Pericenter Passage of the Gas Cloud G2 in the Galactic Center

Gillessen, S.; Genzel, R.; Fritz, Tobias; Eisenhauer, F.; Pfuhl, O.; Ott, Thomas; Schartmann, M.; Ballone, Alessandro; Burkert, Andreas

doi:10.1088/0004-637x/774/1/44

Cited by 88 publications

(171 citation statements)

References 29 publications

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“…So far, mainly two possible structures of G2 have been discussed, with the first scenario being that G2 is a localized overdense region within an extended gas streamer. This agrees with observations reporting that G2 is composed of a compact head and a more widespread tail (Gillessen et al 2013b;Pfuhl et al 2015). The two components are on approximately the same orbit and are connected by a faint bridge in positionvelocity diagrams, in Gillessen et al (2013a) and lasted over one year,while G2 has been stretched substantially along its orbit by tidal shearing caused by the gravitational potential of Sgr A* Article published by EDP Sciences L16, page 1 of 4 (Pfuhl et al 2015).…”

Section: Introductionsupporting

confidence: 89%

No asymmetric outflows from Sagittarius A* during the pericenter passage of the gas cloud G2

Park

Trippe

Krichbaum

et al. 2015

A&A

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The gas cloud G2 that falls toward Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, is assumed to provide valuable information on the physics of accretion flows and the environment of the black hole. We observed Sgr A* with four European stations of the Global Millimeter Very Long Baseline Interferometry Array (GMVA) at 86 GHz on 1 October 2013 when parts of G2 had already passed the pericenter. We searched for a possible transient asymmetric structure -such as jets or winds from hot accretion flows -around Sgr A* that might be caused by accretion of material from G2. The interferometric closure phases remained zero within errors during the observation time. We therefore conclude that Sgr A* did not show significant asymmetric (in the observer frame) outflows in late 2013. Using simulations, we constrain the size of the outflows that we could have missed to ≈2.5 mas along the major axis and ≈0.4 mas along the minor axis of the beam, corresponding to approximately 232 and 35 Schwarzschild radii, respectively; we thus probe spatial scales on which the jets of radio galaxies are thought to convert magnetic into kinetic energy. Because probably less than 0.2 Jy of the flux from Sgr A* can be attributed to accretion from G2, the effective accretion rate is ηṀ 1.5 × 10 9 kg s −1 ≈ 7.7 × 10 −9 M ⊕ yr −1 for material from G2. Exploiting the relation of kinetic jet power to accretion power of radio galaxies shows that the rate of accretion of matter that is finally deposited in jets is limited toṀ 10 17 kg s −1 ≈ 0.5 M ⊕ yr −1 . Accordingly, G2 appears to be mostly stable against loss of angular momentum and subsequent (partial) accretion at least on timescales 1 yr.

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Section: Introductionsupporting

confidence: 89%

No asymmetric outflows from Sagittarius A* during the pericenter passage of the gas cloud G2

Park

Trippe

Krichbaum

et al. 2015

A&A

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“…The left panel of Figure 8 clearly shows that our optimal G1 solution is significantly different from the previously published solution of Pfuhl et al (2015). As a consequence, the best-fit G1 orbit is no longer in agreement with G2ʼs optimal solution (right panel of Figure 8; G2ʼs orbit is thoroughly discussed in Gillessen et al 2012, 2013b, Phifer et al 2013). This can be understood because our data cover almost twice the time baseline presented in Pfuhl et al (2015).…”

Section: Keplerian Orbital Fit Resultsmentioning

confidence: 64%

The Post-periapsis Evolution of Galactic Center Source G1: The Second Case of a Resolved Tidal Interaction with a Supermassive Black Hole

Witzel

Sitarski

Ghez

et al. 2017

ApJ

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We present new adaptive optics (AO) imaging and spectroscopic measurements of Galactic center source G1 from W. M. Keck Observatory. Our goal is to understand its nature and relationship to G2, which is the first example of a spatially resolved object interacting with a supermassive black hole (SMBH). Both objects have been monitored with AO for the past decade (2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014) and are comparatively close to the black hole (a min ∼200-300 au) on very eccentric orbits (e G1 ∼0.99; e G2 ∼0.96). While G2 has been tracked before and during periapsis passage (T 0 ∼2014.2), G1 has been followed since soon after emerging from periapsis (T 0 ∼2001.3). Our observations of G1 double the previously reported observational time baseline, which improves its orbital parameter determinations. G1ʼs orbital trajectory appears to be in the same plane as that of G2 but with a significantly different argument of periapsis (Δω=21°±4°). This suggests that G1 is an independent object and not part of a gas stream containing G2, as has been proposed. Furthermore, we show for the first time that (1) G1 is extended in the epochs closest to periapsis along the direction of orbital motion, and (2) it becomes significantly smaller over time (450 au in 2004 to less than 170 au in 2009). Based on these observations, G1 appears to be the second example of an object tidally interacting with an SMBH. G1ʼs existence 14 yr after periapsis, along with its compactness in epochs further from the time of periapsis, suggest that this source is stellar in nature.

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“…Many papers have since added observations and ideas on individual SMBH orbiters and on the statistics and collective properties of stars in the inner Galactic center region, e.g., Ghez et al (2003a), Ghez et al (2003b), Alexander & Morris (2003), Blum et al (2003), Eisenhauer et al (2005), Gillessen et al (2009), Davies & King (2005), Gillessen et al (2013), Witzel et al (2014) and an extensive review by Genzel et al (2010).…”

Section: Introductionmentioning

confidence: 99%

Lobe Overflow as the Likely Cause of Pericenter Outburst in an SMBH Orbiter

Wilson

Devinney

2015

ApJ

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A very large lobe overflow event is suggested to explain the 0 m . 4 brightening observed in the K band at pericenter passage of the star known as S2, which orbits the Galaxyʼs supermassive black hole (SMBH). Known observed properties of S2 that contribute to lobe filling are (1) the enormous mass ratio, M M SMBH S2 , (2) S2ʼs fast rotation, and (3) S2ʼs large orbital eccentricity. Published estimates have given limiting lobe sizes of order 100-300 R ⊙ but, with S2ʼs fast rotation taken into account, the computed lobe size is much smaller, being compatible with either a main sequence OB star or a stripped evolved star. An important evolutionary consideration that predicts very large pericenter overflows is envelope expansion following mass loss that is characteristic of highly evolved stars. Material removed by lobe overflow at pericenter is replenished by envelope expansion as an evolved star awaits its next pericenter passage. An observational signature of lobe overflow for upcoming pericenter passages would be appearance of emission lines as the ejected gas expands and becomes optically thin. Issues abound regarding the evolutionary states of these stars and how they came to be in the inner Galactic center, e.g., Davies & King (2005), Zhang et al. (2013), Gillessen et al. (2009), Genzel et al. (2010. Stars arriving on nearly parabolic or even hyperbolic orbits could suffer major stripping on initial pericenter passage (Davies & King 2005), with the lost material carrying away enough orbital energy to leave the remnant in an elliptical orbit, filling its limiting lobe at pericenter. Alternatively, a star that has been trapped into a tight orbit while well detached from its lobe could later undergo evolutionary expansion and attain lobe filling. Perhaps a lost binary companion may carry off the requisite orbital energy at first encounter with the SMBH and leave the remaining star bound.Eisenhauer et al. (2005) found most of the brighter inner orbiters to be B0-B9 main sequence stars, with S2 in the range O8-B0, and with rotation velocities typical of B stars in the Galactic disk. Davies & King (2005) argued that they are actually tidally stripped remnants of AGB stars that now superficially resemble main sequence stars and estimated S2ʼs mass at below a solar mass, specifically about 0.8 M ⊙ . All in all, mass estimates for S2 that have been published or correspond to observed (main sequence) spectral types range from 0.8 to more than 20 M ⊙ . Accordingly, we simply adopt 10 M ⊙ for exploratory lobe size computations. BRIGHTENING OF STAR S2 DUE TO LOBE OVERFLOWS2 has continued as the most thoroughly discussed SMBH orbiter, largely due to its having been observed over more than a full orbit, and having brightened by about 0 . 40 m in K band, coincident with pericenter passage (Gillessen et al. 2009). Gillessen, et al. offered seven ideas for explaining the brightening, however they then ruled out four of the ideas and argued against likelihood of the other three. Consequences of lobe overflow-a very comm...

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Pericenter Passage of the Gas Cloud G2 in the Galactic Center

Cited by 88 publications

References 29 publications

No asymmetric outflows from Sagittarius A* during the pericenter passage of the gas cloud G2

No asymmetric outflows from Sagittarius A* during the pericenter passage of the gas cloud G2

The Post-periapsis Evolution of Galactic Center Source G1: The Second Case of a Resolved Tidal Interaction with a Supermassive Black Hole

Lobe Overflow as the Likely Cause of Pericenter Outburst in an SMBH Orbiter

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