Observing the fastest known star in our galaxy

[Martin Meredith]

Back in December I posted about extreme runaway and hypervelocity stars, which are stars that have somehow reached a velocity that will allow them to escape the clutches of our galaxy one day. There I focusing on one called HVS 5 which is believed to have gained exit velocity by an encounter with the super-massive black hole at the centre of the galaxy. 

Out of the 20 or so HVS that have been identified, one stands out as different from the rest: HVS 2. This star is also known as US 708 due to its radial velocity of 708 km/s. At an overall velocity of 2.6 million miles per hour it is the fastest known star in the galaxy. Apart from this claim to fame (and somehow related to it!) HVS2 is special because it is thought not to have been caused by an encounter with the black hole, but instead is believed to be the result of a supernova explosion. Models predict that HVS 2 was a member of a binary star system, one of whose components exploded, sending it out at the enormous speed we now observe.

HVS 2 is in Ursa Major, not far from galaxy UGC 5091. It is pretty faint at Vmag = 18.8 but is within reach of EAA techniques. You can find it at 9h 33' 21"  +44 17 06. Here's a shot from earlier this year.

I waited until this week to complete the story in order to capture another object that links to the HVS 2 story. The favoured ejection scenario for HVS 2 is actually a double-detonation of a white dwarf. The mechanism suggested is that helium is deposited on the white dwarf surface by accretion; this ignites, triggering the explosion of the white dwarf and leading to the ejection at hypervelocity of the helium star. Remarkably, a candidate system capable of such double detonation has been discovered. This is known as CD-30 11223 and lies in Centaurus at a declination of -30 degrees not too far from NGC 5494 (exact position: 14h 11' 16"  -30 53 4). The star is quite bright (Vmag 11.9) and is shown on this single 30s shot from earlier this week:

Admittedly, it is not much to look at, but consider a fascinating paragraph in the discovery article linked to above, pointing out that it is by far the nearest known SN Ia progenitor to Earth, and that when this star explodes (in an estimated 42 million years time), it will be visible from Earth with an apparent magnitude as high as -7.6, as bright as the 1006 supernovae, which itself was the 'brightest stellar event in recorded history'. One for future generations to look forward to. 

For more details, I recommend the web page  of Warren Brown, who discovered the first HVS. It includes a link to his 2015 HVS article in the Annual Review of Astronomy and Astrophysics.

Martin