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MONITORING THE DUSTY S-CLUSTER OBJECT (DSO/G2) NEAR THE GALACTIC CENTERBLACK HOLE: MODEL PREDICTIONS FOR BR� ENERGY SHIFT DURING THE PASSAGE
Michal Zajacek1,2,3, Vladimır Karas3, Andreas Eckart2,1, Monica Valencia-S.21 Max-Planck-Institut fur Radioastronomie (MPIfR), Auf dem Hugel 69, D-53121 Bonn, Germany,
2 I. Physikalisches Institut der Universitat zu Koln, Zulpicher Strasse 77, D-50937 Koln, Germany,3 Astronomical Institute, Academy of Sciences, Bocnı II 1401, 14100 Prague, Czech Republic.
für Radioastronomie
Max-Planck-Institut
Send offprint requests to: M. Zajacek ([email protected]).
The Extremes of Black Hole Accretion, ESAC, 8–10 June 2015 Poster #A11, ‘Spectral Components (AGN)’
Dusty S-cluster Object (DSO/G2) has approached the supermassive black hole at the center of the Galaxy and its passage through theperibothron was monitored by the ESO VLT/SINFONI observations taken in the near-infrared K-band. The profile and the energy shift ofBr-� spectral line can be employed to further constrain the nature of this event. We update and discuss the model predictions for differentscenarios: a core-less cloud versus an enshrouded star with a partially disintegrating envelope, potentially forming a bow shock due tostellar outflow. A comparison of observations with model predictions shows that the DSO is a star rather than a core-less cloud.
IntroductionThe hydrogen emission line Br� (produced by transition from level 7 to level 4) is frequently detectedin active galactic nuclei (AGN) and generally indicates the presence of HII regions and indirectly theoccurrence of massive OB stars that are needed to ionize the nuclear region. Thus it serves as anindicator of young star clusters formed recently during a star-formation episode.
Besides the role of the indicator of the ionized gas in nuclear regions, Br� emission line is frequentlydetected in young stellar systems (YSOs); its occurence is ⇠ 70–75% and it seems to be associatedwith the infall and accretion of material onto YSOs (see e.g. Ilee et al., 2014). In fact the correlationbetween Br� emission-line luminosity and the accretion luminosity of YSOs is found to be relativelytight. The empirical relation between emission line and accretion luminosities is based on varioussignatures of accretion luminosity (H↵ luminosity, optical and UV excess). The recent fit is as follows(Alcala et al., 2014),
log (Lacc/L�) = ⇣1 log [L(Br�)/L�] + ⇣2 . (1)
with ⇣1 = 1.16 ± 0.07 and ⇣2 = 3.60 ± 0.38.
For extragalactic sources we may only detect integrated flux of the nuclear region, whereas for theGalactic centre of the Milky Way individual gaseous components and stars can be resolved and anal-ysed. The recent challenge is the observation of a fast-moving, infrared-excess dusty S-cluster object(hereafter denoted as DSO) that has passed the peribothron in early 2014. Its character was puzzling- mainly two scenarious were discussed: a core-less gas and dust cloud or a dust-enshrouded star.
However, the compact line profile and the constant luminosity of the resolved Br� emission line of thesource provided crucial information to identify the object, see Fig. 2 (right panel). In combinationwith the detection in K-band continuum (Eckart et al., 2014) the character of the DSO must bestellar and it seems to be a young star, possibly accreting material from an inner accretion disc.
Figure 1: Left: Orbital trajectory of the DSO in the S-cluster. Middle: Temporal evolution of theline-of-sight velocity. Right: Temporal evolution of the velocity magnitude. Orbital elements weretaken from the fit in Valencia-S. et al. (2015).
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Figure 2: Left: Illustration of a young star with both the inflow and the outflow. A bow shock isformed due to relative supersonic motion. Right: Observed spectra of Br� emission line during thepre-pericentre (red-shifted profile in red ellipse) and the post-pericentre orbital phase (blue-shiftedprofile in blue ellipse). Spectra adopted from Valencia-S. et al. (2015).
Results: Model of a young star associated with the DSOPassage of a star with circumstellar envelope. The stellar scenario for the DSO was proposed byEckart et al. (2013) and numerically analyzed in Zajacek et al. (2015). The structure of the circum-etellar envelope of a pre-main-sequence star can be complex, with both the inflow and the outflowpotentially present, see Fig. 2 (left panel). The density naturally increases towards the star (materialcan be present in the accretion disc, accretion columns and wind). The material outside the criticaltidal radius is strongly perturbed (see Fig. 5, left panel) and escapes from the star. However, inside thepericentre Roche lobe of the star the density distribution is only slightly perturbed and is stable alongthe orbit.Figure 3: The orbital evolution of a star with a circumstellar envelope whose initial density profiledecreases as r�2. The envelope initially extends from 0.01 AU up to 10 AU from the star whose massis 1.5 M�. The FWHM of velocity dispersion in the envelope (with potentially both the inflow andoutflow) is set to 100 km s�1. The colour-coded axis displays the density of circumstellar matter inarbitrary units. The star is present in the densest region at all epochs (yellow point).
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We perform model calculations of line profiles assuming that the stellar envelope is optically thin forBr� emission, see Fig. 4. The line position and shape are affected by (a) the motion of the star and(b) by the tidal evolution of outer parts of the envelope. For comparison, we compute the evolutionof the line profile for the model of a core-less cloud, which was assumed in Gillessen et al. (2012)and Gillessen et al. (2013). While the stellar model predicts emission with one prominent peak at allepochs, the core-less cloud model predicts a double-peak emission line profile around the pericentrepassage. Observations by Valencia-S. et al. (2015) clearly favour the stellar model, since no prominentdouble-peak emission line was detected around the pericentre passage, see Figs. 2–4.
Due to the tidal field of the supermassive black hole the line profile in all cases is intrinsically asym-metric and skewed, which is not considered in publications where a Gaussian profile is automaticallyassumed.Constraints on a star associated with the DSO. Detection of only one clear peak of Br� emission(see Fig. 4 and analysis in Valencia-S. et al., 2015) indicates that the character of the DSO is stellarrather than nebulous. In that case Br� emission originates in the environment that is not stronglyaffected by the gravitational potential of the SMBH: accretion columns from a disc inside the Rochelobe of the star and/or bow-shock emission due to a supersonic motion of the star (Zajacek et al., 2015)are potential candidates, see Fig. 2 for illustration of different contributions. The left panel in Fig. 5shows the temporal evolution of the tidal (Hill) radius for 1M� star, beyond which the circumstellarmaterial is tidally perturbed and may escape the vicinity of the star. The tidal radius for a star aroundthe SMBH is typically 0.1 AU . rt . 1 AU at the peribothron. Hence, a small accretion disc cansurvive repetitive passages around the SMBH. The right panel in Fig. 5 represents HR diagram of pre-main-sequence stars. The spectral decomposition of the DSO puts an upper limit on its bolometricluminosity (LDSO . 30 L�), which automatically limits the mass as well as the radius of the putativestar: MDSO . 3 M�, RDSO . 10 R�.
Figure 4: Comparison of two scenarios for the temporal evolution of Br� emission of the DSO: starwith envelope (left panel) and core-less cloud (right). Observations by Valencia et al. (2015)favour the stellar model since no clear double-peak Br� emission line was detected around thepericentre passage.
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Figure 5: Left: Temporal evolution of tidal (Hill) radius for orbits of a 1 M� star around the SMBH.Center: Maximum line-of-sight velocity profiles of accretion streams moving from a disc along mag-netic streamlines as a function of the distance from a 2.5 M� star. Right: HR diagram (luminosity–effective temperature) computed for different Solar masses according to Siess et al. (2000).
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Conclusions. Based on the comparison of the observed line profiles near the peribothron passagewith model predictions we propose that the DSO is a star rather than a nebula. The observed char-acteristics of Br� broad emission line are reproduced within the framework of a young-star model,where the emission originates in accretion columns from a disc surrounding the star combined withstellar/disc winds. Additional contribution to the emission may be provided by the bow shock formingahead of the star. The stellar scenario for the DSO is also consistent with the non-detection of fluxenhancement in the radio/mm domain above the quiescent emission of Sgr A*.
Acknowledgements. M. Zajacek is a member of IMPRS Bonn-Cologne and SFB 956 (Conditionsand Impact of Star Formation) and is supported by MPG scholarship.
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