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TOPIC: Polaris system
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RE: Polaris system
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Title: Polaris the Cepheid returns: 4.5 years of monitoring from ground and space
Authors: H. Bruntt, N. R. Evans, D. Stello, A. J. Penny, J. A. Eaton, D. L. Buzasi, D. D. Sasselov, H. L. Preston, E. Miller-Ricci

We present the analysis of 4.5 years of nearly continuous observations of the classical Cepheid Polaris, which comprise the most precise data available for this star. We have made spectroscopic measurements from ground and photometric measurements from the WIRE star tracker and the SMEI instrument on the Coriolis satellite. Measurements of the amplitude of the dominant oscillation (P = 4 days), that go back more than a century, show a decrease from 120 mmag to 30 mmag (V magnitude) around the turn of the millennium. It has been speculated that the reason for the decrease in amplitude is the evolution of Polaris towards the edge of the instability strip. However, our new data reveal an increase in the amplitude by about 30% from 2003-2006. It now appears that the amplitude change is cyclic rather than monotonic, and most likely the result of a pulsation phenomenon. In addition, previous radial velocity campaigns have claimed the detection of long-period variation in Polaris (P > 40 days). Our radial velocity data are more precise than previous datasets, and we find no evidence for additional variation for periods in the range 3-50 days with an upper limit of 100 m/s. However, in the WIRE data we find evidence of variation on time-scales of 2-6 days, which we interpret as being due to granulation.

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Blobrana


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Polaris B
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Title: Polaris B, an optical companion of Polaris (alpha UMi) system: atmospheric parameters, chemical composition, distance and mass
Authors: Igor Usenko, Valentina Klochkova

We present an analysis of high-resolution spectroscopic observations of Polaris B, the optical companion of the Polaris Ab system. The star has a radial velocity V_r of -16.6km/s to -18.9km/s, and a projected rotational velocity vsini=110 km/s. The derived atmospheric parameters are: Teff=6900K; logg=4.3; V_t=2.5km/s. Polaris B has elemental abundances generally similar to those of the Cepheid Polaris A (Usenko et al. 2005a), although carbon, sodium and magnesium are close to the solar values. At a spectral type of F3V Polaris B has a luminosity of 3.868L_sun, an absolute magnitude of +3.30mag, and a distance of 109.5pc. The mass of the star is estimated to be 1.39M_sun, close to a mass of 1.38 ±0.61M_sun for the recently-resolved orbital periods companion Polaris Ab observed by Evans et al. (2007).

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Blobrana


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RE: Polaris system
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Title: Polaris: Mass and Multiplicity
Authors: Nancy Remage Evans, Gail Schaefer, Howard E. Bond, Edmund Nelan, Giuseppe Bono, Margarita Karovska, Scott Wolk, Dimitar Sasselov, Edward Guinan, Scott Engle, Eric Schlegel, Brian Mason

Polaris, the nearest and brightest classical Cepheid, is a member of at least a triple system. It has a wide (18'') physical companion, the F-type dwarf Polaris B. Polaris itself is a single-lined spectroscopic binary with an orbital period of 30 years (Kamper, 1996, JRASC, 90, 140). By combining Hipparcos measurements of the instantaneous proper motion with long-term measurements and the Kamper radial-velocity orbit, Wielen et al. (2000, A&A, 360, 399) have predicted the astrometric orbit of the close companion. Using the {\it Hubble Space Telescope} and the Advanced Camera for Surveys' High-Resolution Channel with an ultraviolet (F220W) filter, we have now directly detected the close companion. Based on the Wielen et al. orbit, the Hipparcos parallax, and our measurement of the separation (0.176'' ±0.002''), we find a preliminary mass of 5.0 ± 1.5 Solar masses for the Cepheid and 1.38 ± 0.61 solar masses for the close companion. These values will be refined by additional HST observations scheduled for the next 3 years.
We have also obtained a Chandra ACIS-I image of the Polaris field. Two distant companions C and D are not X-rays sources and hence are not young enough to be physical companions of the Cepheid. There is one additional stellar X-ray source in the field, located 253'' from Polaris A, which is a possible companion. Further investigation of such a distant companion is valuable to confirm the full extent of the system.

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Astronomers using the Hubble Space Telescope have photographed the close companion to Polaris, known also as the North Star, for the first time.

"Hubble's exceptional pointing capabilities combined with the wonderful performance of its instruments allow scientists to see the universe in finer detail than ever before. It is that clear vision that makes these types of images possible" - Michael Moore, Hubble program executive.

The North Star is actually a triple star system in the constellation Ursa Minor. While one companion is easily viewed with small telescopes, the other hugs Polaris so tightly that it has never been seen until now.

"The star we observed is so close to Polaris that we needed every available bit of Hubble's resolution to see it" - Nancy Evans, astronomer at the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.


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Position (2000): R.A. 02h 31m 49s.08 Dec. +89° 15' 50".8
The left frame shows Polaris's location very close to the position of Earth's north celestial pole in Ursa Minor (the Small Bear).
The upper right image shows Polaris A and its distant companion Polaris B, as viewed by Hubble. They are separated by approximately 240 billion miles. Polaris B, the wide companion, is visible in small telescopes, and was first noticed by William Herschel in 1780. The close companion, Polaris Ab was known to exist from its gravitational tug on Polaris A, but has only been seen directly now using Hubble (lower right image).

The companion proved to be less than two-tenths of an arcsecond from Polaris. The angle is equivalent to the apparent diameter of a penny located 19 miles away. At the system's distance of 430 light-years from Earth, that translates into a separation of about 2 billion miles.

"The brightness difference between the two stars made it even more difficult to resolve them. With Hubble, we've pulled the North Star's companion out of the shadows and into the spotlight" - astronomer Howard Bond of the Space Telescope Science Institute, Baltimore.

Polaris is a super-giant more than two thousand times brighter than the sun, while its companion is a dwarf star.

"Our ultimate goal is to get the accurate mass for Polaris. To do that, the next milestone is to measure the motion of the companion in its orbit" - Nancy Evans.

Astronomers want to determine the mass of Polaris, because it is the nearest Cepheid variable star. Cepheids' brightness variations are used to measure the distances of galaxies and the expansion rate of the universe. It is essential to understand their intrinsic physics makeup and evolution. Knowing their mass is the most important ingredient in this understanding.
The researchers plan to continue observing the Polaris system for several years. The movement of the small companion during its 30-year orbit around the primary should be detectable. The researchers presented their data during the 207th meeting of the American Astronomical Society in Washington.

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