Astronomers are announcing refinements and new findings relating to the 15 Sagittae system, which contains a younger version of our Sun, and a cool dim brown dwarf companion. These results are being presented by Michael J. Dulude, Dr. Edward F. Guinan, Laurence E. DeWarf, and Dr. George McCook of Villanova University, Villanova, PA., and Dr. Ignasi Ribas, of the Institut d’Estudis Espacials de Catalunya, Bellaterra, Spain to the American Astronomical Society meeting in San Diego, California. These findings are of particular interest because they refine the age and mass of the brown dwarf companion. This study furthers our understanding of the young Sun and its (sometimes drastic) effects on the early solar system at a time when life was just gaining a foothold on the Earth and Mars was possibly much warmer and wetter than it is today.

 

Determination of an Accurate Age for 15 Sagittae

and its Brown Dwarf Companion

15 Sagittae (HR 7672, 15 Sge A) is a 5^th magnitude yellow star located 58 light years away from us, in the constellation Sagittae-the arrow. This young solar proxy has been a part of a long-term monitoring project (the Villanova Sun in Time program) for over 15 years. By using solar-like stars as “proxies” for the Sun at different times during its life, an evolutionary timeline for the Sun can be pieced together.  Using data obtained from several Earth-orbiting observatories, and ground-based photometry obtained from the 0.8 meter (31.5 inch) Five College Consortium telescope on Mt. Hopkins, Az., astronomers have determined the age of 15 Sge A to be 1.8 ± 0.15 Giga (Billion) years. This result has a number of important implications, for the 15 Sge and its brown dwarf companion, as well as our own solar system.

 

Recently, Liu et al. (2002) discovered a cool dim brown dwarf orbiting 15 Sge A at a distance of between 7 and 21 AU. Adopting an age of 1.8 ± 0.15 Gyrs, the Villanova astronomers were able to narrow down the mass of the brown dwarf to between 67 and 71 Jovian masses. 15 Sge A (and by extension, the Sun ~2.8 billion years ago) had ~5-6 times stronger X-ray emissions and emitted ~2-3 times more ultraviolet radiation than the Sun today. These enhanced XUV emissions probably would have little effect on the brown dwarf companion of 15 Sge because of its large distance (7-21 AU) from the star.

 

Evolution of the Sun’s Magnetic Activity

and Coronal XUV Emissions Over Time

Using solar proxies from the Sun in Time program, the early magnetic activity and resulting high-energy coronal X-ray and chromospheric ultraviolet emissions of the Sun can be determined. (See Ribas et al. 2005 Astro-ph/0412253) These studies show that the young Sun rotated >10 times faster than the present Sun, resulting in a more energetic magnetic dynamo. However, stellar evolution theory indicates that the overall optical brightness of the young Sun was diminished up to 30% for other reasons. Thus, the young Sun was dimmer at optical wavelengths, but had XUV coronal emissions that were several hundred times stronger than the present Sun. Over time, the Sun spins down, loosing angular momentum via magnetic winds, and its dynamo-related magnetic activity and corresponding XUV emissions greatly decrease. On the other hand, the nuclear reactions in the core of the Sun accelerate causing the luminosity of the Sun to (at first) monotonically increase with age. (See below diagrams)

 

Effects of the Young Sun’s Strong XUV

Emissions on the Early Solar System

15 Sge is extremely important solar proxy because of its age. It serves as an important proxy for our Sun some 2.8 billion years ago, at a time in the early solar system when primitive life had established a firm foothold on Earth and when Mars may have been warm, wet, and suitable for life. Using the data for 15 Sge together with all of the solar proxies in the Sun in Time program, the XUV irradiances of the young Sun have been determined. (See Ribas et al. 2005 Astro-ph/0412253) These XUV irradiances of the young Sun have been used to estimate possible effects on the environments of Solar System planets. The large XUV fluxes of the younger Sun may have had major effects on the younger planets’ paleoatmospheres and environments. Also, the stronger FUV fluxes of the early Sun could have been influential in photochemical reactions that could profoundly affect life on Earth and Mars.

Some examples of the strong influence of the early active Sun on Mercury, Earth and Mars are provided in the below illustrations and diagrams.

The effects of the young Sun’s strong XUV irradiance and enhanced winds would be most profound on planets nearest to the Sun, such as Mercury, Earth and Mars.

Mercury

Mercury’s density and mass do not follow the trend of all the other terrestrial planets. One possible explanation is that Mercury’s lighter mantle/crust was ablated away by the strong (>1,000 times present values) winds and the early Sun’s higher extreme ultraviolet fluxes leaving little more than a dense iron core. (See below diagrams)

Earth

A young and active Sun may have played a major role in the evolution of the Earth’s atmosphere and possibly the origin and evolution of life. It is possible that the large FUV fluxes from the young Sun may have helped create some of the Earth’s first organic compounds, played a part in the large-scale destruction of methane, and assisted in the creation of ozone in the Earth’s atmosphere (See below diagram)

Mars

Mars’ protective magnetosphere collapsed when its core solidified some 3.5 billion years ago. There is extremely strong evidence that prior to this point, some amount of liquid water flowed across the Martian surface. Without a protective magnetic field, the outer Martian atmosphere was subjected to the ionizing effects and strong winds from the Sun, and its atmosphere began to erode. Water disassociated into 2H+O, where the lighter hydrogen was lost to space (via ion pickup), while most of the heavier oxygen combines with iron on its surface (Lammer 2003, Icarus, 165, 9L). Whatever water remained quickly froze due to plummeting temperatures and the absence of any greenhouse effect. (See below diagrams)