PSF is part of the basic processing pipeline, but if accuracies approaching those predicted by photon statistics (as limited by seeing, detector quantization, etc.), then exciting and fundamentally important scientific results can be obtained. B.1 The Parallax Survey LSST offers the opportunity to measure the parallax of every object in its field of view. Although this sounds both obvious and trivial, this project has been beyond the reach of the astronomers until the Hipparcos satellite (Perryman et al. 1977) provided these data for the bright stars. Many, most notably Murray (1986) attempted the measurement of the parallaxes for a large number of faint stars using Schmidt telescope survey plates and automated plate measuring machines, but the relatively large errors were encountered. Indeed, the Yale Parallax catalog (van Altena 1995) lists values for only 8,112 stars. LSST will change everything. It will measure the positions for an estimated 1e10 stars several times per year, and the pipeline will produce measurements and uncertainty estimators for the position, parallax, and proper motion of every star it observes. This will be the dawn of a new age in our understanding of the nearby stars. LSST will provide the first distance-limited catalog of the distances to all stars in the sky subject only to apparent magnitude and celestial coverage limitations. In the past, studies of solar neighborhood have had to use magnitude, color, or proper motion as a surrogate for distance, at least in the selection process. No more. The LSST catalog will provide the distances, and this will remove almost all of the selection effects that are present in the current studies. This is one of the few times when the observational data (position as a function of time) measure the astrophysical parameter (distance) directly. The LSST survey of the nearest stars (within 10 pc) will be of great importance to a large segment of the community, and should be available after the first year of operation. If LSST can observe the available sky twice per lunation, then the parallax and proper motion can be separated after a few months, and the large signal from a nearby star can be detected. Of course, more observations will improve the determination and weed out the occasional binary system that fits the detection scheme, but parallaxes with enough accuracy (10% or better) for astrophysical follow-ups will be available in almost real time. It is proposed to make this list available as quickly as possible so that further studies can be started before the stars leave the evening sky. No more will the astrometric studies of newly identified nearby stars (e.g., Dahn et al 2002 and Vrba et al 2003) await the curious process of photometric discovery. Astrometry will be used to select the nearby stars without any constraints on photometry or spectroscopy. The discovery of L-dwarfs, T-dwarfs, and other objects with low luminosity are just previews of the new types of objects waiting to be discovered. B.2 The Wiggle Survey Binary stars offer one of the few opportunities to measure the mass of a star, but searches for binaries are rarely done. Usually, binaries are discovered by accident (corrupting an astrometric or spectroscopic analysis) or through directed surveys looking for companions around specific types of stars (e.g., planets around solar-type stars). Once the binary nature of a system is demonstrated, many further observations are needed if an orbital solution is to be derived. The Washington Double Star Catalog (WDS; Mason et al 2003) shows 5 systems with semi-major axes larger than 0.7 arcsec and periods less than 20 years. Indeed, the WDS lists orbits for fewer than 2000 systems. LSST will change all of this. The residuals from the fits for position, proper motion, and parallax will be searched for the signature of Keplerian motion. For systems whose orbital period is shorter than about a quarter of the available epoch difference, the periodic nature of orbital motion can be sensed and parameterized. Once adequate coverage is available, an orbit can be computed. Modern Fourier techniques can do this in an impersonal manner on all stars in the LSST archive. A very exciting aspect of LSST is that the astrometry will be done in each of the survey passbands, and the amplitude of the astrometric perturbation can be measured as a function of color. This will allow a statistical identification of the components involved, and allow the list of all binaries to be culled so that the important ones can be handed to the user community for additional observations. Perhaps the greatest contribution of the Wiggle survey will be in combination with the Parallax survey for the understanding of the star formation process. The LSST data will provide an almost complete inventory of systems within a nearby volume, and the wiggles and common proper motion data can be used to identify the major components of each system. ================= Contributions to Requirements Document 1) Cadence needs to be 2 visits per field per lunation in the same color during the first year. This flows from the Nearby Star Parallax survey's need to separate parallax from proper motion on the basis of the nominal 6 months of data per field during the first year. After the first year, 1 visit per field per color is sufficient to measure parallax, proper motion, and wiggles from unseen companions. 2) Sub-pixel sensor characterization. The astrometric accuracies need to be at the hundredth of a pixel level per observation, and at the thousandth of a pixel level for the mean of several observations. Once the LSST sensors have been selected, a detailed laboratory and observing campaign will be needed to understand the technology and to verify centroiding algorithms at the milli-pixel level. 3) Telescope and PSF stability. Upon review, astrometric science is fully enabled if the requirements imposed by the Dark Matter programs are implemented. 4) Astrometric Tie to Bright Stars. The system of J2000 is based on Hipparcos stars (V <= 10), and some method must exist to follow the mapping of the coordinate system from these bright stars to the deep exposures. Neutral density filters might be needed, or perhaps the sensor technology allows for magnitude compensation, but astrometry must levy a requirement that a path for this mapping must exist. 5) The Streams in the Galactic Halo project needs photometric data in non-Survey passbands to assist in measuring the luminosity and abundance of the stars. Since variability is not an issue for most stars, these data can be taken once, presumably during the pre-Survey commissioning period. ================= Questions that need SWG attention: What the relative merits of more colors to do color-color selection vs. more in one passband to do more precise astrometry? The clear purpose of multicolor data is to separate stellar population (i.e. foreground dwarfs from halo giants, etc.). Some recent results using the SDSS filters suggest that broad band colors might do a reasonable job. The preferred narrow-band filters are probably out of the question for LSST. Could they be done (at least in part) by a dedicated smaller telescope? This topic needs more work on understanding how much can be gleaned from a relatively standard and how much cannot. ================= Clewley, L., et al. 200, MNRAS, 337, 87. Dahn, C.C. et al (2002), AJ 124, 1170. Geza, G., Evans, N.W., and Gates, E.I. 1998, ApJL, 502, 29. Ivezic, Z. et al. 2000, AJ, 120, 963. Kochanek, C.S. 1996, ApJ, 457, 228. Mason, B.D., Wycoff, G.L., Hartkopf, W.I. (2003) http://ad.usno.navy.mil/wds/ Morrison, H. et a. (2001) astro-hp/0111097. Murray, C.A., Corben, P.M., Argyle, R.W. (1986) in Astrometric Techniques ed. H. Eichhorn and R. Leacock (Dordrecht, Reidel), p213. Perryman, M.A.C. et al. (1997) The Hipparcos and Tycho Catalogs (Noordwijk, European Space Agency SP-1200). van Altena, W.F., Lee, J.T., Hoffleit, E.D. (1995) The General Catalog of Trigonometric Catalogs (New Haven, Yale). Vrba, F.J. et al, (2003) in preparation. Wilkinson, M.I., and Evans, N.W. 1999, MNRAS, 310, 645. LSST LSST LSST LSST LSST Mailing List Server LSST LSST LSST LSST LSST LSST LSST LSST This is message 154 in the lsst-general archive, URL LSST http://www.astro.princeton.edu/~dss/LSST/lsst-general/msg.154.html LSST http://www.astro.princeton.edu/cgi-bin/LSSTmailinglists.pl/show_subscription?list=lsst-general LSST The index is at http://www.astro.princeton.edu/~dss/LSST/lsst-general/INDEX.html LSST To join/leave the list, send mail to lsst-request@astro.princeton.edu LSST To post a message, mail it to lsst-general@astro.princeton.edu LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST LSST