Sirius: The Search for Companions Continues

With a V magnitude of -1.46, Sirius is by far the brightest star in our nighttime sky. Located in the constellation of Canis Major (the Greater Dog) and popularly known as “the Dog Star”, this bright star has been part of sky lore in many cultures across the world dating back to at least Neolithic times with its appearance marking important events in the year. In ancient Greece which has strongly influenced Western culture, for example, the initial brief appearance of Sirius on the eastern horizon just before sunrise (known as a heliacal rising) marked the beginning of the hottest months of the year. Because of this association, the Greeks named this star Seirios (in Greek, Σείριος) which means “sparkling” or “scorcher” from which we derive its modern name. The heliacal rising of Sirius in the summer and its association with Canis Major is the origin of the expression “the dog days of summer”.

While Sirius has been part of folklore since before recorded history, it has also played a role in modern astronomical lore in recent centuries. 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 Ptolemy. Halley had discovered what we now call “proper motion” of the stars.

This picture is an artist’s impression showing how the binary star system of Sirius A and its diminutive white dwarf companion, Sirius B, might appear to an interstellar visitor. (NASA/ESA)

During the early 19^th century, astronomers were able to measure the parallax of Sirius showing it to be among the closest stars to the Sun. Careful analysis of its motions led to the discovery in 1844 by Frederich Bessel of a slight wobble indicating the presence of an unseen solar-mass companion orbiting Sirius every half century. Sirius became the first “astrometric binary” to be discovered. It would be nearly two decades before this companion, a dim white dwarf known as Sirius B, was finally observed visually. After the confirmation of the existence of Sirius B, astronomers have sought additional unseen companions – a search which continues to this day.

Background

Sirius, also known by its Bayer designation α Canis Majoris, is a hot A1V main sequence star with a mass of 2.0 times that of the Sun, 1.7 times its radius and 25 times its luminosity. The best measurements of its parallax indicates that it is 8.60 light years away making it 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 Sirius being the brightest star in the nighttime sky. Models of stellar evolution indicate that Sirius is a young star with an age estimated to be only about 225 to 250 million years.

Based on an 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 object with a period of about a half a century. 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.

Observations over the coming decades confirmed those of Bessel and Clark showing that Sirius B was in a 50.09-year orbit with a semimajor axis of 19.8 AU. With an orbital eccentricity of 0.592, the distance between Sirius B and its much brighter primary, formally designated Sirius A, varies from 8.1 to 31.5 AU – roughly equivalent to the distance of Saturn and Neptune, respectively, in our own Solar System. With a mass of 0.98 times that of the Sun but with only 0.026 times the luminosity, 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 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 12,000 kilometers and a surface temperature of 25,000 K. With so much mass squeezed into such a small volume, Sirius B has a density of 2.3 million times that of water and a surface gravity 350,000 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. According to the work of Liebert at al., “The Pup” was formed around 124 million years ago when the progenitor star, with a mass of about five times that of the Sun, ran out of hydrogen in its core and quickly evolved off the main sequence. When it was in its prime as a main sequence star, Sirius B would have been one and a half orders of magnitude brighter than the primary of today’s system and, if it were still around today, would shine at a V magnitude of around -5.

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 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 are taken into account, the French astronomers found a six-year periodicity with an amplitude of only 0.09 arc seconds. Unfortunately, there are no stable orbits around Sirius A with a period greater than about four years 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 times that of the 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.

New Observations

One method to detect such a large companion orbiting a bright star would be the precision radial velocity measurement technique which has been used so successfully to detect extrasolar planets and brown dwarfs over the last two decades. Unfortunately, hot A-type stars do not have the large number of stable spectral features that cooler M through F type stars possess which are required for this technique to work best. Combined with natural noise from surface activity of such a young star and its high mass, radial velocity measurements of Sirius simply do not have the required accuracy.

Since a 225 to 250 million year old brown dwarf with a mass of 50 M_J like that predicted by Benest and Duvent would still be radiating a lot of heat left over from its formation, it should be detectable by direct imaging in the infrared (IR) where the difference in brightness with Sirius A would be significantly less. Over the last quarter of a century, several teams of astronomers using increasingly sophisticated instruments have attempted to search for close-orbiting substellar companions with no luck. The most recent attempt at direct imaging is described in a paper just accepted for publication with Arthur Vigan of the European Southern Observatory (ESO) as the lead author.

The SPHERE instrument is shown shortly after it was installed on ESO’s VLT Unit Telescope 3. The instrument itself is the black box, located on the platform to one side of the telescope. (ESO)

For the new observations, Vigan et al. used the SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) instrument on ESO’s VLT in Paranal Observatory in Chile. Fitted to the Nasmyth focus of one of VLT’s four 8.2-meter telescopes, SPHERE with a built-in adaptive optics system is specifically designed to perform high contrast imaging near bright stars to observe and characterize young extrasolar planets in near-IR wavelengths. Originally, Vigan and his team were allotted two hours of observation time with this instrument which would have been evenly divided between Sirius A and B. Unfortunately, stray light from the very bright Sirius A only 9.9 arc seconds away prevented SPHERE’s adaptive optics system from working properly when looking at Sirius B so all of the observation time was focused on the A component.

Vigan et al. made four separate observation runs on the night of December 7, 2014 totaling one hour and 49 minutes. Observations were made in the Y, J, H and K bands running from wavelengths of 1.0 to 2.2 μm with typical seeing of 0.43 arc seconds. After extensive processing of the data to remove the effects of light from Sirius A, the astronomers could find nothing in the range of 0.2 to four arc seconds from Sirius. Further analysis of the data, which included injecting simulated signals from planets of various brightnesses into their data to test the detection limits, set the tightest upper limits on close-orbiting companions to date.

This plot shows the detection limits as a function of projected distance from Sirius A for the observations by Vigan et al. using VLT’s SPHERE system in December 2014. Click on image to enlarge. (Vigan et al.)

Based on models of giant planet and brown dwarf evolution as well as the assumed 225 to 250 million year age of Sirius, Vigan et al. concluded that their observations were capable of detecting an object more massive than 11 M_J at a projected distance of 0.5 AU, 6 to 7 M_J in the 1 to 2 AU range and about 4 M_J at 10 AU. Since these observations were made on only one night, there is a chance that an orbiting body could have had a projected distance at this time too close to Sirius to be detected. Vigan et al. performed a Monte Carlo orbital analysis and concluded that they can reject the presence of a 8 M_J exoplanet in an orbit with a semimajor axis 1 AU (equivalent to an orbit with a period of 0.8 years) at a 50% confidence level. This probability soars to 99% for companions with masses greater than 11 M_J in orbits with semimajor axis values larger than 2 AU.

Future observations will be required to tighten these constraints further but, when combined with the negative results of other recent searches, it seems increasingly unlikely that the 50 M_J brown dwarf hypothesized by Benest and Duvent to explain the astrometric observations actually exists. As has happened all too frequently in recent years, the tiny astrometric signature may be nothing more than instrument artifacts (see “The Search for Planets Around Barnard’s Star”). Despite the negative result, Sirius could still host a number of planets as large or larger than Jupiter that could easily evade detection opening the prospects of what future observations with still more capable instruments might one day find orbiting our neighbor. Until then, the search for additional (now most likely, planetary) companions of Sirius continues.

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Related Video

Here is a short ESA video zooming in to a Hubble Space Telescope view of Sirius B.

Related Reading

“The First Observational Evidence for Extrasolar Planets”, Drew Ex Machina, October 21, 2014 [Post]

“The Search for Planets Around Alpha Centauri”, Drew Ex Machina, August 11, 2014 [Post]

“The Search for Planets Around Alpha Centauri II”, Drew Ex Machina, September 25, 2014 [Post]

“The Search for Planets Around Proxima Centauri”, Drew Ex Machina, February 23, 2015 [Post]

“The Search for Planets Around Barnard’s Star”, Drew Ex Machina, April 23, 2015 [Post]

General References

Richard Hinkley Allen, Star Names: Their Lore and Meaning, Dover Publications, 1963

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

James Liebert et al., “The Age and Progenitor Mass of Sirius B”, The Astrophysical Journal, Vol. 630, No. 1, pp. L69-L72, September 1, 2005

A. Vigan et al., “High-Contrast Imaging of Sirius A with VLT/SPHERE: Looking for Giant Planets Down to One Astronomical Unit”, arXiv 1509.00015 (accepted for publication in Monthly Notices of the Royal Astronomical Society), August 31, 2015 [Preprint]