EVIDENCE OF ACCRETION-GENERATED X-RAYS IN THE YOUNG ...

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3.1.3 X-ray Production & Accretion One of the possible X-ray generation mechanisms is accretion of material from a circumstellar disk. Calvet & Gullbring (1998) determined that material near free-fall velocity can be shock-heated due to the large velocity gradient. They find that the plasma temperature, according to strong-shock theory, is given by TS ⇡ 3.5 M M ✓ R R ◆ 1 [MK]. (3.1) Substituting values for the radius and mass of typical TTSs (M = 0.1–1 M and R =0.5–2R ) into Equation 3.1, the expected shock temperature is TS ⇡ 0.4–4 MK, which corresponds to an X-ray temperature of kTX ⇡ 0.035–0.35 keV. Stassun et al. (2006) searched for a possible link between X-ray production and accretion using COUP data and time-correlated BVRI photometry of young stars in the ONC. With accretion known to produce increases in optical flux, variability in optical flux (due to accretion) that is correlated with variability in X-ray flux would suggest that both flux variations are the result of accretion. Stassun et al. (2006) found that only 5% of their sample showed any correlation/anti-correlation between X-ray and optical variations, suggesting that accretion hotspots in the stellar photosphere are not dominant sites of X-ray production. However, it is still unclear as to why there is sometimes a correlation between optical and X-ray flux in some objects and not others when there are no observable characteristics that distinguish the stars in the 95% group from those in the 5% group. 29
3.1.4 Gleaning Other Information from X-rays X-ray line profiles have also been used to determine characteristics of stars. Helium- like triplets (spectral features usually comprised of three spectral lines - a forbidden, an intercombination, and a resonance feature) can be used to determine plasma characteristics such as electron temperature and density. For example, the K-shell helium-like 3 transitions consist of a resonance line, two intercombination lines, and a forbidden line. The forbidden transition, which is observed to occur only at very low densities, can be collisionally excited to the excitation level of the intercombination lines. As density increases, the likelihood that an electron in the excited forbidden state will be excited to an intercombination energy state increases as the likelihood that an electron in the excited forbidden level will radiate spontaneously before being collisionally excited/de-excited decreases. Thus, the ratio of the forbidden to intercombination line strengths provides a proxy for density. Kastner et al. (2002) observed the wTTs TW Hydra in 2000 with the Chandra X-ray Observatory using the High Energy Transmission Grating System and ACIS-S (CXO instruments will be in §3.2) . TW Hydra is known to be accreting due to the presence of hydrogen-balmer emission; however, determining the accretion rate from this line profile alone is often di cult to do with low uncertainty. The He-like Ne IX was observed in the high-resolution X-ray spectrum, and the relative line strengths were used to determine the electron density. After modeling the X-ray spectrum and deriving other plasma parameters such as emission measure, Kastner et al. (2002) were able to determine that the filling factor (the percentage of the stellar surface 3 Helium-like refers to ions that have only have two bound electrons. 30
Page 1 and 2: EVIDENCE OF ACCRETION-GENERATED X-R
Page 3 and 4: 4.3.2. Spectral Modeling . . . . .
Page 5 and 6: the first time through a telescope,
Page 7 and 8: List of Tables Table Page 1. 2002-2
Page 9 and 10: 19. Correlation Between Changes in
Page 11 and 12: on Earth, nay, the entire solar sys
Page 13 and 14: known as a molecular cloud. These s
Page 15 and 16: internal pressure will be overcome.
Page 17 and 18: With advancements in telescope and
Page 19 and 20: the circumstellar disk, such as the
Page 21 and 22: perturbations or disk instabilities
Page 23 and 24: CHAPTER II ERUPTING YOUNG STARS 2.1
Page 25 and 26: (1997). Once the TTS spectral class
Page 27 and 28: Annu. Rev. Astro. Astrophys. 1996.3
Page 29 and 30: FUors have typical accretion rates
Page 31 and 32: and radiative equilibrium and that
Page 33 and 34: CHAPTER III X-RAY ASTRONOMY: A TOOL
Page 35 and 36: of the X-ray luminosity (LX) totheb
Page 37: filtered light curves (which had la
Page 41 and 42: Fig. 9a ~6.4 keV ~6.7 keV Figure 8:
Page 43 and 44: 34 Figure 9: Schematic of the Chand
Page 45 and 46: Figure 10: Schematic of the Chandra
Page 47 and 48: Top View Rowland Circle Grating fac
Page 49 and 50: electrons eventually exits the outp
Page 51 and 52: one to e↵ectively determine the e
Page 53 and 54: pixels or pixel columns. • Parame
Page 55 and 56: deal of built-in flexibility in how
Page 57 and 58: accrete most of their mass (Hartman
Page 59 and 60: 4.2 Observations & Data Reduction O
Page 61 and 62: were grouped into energy bins with
Page 63 and 64: Since V1647 Ori was imaged with bot
Page 65 and 66: 4.3 Results 4.3.1 Short-Term and Lo
Page 67 and 68: overall X-ray light curve, the soft
Page 69 and 70: Count Rate (cts/s) Count Rate (cts/
Page 75 and 76: The highest measured X-ray mean cou
Page 77 and 78: during short-term variability, howe
Page 79 and 80: Change in Mean Hardness Ratio 1 0.5
Page 81 and 82: These results suggest that we can i
Page 83 and 84: Table 3. Model Fits for 2008-2009 C
Page 85 and 86: For the CXO observations of V1647 O
Page 87 and 88: normalized counts s −1 keV −1
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Determining a robust model for the
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April 4 XMM observation (109 eV) an
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s −1 ) Log Observed Flux (ergs cm
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tures of a few million Kelvin (kTX
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2004 March, especially if the large
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and AK =0.5on2004Feburary18,allofwh
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In order to test whether CXO would
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• and X-ray luminosities that are
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With V1647 Ori being observed inten
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level approximately one year after
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of an object in which this process
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eduction. In §3, we discuss the re
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Table 5. Chandra ACIS Observations
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5.3 Results from X-ray Observations
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y an intervening column of hydrogen
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Table 6. Best-Fit Models for EX Lup
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component plasma, subject to a sing
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does require a third, heavily-absor
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5.3.3 The Temporal Evolution of the
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Figure 29 shows the three CXO spect
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From 2008 June to August, the plasm
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June, however, the X-ray flux from
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to test this light curve for possib
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We note that in the above analysis,
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5.5 Discussion & Conclusions The op
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CHAPTER VI DISCUSSION AND CONCLUSIO
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sensitive line diagnostics that wou
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Bell, K. R., Lin, D. N. C., & Ruden
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Hartmann, L. & Kenyon, S. J. 1996,
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Orlando, S., Peres, G., & Reale, F.
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Venkat, V. & Anandarao, B. G. 2011,
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EVIDENCE OF ACCRETION-GENERATED X-RAYS IN THE YOUNG ...

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