Tuesday, March 31, 2015

Surviving Population III Stars in the Galaxy

Population III stars are the first generation of stars to form in the universe. At that time, the universe only had hydrogen and helium since stars have yet to exist to forge the heavier elements. As a result, Population III stars are composed entirely of hydrogen and helium (i.e. no metals). Although a number of extremely metal-poor stars have been found, no Population III star has yet been directly observed.

Population III stars are predicted to be extremely massive and hot. These stars have short lifespans as they burnout rapidly. Nevertheless, simulations of the formation of Population III stars show that in the initial mass distribution of these stars, the least massive ones may be less massive than the Sun. If Population III stars with less than 0.8 times the Sun’s mass formed in the early universe, the lower mass and correspondingly longer lifespans means that they should still be present today.


Although these low-mass Population III stars started out metal-free, over the course of their long lives, the can accrete sufficient metal-enriched material from the interstellar medium to pollute and mask their primordial nature. The interstellar medium is comprised of gas and dust. A study by Jarrett L. Johnson (2015) shows while gas can accrete onto low-mass Population III stars, the accretion of dust can be inhibited as pressure from radiation emitted by these stars can push dust particles away. If only gas accretion occurs, it can create a unique chemical signature where there is an enhancement in elements that came from gas accretion and depletion in elements that came from dust accretion.

Carbon and oxygen tend to be in the gas phase while iron tends to be in the dust phase. If low-mass Population III stars are only polluted by gas accretion, then they are expected to be enriched in carbon and oxygen, and depleted in iron. Such a chemical signature is similar to a category of stars known as carbon-enhanced metal poor (CEMP) stars. CEMP stars are some of the most metal-poor stars known and some fraction of them might turn out to be Population III stars that have been polluted by gas accretion.

Reference:
Jarrett L. Johnson (2015), “The chemical signature of surviving Population III stars in the Milky Way”, arXiv:1411.4189 [astro-ph.GA]

Saturday, March 28, 2015

Potassium in the Atmosphere of a Hot Jupiter

Observations have shown that there is a lot of diversity in the atmospheres of hot Jupiters. Some hot Jupiters have atmospheres that are obscured by high altitude hazes, while others have relatively clear skies. When a transiting hot Jupiter passes in front of its host star, the amount of starlight it obscures depends on the composition of its atmosphere. This is because the presence of atmospheric species such as sodium and potassium let less light through at certain wavelengths. Since the inferred size of a hot Jupiter is based on the amount of starlight it obscures, the planet’s size can appear slightly larger at wavelengths where the planet’s atmosphere is more opaque. 

Figure 1: Artist’s impression of a planet transiting a star. Image credit: ESA/ATG medialab.

HAT-P-1b is a hot Jupiter in a close-in 4.47 day orbit around a Sun-like star. Being so close to its host star, its dayside is heated to temperatures well over 1,000 °C. HAT-P-1b has 1.319 times the radius and 0.525 times the mass of Jupiter, giving it an average density of only 0.345 g/cm³. A study by Wilson et al. (2015) reports the detection of potassium in the atmosphere of HAT-P-1b. Four transits of HAT-P-1b were observed - two at wavelengths that are not affected by the presence of potassium (6792 Å and 8844 Å) and two at wavelengths where potassium is more opaque (7582.0 Å and 7664.9 Å).

The measured planet-to-star size ratio is 0.1176 ± 0.0013 at 6792 Å and 0.1168 ± 0.0022 at 8844 Å. At the other two wavelengths, the planet’s atmosphere becomes more opaque, causing the planet to appear larger with a planet-to-star size ratio of 0.1248 ± 0.0014 at 7582.0 Å and 0.1268 ± 0.0012 at 7664.9 Å. Basically, the detection of potassium is based on the observed increase in the planet-to-star size ratio, Δ0.0073 ± 0.0017 at 7582.0 Å and Δ0.0094 ± 0.0016 at 7664.9 Å. The strong detection of potassium is due to the high temperatures on HAT-P-1b which keep the upper atmosphere of the planet puffed up. When a transit of HAT-P-1b is observed at a wavelength where potassium is opaque, the planet’s puffed up upper atmosphere allows less light of that particular wavelength through, resulting in a larger planet-to-star size ratio.

Figure 2: Transit lightcurve of HAT-P-1b observed in the 6792 Å and 8844 Å wavelengths. Wilson et al. (2015).

Figure 3: Transit lightcurve of HAT-P-1b observed in the 7582.0 Å and 7664.9 Å wavelengths where potassium is more opaque. The half transits were obtained on 19 November 2010 whilst the full transits were obtained on 26 November 2013. Wilson et al. (2015).

Reference:
Wilson et al. (2015), “GTC OSIRIS transiting exoplanet atmospheric survey: detection of potassium in HAT-P-1b from narrowband spectrophotometry”, arXiv:1503.07165 [astro-ph.EP]

Friday, March 27, 2015

Hints of Earth-Size Planets around Alpha Centauri B

Alpha Centauri Bb is a possible Earth-size exoplanet in a 3.2 day orbit around Alpha Centauri B, a K-type star located 4.37 light-years away. The discovery of Alpha Centauri Bb was announced by Dumusque et al. (2012) from measurements made using the High Accuracy Radial Velocity Planet Searcher (HARPS) instrument at the La Silla Observatory in Chile. The HARPS instrument measures the tiny gravitational wobbles an orbiting planet exerts on its host star. Since then, more attempts have been made to try and confirm the existence of Alpha Centauri Bb.

Figure 1: Artist’s impression of a hot Earth-size planet. Image credit: Invader Xan.

Because Alpha Centauri Bb orbits so close to its host star, the probability that it transits in front of its host star as observed from Earth is estimated to be between ~10 to 30 percent. Using the Hubble Space Telescope (HST), an attempt was made by Demory et al. (2015) to detect the transit of Alpha Centauri Bb in front of its host star Alpha Centauri B. The search involved observing Alpha Centauri B twice in 2013 and 2014, for a total of 40 hours. With a confidence level of 96.6 percent, the search ruled out any transits of Alpha Centauri Bb.

Nevertheless, a single transit-like event was found in the data that was collected. This transit-like event lasted for 3.8 hours and it is consistant with the presence of another Earth-size planet on a longer period orbit around Alpha Centauri B. The Alpha Centauri system, consisting of Alpha Centauri A, Alpha Centauri B and the red dwarf Proxima Centauri, are the closest stars to the Sun. The search for nearby alien worlds is a fascinating prospect and additional observations will be necessary to verify the presence of multiple Earth-size planets around Alpha Centauri B.

Figure 2: Light curve of the single transit-like event indicating the possible existence of an Earth-size planet on a longer period orbit around Alpha Centauri B. Demory et al. (2015).

References:
- Dumusque et al., “An Earth-mass planet orbiting α Centauri B”, Nature 491, 207-211 (08 November 2012)
- Demory et al. (2015), “Hubble Space Telescope search for the transit of the Earth-mass exoplanet Alpha Centauri Bb”, arXiv:1503.07528 [astro-ph.EP]

Wide Pair of Neutron Stars

A pulsar is basically a rotating neutron star that emits a bipolar beam of electromagnetic radiation. This beam of electromagnetic radiation is only visible when directed at the observer. As the neutron star rotates, the beam periodically sweeps pass the observer, resulting in a pulsed appearance of the emission. Hence the term “pulsar” denotes such a neutron star. Of all the pulsars known to date, ~10 percent are in binary systems with white dwarf, neutron star or main sequence star companions.


PSR J1930-1852 is a pulsar in a double neutron star system (DNS). Both neutron stars orbit each other every 45 days, making this system the widest known pair of neutron stars. The rest of the known DNS systems are much more tightly bound and have orbital periods of a few hours to a few days. PSR J1930-1852 itself spins at a rate of over 300 times per minute. Although this may seem fast, it is actually extremely slow for a pulsar. The combined mass of PSR J1930-1852 and its companion neutron star is 2.6 times the Sun’s mass. PSR J1930-1852 is estimated to contain less than 1.3 times the Sun’s mass while its companion is estimated to contain more than 1.3 times the Sun’s mass.

Neutron stars form from the collapsed cores of massive stars in supernova explosions. DNS systems are rare because the formation of such a binary system requires it to survive two supernova explosions. PSR J1930-1852 is likely to have formed before its companion star went supernova. During that time, it accreted material from its companion star and spun-up. This process of accretion and spin-up is known as recycling. However, for PSR J1930-1852, the recycling process was shorter than average in duration and/or inefficient. The companion star went supernova before PSR J1930-1852 spun-up sufficiently.

Reference:
Swiggum et al. (2015), “PSR J1930-1852: a pulsar in the widest known orbit around another neutron star”, arXiv:1503.06276 [astro-ph.HE]

Thursday, March 26, 2015

Birth of a Quadruple Star System

Roughly half of all stars reside in multiple star systems - binaries, triples, quadruplets, quintuples, etc. There appears to be a higher prevalence of multiplicity for stars that are still forming as compared to fully-formed stars. This is because dynamical interactions tend to scatter apart multiple star systems. Observations of Bernard 5, a cloud of gas ~800 light-years away, show the presence of a quadruple star system in its beginning stages of formation.

Figure 1: Barnard 5, embedded in dust (blue) as seen with ESA’s Herschel Space Observatory, in infrared light. Credit: Bill Saxton, NRAO/AUI/NSF.

The quadruple system in Bernard 5 consists of one young protostar and three dense condensations of gas. All four objects are currently gravitationally bound. The protostar, B5-IRS1, is a low-mass star estimated to be roughly 0.1 times the Sun’s mass. As for the three gas condensations, they are expected to gravitationally collapse to form stars one-tenth to one-third the mass of the Sun in ~40,000 years.

At present, the project separations between the protostar and the three condensations, B5-Cond1, B5-Cond2 and B5-Cond3, are 11,400 AU, 3,300 AU and 5,100 AU, respectively. The quadruple system is unstable and will likely disperse on a timescale of roughly 500,000 years. Nevertheless, the closest pair, B5-IRS11 and B5-Cond2, will likely remain gravitationally bound as a binary system.

Figure 2: Artist’s conception of the B5 complex as seen now (left), and as it will appear as a quadruple star system in ~40,000 years (right). Credit: Bill Saxton, NRAO/AUI/NSF.

Figure 3: Dust continuum emission maps of Barnard 5. Pineda et al. (2015).

Reference:
Pineda et al., “The formation of a quadruple star system with wide separation”, Nature 518, 213-215 (12 February 2015)

Sunday, March 22, 2015

Accreting Material from a Hot Jupiter

HD 189733 b is a hot Jupiter circling a K-type star that is somewhat less massive and less luminous than the Sun. Like Jupiter, HD 189733 b is a gas giant planet comprised mostly of hydrogen and helium. However, HD 189733 b orbits very close to its host star, about 30 times closer than Earth is from the Sun. The planet has an orbital period of only 2.22 days. During each orbit, HD 189733 b also transits across the face of its host star. HD 189733 b is slightly more massive and slightly larger in size than Jupiter. Because it is so close to its host star, HD 189733 b is subjected to very intense irradiation, about 275 times more insolation than Earth gets from the Sun. Due to the intense heating, the outer atmosphere of HD 189733 b is known to be evaporating into space.

Figure 1: Artist’s impression of HD 189733 b. Credit: NASA’s Goddard Space Flight Center.

The host star of HD 189733 b was observed with the Cosmic Origin Spectrograph (COS) on the Hubble Space Telescope (HST). The observations revealed the presence of an active spot on the surface of the star. An indication that HD 189733 b is responsible for this feature on its host star is that the active spot comes in and out of view in phase with the orbital motion of HD 189733 b. The active spot is situated 70 to 90 degrees ahead of the sub-planetary point. Essentially, the sub-planetary point is the point on the star’s surface where HD 189733 b appears directly “overhead”. Magneto-hydrodynamic (MHD) simulations point out that the active spot is consistant with material evaporating from HD 189733 b that is steadily accreting onto the surface of the star. The rate at which material is accreted from HD 189733 b is estimated to be ~200 million kg/s.

Figure 2: MHD simulations showing the interactions between HD 189733 b and its host star. The star rotates counter-clockwise and the planet orbits the star along the same direction. The two “+” symbols shown on the left panel indicate the location of the star (red disk) and the planet (green disk). A close up of the impact region is depicted in the right panel, where the motion of the accreting plasma is marked with arrows. The plasma is funnelled by the magnetized stellar wind in an almost radial trajectory close to the star (A), it forms a “knee” structure that consists of hot and dense plasma (B), and then accretes in a spot ahead of the orbital phase (C). The knee (B) of the stream and the active spot (C) are the main sites emitting the observed enhanced flux of ultraviolet and X-rays. Pillitteri et al. (2015).

Reference:
Pillitteri et al. (2015), “FUV variability of HD 189733. Is the star accreting material from its hot Jupiter”, arXiv:1503.05590 [astro-ph.SR]

Saturday, March 21, 2015

Hypervelocity Star Ejected by a Supernova


Hypervelocity stars are a class of stars that travel at such high velocities that the galaxy’s gravitational field cannot contain them. These stars eventually leave the galaxy and continue on into intergalactic space. Many hypervelocity stars have trajectories that start out near the center of the galaxy and were likely yanked out by interactions with the supermassive black hole residing there.

US 708 is a hypervelocity star estimated to be travelling at a velocity of about 1,200 km/s. It is currently the fastest unbound star known in the galaxy. Basically, the observable motion of a star is comprised of two components - radial motion and proper motion. Radial motion is motion along the line of sight and proper motion is motion across the plane of the sky.

Measurements of both the radial and proper motions were used to determine the true velocity of US 708. The radial motion of US 708 was estimated from measurements of the Doppler shift in the star’s spectral lines and the measurements show that the star is moving away rapidly with a speed of about 900 km/s. As for the star’s proper motion, it was estimated based on archival images showing the past positions of US 708.

Spectra of US 708 showing the redshift in the spectral lines due to the star’s large radial velocity. S. Geier et al. (2015).

Reconstruction of the trajectory of US 708 to trace its origin show it did not come from anywhere near the center of the galaxy. This indicates US 708 was not flung out by the galaxy’s supermassive black hole. Instead, US 708 is believed to have once been part of a tight binary system with a more massive companion white dwarf star. US 708 is itself a compact sub-dwarf O-type star, basically the helium core of a former red giant star that was stripped of its hydrogen envelope due to interactions with its companion.

US 708 was close enough to its companion that its companion could accrete material from it. As a result, the companion grew in mass until it reached a critical mass where it exploded in a thermonuclear supernova. This event is estimated to have occurred about 14 million years ago and it liberated US 708 from its tight orbit. From there, US 708 shot out in a straight line with the same high velocity it once orbited its companion and became a hypervelocity star. The current velocity of US 708 suggests that the tight binary system it once belonged to had an orbital period of only 10 minutes.

Another indication US 708 was once part of a tight binary system can be seen from its fast rotation. The projected rotational velocity of US 708 exceeds ~100 km/s. The fast rotation is due to the fact that in a tight binary system, the two stars are tidally-locked where the rotational period of each star is equal to the orbital period. Being so near to a supernova, the surface of US 708 may be significantly enriched with heavy elements from the supernova.

Reference:
S. Geier et al. (2015), “The fastest unbound star in our Galaxy ejected by a thermonuclear supernova”, arXiv:1503.01650 [astro-ph.SR]

Friday, March 20, 2015

A Yellow Hypergiant with a Close Companion

Yellow hypergiants (YHGs) are extremely rare stars. Only several of them have been identified in the galaxy so far. These stellar oddities are stars that are at a particularly unstable and rapidly changing phase of stellar evolution. They are known to expel prodigious amounts of material, creating complex circumstellar environments. An international team of collaborators using ESO’s Very Large Telescope Interferometer (VLTI) has found that the star HR 5171 is the largest YHG known.

HR 5171 measures more than 1,300 times the Sun’s diameter, and it is one of the biggest and rarest stars in the galaxy. If placed in the solar system, its visible surface would extend all the way to between the orbits of Jupiter and Saturn. HR 5171 is located about 12,000 light years away and it shines with roughly a million times the Sun’s luminosity.

Artist’s impression of the yellow hypergiant star HR 5171. Image credit: ESO.

Observations using the VLTI also show that HR 5171 has a smaller companion star going around it every 1,300 days. The companion star is close enough that it is actually in contact with the main star, resulting in what known as a contact binary. The presence of the companion star distorts the overall shape of HR 5171, causing the observed flux to modulate by 17 ± 5 percent.

Despite its smaller size, the companion star is expected to have a significant influence on the subsequent evolution of HR 5171. Piror to the recent observations by the VLTI, observations of HR 5171 over the past 40 years have shown that the star is cooling as it enlarges. Future observations of HR 5171 will provide a fascinating insight into the evolution of these rare stellar behemoths.

Reference:
Chesneau et al. (2014), “The yellow hypergiant HR 5171 A: Resolving a massive interacting binary in the common envelope phase”, arXiv:1401.2628 [astro-ph.SR]

Thursday, March 19, 2015

An Extreme Planetary System around a Hot Star

KIC 10001893 is one of 19 subdwarf B (sdB) stars observed by NASA’s Kepler space telescope during its primary mission. An sdB star is a type of very hot star, typically containing ~0.5 times the Sun’s mass. It represents one of the final stages in the evolution of some stars. An sdB star forms when a red giant star loses its outer hydrogen layers before helium in its core starts fusing into carbon. As a consequence, an sdB star is almost entirely comprised of helium, with only a thin outer layer of hydrogen.

R. Silvotti et al. (2014) report the detection of three Earth-size planet candidates in very close-in orbits around KIC 10001893. The three planet candidates have orbital periods of 5.273, 7.807 and 19.48 hours. KIC 10001893 is a very hot star with an estimated surface temperature around 26,700 K. For comparison, the Sun’s effective surface temperature is only 5,778 K. Being so close to such a hot star, the day sides of the three planet candidates are expected to be heated to extraordinary temperatures, possibly up to several thousand degrees K.

Figure 1: Artist’s impression of a superheated planet.

The extreme planetary system around KIC 10001893 resembles the two Earth-size planets previously detected by S. Charpinet et al. (2011), dubbed Kepler-70b and Kepler-70c, in tight orbits around Kepler-70, another sdB star similar to KIC 10001893. Like the two planets around Kepler-70, the planetary system around KIC 10001893 probably formed when the star evolved into a red giant star and engulfed three massive gas giant planets, each having a few times the mass of Jupiter.

Once inside the red giant star, drag removes energy from the orbits of the three giant planets and they spiral towards the core of the star. At the same time, the gaseous envelopes of each of the three giant planets are stripped away, leaving behind their rocky cores. The spiral-in of the giant planets deposits orbital energy in the outer layers of the red giant star, aiding in the expulsion of these layers. Eventually, all that remains is a hot sdB star hosting three small remnant rocky cores in very close-in orbits.

Figure 2: (a) The two planets around Kepler-70 were initially massive gas giant planets orbiting much farther from their host star. (b) When the host star expanded to become a red giant star, its outer layers engulfed the two planets. This process stripped away the outer gaseous layers of the two planets and at the same time caused the orbits of the two planets to spiral inwards. (c) The two in-spiralling planets probably deposited sufficiently energy within the red giant star to aid the star in expelling its outer layers. The end result is a hot sdB star with two rocky remnants of the initial planets in extremely tight orbits. S. Charpinet et al. (2011).

Alternatively, based on a study by Bear & Soker (2012), the planetary system around KIC 10001893 could also have formed from the destruction of a single massive gas giant planet. The spiral-in of the giant planet within the red giant star releases orbital energy which unbinds the outer layers of the red giant star, leaving behind the core which then becomes a hot sdB star. The spiral-in also brings the giant planet close enough to the sdB star to be tidally destructed where its gaseous envelope is completely stripped away and its dense rocky core is broken up into several Earth-size fragments, 3 of which still circle KIC 10001893.

References:
- R. Silvotti et al. (2014), “Kepler detection of a new extreme planetary system orbiting the subdwarf-B pulsator KIC10001893”, arXiv:1409.6975 [astro-ph.EP]
- S. Charpinet et al. (2011), “A compact system of small planets around a former red-giant star”, Nature, 480, 496
- Bear & Soker (2012), “A tidally destructed massive planet as the progenitor of the two light planets around the sdB star KIC 05807616”, arXiv:1202.1168v1

Wednesday, March 18, 2015

Event Horizon of a Supermassive Black Hole

Observations of the supermassive black hole (SMBH) at the center of the supergiant elliptical galaxy M87 made using the Event Horizon Telescope and the Hubble Space Telescope indicates the presence of an event horizon and rules out the existence of a physical surface. Indeed, a black hole does not have a physical surface. Instead, the event horizon is a non-physical boundary around a black hole where the escape velocity becomes greater than the speed of light. Anything which crosses the event horizon, including light, is no longer visible to outside observers.


Accretion of material onto compact objects with physical surfaces such as white dwarfs and neutron stars lead to very different observational outcomes. As accreted material violently crashes down onto a physical surface, the remaining kinetic energy of the accreted mass is thermalized and radiated away as radiation, generating a considerable flux of infrared and optical emission.

The SMBH at the heart of M87 has an estimated ~6 billion times the Sun’s mass and it is accreting material at a prodigious rate. Measurements of the flux from the SMBH show it is at least an order of magnitude below what is expected if a physical surface were present. This indicates that the central compact object in M87 is a supermassive black hole with an event horizon and not some very massive exotic object with a physical surface. Once accreted material falls through the event horizon of a black hole, it is no longer visible to the outside universe.

Reference:
Broderick et al. (2015), “The Event Horizon of M87”, arXiv:1503.03873 [astro-ph.HE]