SoylentNews

A Three-Dimensional Map of the Milky Way Using classical Cepheid Variable Stars

posted by chromas on Friday August 02 2019, @11:20AM   
from the Home-is-where-your-planet-is dept.

aristarchus writes:

As anyone who has been paying attention knows, Edwin Hubble's discovery of the "standard candles" of Cepheid variable stars expanded our universe by magnitudes. Prior to Hubble, "nebulae" that were galaxies, like the Andromeda Nebula, were thought to be within our local neighborhood. But now we know, etc., etc..

But now, we have a new model of our own galaxy (γαλαξίας in Greek: The myth is that somehow Heracles managed to be suckling at the breast of Hera, who when she realized who the infant was, ripped him from her breast and spewed milk across the heavens, the Milky Way), 3D-imaged with Cepheid variables. Article is at Science Mag^[$], as well as elsewhere.

Cepheids help to map the Galaxy

Cepheid variable stars pulsate, which allows their distances to be determined from the periodic variations in brightness. Skowron et al. constructed a catalog of thousands of Cepheids covering a large fraction of the Milky Way. They combined optical and infrared data to determine the stars' pulsation periods and mapped the distribution of Cepheids and the associated young stellar populations across the Galaxy. Their three-dimensional map demonstrates the warping of the Milky Way's disc. A simple model of star formation in the spiral arms reproduced the positions and ages of the Cepheid population.

Science, this issue p. 478^[$].

Science 02 Aug 2019:
Vol. 365, Issue 6452, pp. 478-482
DOI: 10.1126/science.aau3181^[$]

Also at Popular Science.

---

Original Submission

• gaps (Score: 2) by takyon on Friday August 02 2019, @01:00PM

  by takyon (881) <takyonNO@SPAMsoylentnews.org> on Friday August 02 2019, @01:00PM (#874592) Journal

  Follow up to this article [soylentnews.org].

  We haven't actually seen all of the stars in the Milky Way in great detail. The Gaia [wikipedia.org] mission will help fill in the gaps by characterizing around 1 billion stars. LSST [wikipedia.org] will also help, especially with finding the dimmer objects, such as red and brown dwarfs, and anything near the center of the Milky Way:

  https://arxiv.org/pdf/1708.04058.pdf [arxiv.org]

  *SPOILER* (click to show) *SPOILER* (click to hide)

  Many populations of great importance to Astronomy exist predominantly in or near the Galactic Plane, and yet are sufficiently sparsely-distributed (and/or faint enough) that LSST is likely to be the only facility in the forseeable future that will be able to identify a statistically meaningful sample. Some (such as the novae that allow detailed study of the route to Type Ia Supernovae) offer unique laboratories to study processes of fundamental importance to astrophysics at all scales. Others (like slow-microlensing events heralding an unseen compact object population; e.g. Wyrzykowski et al. 2016) offer the only probe of important populations.

  [...] Since most black hole candidates have been identified near the plane in the inner Milky Way (68% and 92% identified within 5◦ and 10◦ of the Plane, respectively), this science case requires that LSST observe the plane with sufficient cadence to detect the ∼hundreds of quiescent black-hole binaries by virtue of their variability. The natural choice for a survey for low-luminosity black hole binaries would be to extend the Wide-Fast-Deep survey throughout the Plane in the direction of the inner Milky Way.

  [...] Only ∼ 15 novae (explosions on the surfaces of white dwarfs) are discovered in the Milky Way each year, while observations of external galaxies show that the rate should be a factor of ∼ 3 higher (Shafter et al. 2014). Evidently, we are missing 50–75% of novae due to their location in crowded, extinguished regions, where they are not bright enough to be discovered at the magnitude limits of existing transient surveys.

  [...] The study of the halo of the Milky Way is of the highest importance, not only to understand the formation and early evolution of our own galaxy, but also to test current models of hierarchical galaxy formation. LSST will provide an unprecedented combination of area, depth, wavelength range and long time-baseline for imaging data, allowing detailed studies of the present-day structure of this old Galactic component.

  [...] Populations near, or brighter than, LSST’s nominal saturation limit (r ∼ 16 with 15s exposures) are likely to be crucial to a number of investigations for Milky Way investigations, whether as science tracers in their own right, or as contaminants that might interfere with measurements of fainter program objects (due, for example, to charge bleeds of bright, foreground disk objects). Quantitative exploration of these issues now requires involvement from the community (e.g. to determine LSST’s discovery space for bright tracers in context with other facilities and surveys like ZTF, Gaia and VVV), and the project (e.g. to determine the parameters of short exposures that might be supported by the facility).

  But brighter objects could be observed without impacting the LSST's main science goals:

  *SPOILER* (click to show) *SPOILER* (click to hide)

  The current LSST requirements stipulate a minimum exposure time of 5 seconds, with an expected default exposure time of 15 seconds. This document advocates for decreasing the minimum exposure time requirement from 5 to 0.1 seconds. This would increase the dynamic range for bright sources (compared to the default 15 sec time) by about 5 magnitudes, to a total of 13 astronomical magnitudes (where dynamic range is the difference between the brightest unsaturated source and the faintest point source detectable at 5 sigma). This is a large factor, and would enable a wide range of science goals, outlined below. One interesting aspect of this is that it would allow us to operate the LSST system during twilight times that would otherwise saturate the array due to background sky brightness. This would allow a number of the goals described below to be carried out without impacting the primary survey by conducting observations during twilight sky conditions that would saturate the array at longer exposure times.

  [...] We stress that this twilight SN followup campaign can be accomplished without impacting the main survey, during the roughly 20 minutes per night of twilight that would otherwise unusable at the default exposure time. We would use the brighter twilight time to obtain pointed observations on nearby supernovae, motivated by the importance of photometric uniformity described above.

  We could also use the added twilight time to conduct a bright star survey, and the precise astrometry and photometry from LSST can then be used in conjunction with archived data ranging from 11th to 27th AB magnitudes. This short-exposure domain would extend the LSST dynamic range in fluxes by two orders of magnitude, towards the bright end. Moreover, obtaining precise positions, fluxes and variability at these brighter magnitudes would greatly increase the overlap with the historical archive of astronomical information, including from digitized plate data. We would be able to obtain astrometric and color information to high precision, as well time series for variability studies.

  An example of an application to Milky Way structure studies comes from RR Lyrae variable stars. With a saturation magnitude of around 16th in the standard LSST survey, RR Lyrae closer than 20 kpc will be saturated in the standard LSST images. So we will lose nearly all Galactic RR Lyrae. Extending the survey’s bright limit to 11th magnitude will allow us to collect light curves for RR Lyrae beyond ∼ 100 parsecs, collecting essentially all Southern hemisphere Galactic RR Lyrae.

  Another application for stellar population studies is measuring the fraction of binary stars as a function of stellar type, metallicity, age and environment. By conducting a variability survey in the 11-18 magnitude range we can capitalize on temperature and metallicity data already in hand for many of these objects.

  Some future follow-up missions will be needed before we know where 100 billion to 1 trillion Milky Way objects are.

  --
  [SIG] 10/28/2017: Soylent Upgrade v14 [soylentnews.org]

  Starting Score:	1		point
  Karma-Bonus Modifier		+1
  Total Score:		2