New Hubble Observations of the Sirius System

Usually NASA’s Hubble Space Telescope (HST) is associated with making observations of incredibly dim objects frequently hundreds of millions or even billions of light years away. But in addition to studying such objects, HST has also been used to observe much brighter targets much closer to home. One such target has been Sirius – a nearby star system which includes the brightest star in our nighttime sky along with a dim white dwarf companion. Almost two decades of HST observations of Sirius in combination with earlier data have now been analyzed to help astronomers pin down the orbits of these stars and derive the most accurate masses available to date in an effort to improve our understanding of this pair of stars with such disparate properties as well as address the ongoing question of a third “companion” in the system.

A view of NASA’s Hubble Space Telescope in orbit above the Earth after its second servicing mission in February 1997 – just weeks before its first observations of Sirius. (NASA)

Background

Sirius, popularly known as “the Dog Star”, is a V magnitude -1.46 star located in the constellation of Canis Major. Also known by its Bayer designation α Canis Majoris, Sirius is a hot spectral type A1V main sequence star with a luminosity that is 25 times that of the Sun. At a distance of only 8.61 light years, Sirius is the seventh closest star system to the Sun currently known. It is this relative closeness combined with its high intrinsic brightness that is responsible for making Sirius the brightest star in our nighttime sky.

Edmond Halley (seen here in a portrait by Richard Phillips painted before 1722) was the first astronomer to recognize the proper motion of Sirius and other “fixed stars”.

In 1718 English astronomer Edmond Halley (1656-1742), after whom Comet Halley is named, discovered that Sirius, along with the “fixed stars” Aldebaran and Arcturus, were actually moving slowly across the sky after he noted the differences in the apparent position of these stars in the sky at his time compared to those recorded 18 centuries earlier by the famed ancient Greek astronomer Claudius Ptolemy (c100-c170). Halley had discovered what we now call the proper motion of the stars which, for Sirius, amounts to 1.3 arc seconds per year, according to modern measurements.

Friedrich Bessel (shown here in an 1839 portrait by Danish artist Christian Albrecht Jensen) “discovered” Sirius B in 1844 by the wobbles it produced in the motion of Sirius A.

Based on a detailed analysis of the motion of Sirius across the sky, German astronomer Friedrich Bessel (1784-1846) noted a periodic wobble with an amplitude of a couple of arc seconds. Without any other explanation possible, in 1844 Bessel hypothesized that Sirius was orbited by an unseen, solar-mass (or ~1 M_Sun) object with a period of about a half a century. Bessel had detected the first “unseen companion”. This hypothesis was confirmed in 1862 by American telescope maker and astronomer, Alvan Graham Clark (1832-1897), when he observed a faint star with a V magnitude of only 8.2 a few arc seconds from Sirius while he was testing the new 47-centimeter refracting telescope his family’s firm was building for the Dearborn Observatory (the largest telescope of its type at that time) at his workshop in Cambridge, Massachusetts. Subsequent observations with a smaller telescope confirmed that what would become known as Sirius B was real and not an instrument artifact.

Alvan Graham Clark made the first visual observation of Sirius B in 1862 as he was testing the 47-cm telescope his firm was constructing for the Dearborn Observatory.

With a mass comparable to that of the Sun but with a luminosity only 0.024 times the Sun’s, Sirius B presented a bit of a mystery to astronomers in the decades after its discovery since stars of its mass and color were typically much brighter. We now know that Sirius B is a type DA2 white dwarf star – the inert, hot core of a Sun-like star left over after it could no longer sustain fusion at the end of its life. Unable to support its mass with the internal heat of fusion reactions, Sirius B has compressed itself into an Earth-sized sphere with a diameter of only 11,300 kilometers and a surface temperature of 25,300 K. With so much mass squeezed into such a small volume, Sirius B has a mean density 2.7 million times that of water and a surface gravity 430,000 times greater than Earth’s. Given its small size and dimness compared to its primary, the Dog Star, Sirius B is sometimes affectionately referred to as “The Pup”.

This is an artist’s depiction showing a size comparison of the white dwarf, Sirius B, and Earth. (NASA/ESA)

Sirius B was only the second white dwarf discovered after 40 Eridani B was found in 1783 by German-born British astronomer William Herschel (1738-1822). It would be 1917 before the third recognized white dwarf, the nearby van Maanen’s Star, was discovered by Dutch-American astronomer Adriaan van Maanen (1884-1946) (see “The First Observational Evidence for Extrasolar Planets”). And it would be decades more before the physics of these objects and their place in stellar evolution were understood. Being the closest known white dwarf as well as being located in a binary system with a fairly short orbital period (which allows its mass to be precisely measured), Sirius B is one of the best studied white dwarf stars. New observations of this object continue to let astronomers probe their understanding of this class of object.

With one dim companion known in the Sirius system, by 1894 there was speculation that hints of additional wobbles in the motions of the two stars might indicate the presence of yet another dim companion. In the 1920s there were reported observations of a dim companion with a V magnitude of about 12 possibly in a two-year orbit around Sirius. Radial velocity measurements of Sirius A between 1899 and 1926 hinted at a small companion in a 4.5-year orbit. None of these findings were ever confirmed, however. And even in the decades that followed, continued analysis of astrometric measurements of the motions of Sirius A and B had mixed results with some researchers failing to find any evidence of a purported additional wobble.

This plot shows the individual data points and the orbit of Sirius B derived by Benest and Duvent in 1995. The scales on the plot axes are in arc seconds centered on Sirius A. Click on image to enlarge. (Benest & Duvent)

The definitive analysis of the 20^th century for the astrometric observations of Sirius was published by French astronomers Daniel Benest and J.L. Duvent in 1995. Using all of the published data on the positions of Sirius A and B from 1862 to 1979, Benest and Duvent derived the orbits of these two stars about the system’s barycenter. Looking at the small differences or residuals remaining after the orbital motions of Sirius A and B were taken into account, the French astronomers found a six-year periodicity with an amplitude of only 90 milliarc seconds. Unfortunately, there are no stable orbits around Sirius A with a period greater than about two to four years (depending on the criteria used) because of the presence of the B-component in its elliptical orbit so the straightforward interpretation of a third object in an orbit with a period of about six years (corresponding to a semimajor axis of four AU) was not physically likely. Benest and Duvent concluded, however, that there might be a small companion with a mass of 0.05 M_Sun (or about 50 times that of Jupiter or M_J) in a close orbit around Sirius A that might carry it up to about three arc seconds (equivalent to a projected distance of 8 AU) from its bright host as seen from the Earth. Numerous attempts to observe this hypothesized 50 M_J object have all failed to find anything (see “Sirius: The Search for Companions Continues”).

New Observations & Analysis

The newest analysis of the dynamics of the Sirius system with Howard Bond (Penn State/STScI) as the lead author has recently been accepted for publication in The Astrophysical Journal. Because of the proximity of the Sirius system and its importance in refining our knowledge of both massive main sequence stars and white dwarfs, this team of astronomers started making regular observations of Sirius using NASA’s Hubble Space Telescope (HST) beginning in 2001. Starting with earlier WFPC2 (Wide Field Planetary Camera 2) imagery from March and May 1997 of Sirius, the team added ten more WFPC2 observation sessions between October 2001 and January 2008. Following the replacement of WFPC2 during the STS-125 servicing mission in May 2009, five more observation sessions were conducted between September 2010 and August 2016 using the new WFC3 (Wide Field Camera 3).

This Hubble Space Telescope image, taken on October 15, 2003, shows Sirius A along with its faint faint companion, Sirius B. Astronomers overexposed the image of Sirius A (center) so that the dim Sirius B (the dot in the lower left) could be seen (along with diffraction effects from the telescope and its imaging system). (NASA, ESA, H. Bond -STScI and M. Barstow – University of Leicester)

Since the 19 years of HST imagery followed the motions of Sirius A and B through less than half of their orbits about each other, Bond et al. determined that the addition of historical ground-based observations which covered more of the orbit would greatly improve the accuracy of their orbital analysis despite their lower astrometric accuracy. Between 1965 and 1984, American astronomer Irving Lindenblad (1929-2011) had conducted a long term photographic astrometry program of the Sirius system. Lindenblad used the 66-centimeter refractor at the US Naval Observatory (USNO) in Washington, DC fitted with a special hexagonal aperture mask to aid in photographing both stars simultaneously despite the huge differences in brightness and the limitations of the photographic plates then in use. While Lindenblad’s observations of Sirius up to 1972 had been previously analyzed and published, the next dozen years of photographs were never measured and analyzed. In order to recover these valuable data, a member of the team and coauthor of Bond et al., Miranda Seitz-McLeese (USNO), digitized and measured 166 photographic plates Lindenblad had acquired on 66 different nights between 1970 and 1984. Although the typical position uncertainty of the USNO measurements was a factor of several greater than those of HST, the combined data set now covered almost and entire orbit resulting in a net increase in the accuracy of the analysis. Finally, Bond et al. included in their analysis an additional 2,350 historical measurements of the separation and position angle of Sirius A and B stretching back to Clark’s original observations of 1862 increasing coverage to three full orbits of this binary system.

A sample digitized image of a photograph of Sirius (with Sirius B to the lower right) acquired using the 66-cm refractor at the US Naval Observatory in Washington, DC. (Bond et al.)

The analysis of Bond et al. found that the orbit of Sirius A-B has a period of 50.1284±0.0043 years, an eccentricity of 0.59142±0.00037 and a semimajor axis of 7.4957±0.0025 arc seconds. Using the value for the absolute parallax adopted by Bond et al., 0.3789±0.0014 arc seconds (yielding a distance of 8.61±0.03 light years) based on the weighed mean of Hipparcos and a compilation of ground-based measurements from the Yerkes and Allegheny Observatories, the actual size of the semimajor axis is 19.783±0.073 AU. The dynamical masses for the two stars were found to be 2.063±0.023 M_Sun and 1.018±0.011 M_Sun for A and B, respectively. The uncertainties in both of the derived masses are dominated by the uncertainty in the adopted absolute parallax measurement with the other sources of error being an order of magnitude or more smaller. In the future, more accurate measurements of the parallax of Sirius on its own will help drive down the current mass uncertainties in this analysis by up to a factor of several. All of these parameters agree well with earlier results.

All of the historical and HST data from 1862 to 2016 showing the position of Sirius B in its orbit around Sirius A used by Bond et al. in their analysis. Click on image to enlarge. (Bond et al.)

A comparison of this mass and other properties for Sirius B with models of how white dwarfs cool and evolve indicate a “cooling age” of about 126 million years (i.e. the time since the white dwarf formed) and are consistent with a carbon-oxygen core. Including its time before evolving into a white dwarf, the total age of Sirius B is estimated to be 228±10 million years. The progenitor of the current white dwarf would have had a mass of 5.06 +0.37/-0.28 M_Sun and, when it was a main sequence star, would have been on the order of 30 times brighter than Sirius A is today shining with a V magnitude brighter than -5, if it still existed today. Assuming a metallicity (i.e. a concentration of elements heavier than helium which astronomers dub “metals”) that is 85% that of the Sun, the properties of Sirius A are consistent with a star in the 237 to 247 million year age range with an uncertainty of ±15 million years – in rough agreement with the total age of Sirius B.

These consistent ages for Sirius A and B have presented yet another problem for astronomers. Back in 1909, Danish astronomer Ejnar Hertzsprung (1873-1967), who had co-developed astronomy’s famous Hertzsprung-Russel diagram, suggested that Sirius shared its motion with similar type stars in Ursa Major and that they were all part of what became called the Ursa Major Group (UMaG) – a lose group of stars which were born about the same time and have since drifted from their stellar nursery. While much of the literature of the last century seemed to support this association, more recent work using improved data on the distances and motions of all of the proposed members has cast serious doubt that Sirius is actually part of this group after all. The estimated 500±100 million year age of the UMaG is twice as great as the improved ages of the components found by Bond et al. as well as those in some earlier work. Unless there were some significant interaction between Sirius A and B during their earlier history which could skew the derived ages of these stars (e.g. the transfer of mass between Sirius A and B, despite the fact the current orbit never brings them closer together than 8.1 AU), it now seems even less likely that Sirius is a member of UMaG.

In addition to improving our knowledge of the properties of Sirius A and B as well as helping to refine our understanding of their evolution, the HST observations can also address the question of a third object in this system. Although the HST imaging campaigns in support of this project were not optimized for searching for dim companions, there is no evidence for such an object visible in the images as has been the case with earlier ground-based searches. An analysis of the residuals in the astrometric measurements of Sirius A and B after their orbital motion is taken into account was performed to determine if an object was present. While there is an outlying data point from 2006 (likely the result of a known issue with HST’s roll angle knowledge), this astrometric analysis shows that there is no evidence for the presence of any periodic perturbations in the motions of either star down to the 5 milliarc second level.

A plot showing the residuals from the position offsets of Sirius B from A for the HST position measurements in right ascension (top) and declination (bottom) after the effects of orbital motion have been removed. There is no hint of periodic motion down to the 5 milliarc second (mas) level. Click on image to enlarge. (Bond et al.)

This new result of Bond et al. categorically excludes the possible existence of the 50 M_J object hypothesized by Benest and Duvent over twenty years ago. In the case of Sirius A, objects with masses in excess of ~35 M_J and ~15 M_J have been excluded for orbital periods of one half and two years, respectively. For Sirius B, objects with ~25 M_J and ~10 M_J have been excluded for orbital periods of 0.5 and 1.8 years (the maximum period considered by Bond et al. as being stable), respectively. While various direct imaging campaigns have set even more stringent upper limits for the mass of any objects orbiting the stars of the Sirius system, it appears that the objects suspected by some over the last century simply do not exist. In the mean time, the search for brown dwarfs and even smaller exoplanets in the Sirius system continues.

Follow Drew Ex Machina on Facebook.

General References

A Benest and J.L. Duvent, “Is Sirius a Triple Star?”, Astronomy & Astrophysics, Vol. 299, pp. 621- 628, July 1995

Howard E. Bond et al., “The Sirius System and its Astrophysical Puzzles: Hubble Space Telescope and Ground-Based Astrometry”, arXiv 1703.10625 (accepted for publication in The Astrophysical Journal), March 31, 2017 [Preprint]