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

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clusters. They found a strong correlation between the age of the cluster and the fraction of stars in each of the clusters that show infrared excesses characteristic of the presence of circumstellar disks. Starting with the youngest observed cluster ( 80% of cluster members possessed circumstellar disks), Haisch et al. (2001) found that the cluster disk fraction decreased to 50% with an increase in age of approximately 3 Myr, suggesting that the overall disk lifetime of young stars is roughly 6 Myr. Wolk & Walter (1996) observed 39 stars with strong infrared excesses and found that some appeared to have “transition disks,” circumstellar disks that are in the process of dispersal and that are transitioning from optically thick to optically thin. By multiplying the fraction of stars observed to have transition disks with the mean age of the stars, they deduced that circumstellar disks have an optically-thick lifetime (the time it takes for the disk to go from optically thick to optically thin) of ⇠10 5 years. After that, the disks are dominated by micron-size dust grains. The presence of disks around stars older than a few million years is thought to be due to collisions of larger bodies within the disks. If these debris disks were primordial, then the detected dust out to ⇠10 AU should have been depleted in ⇠10 5 years due to Poynting-Robertson drag (Mamajeck et al., 2004). The lifetimes of optically thick and optically thin disks help to constrain planet formation theories in that we now know the typical periods of time during which there are reservoirs of material out of which planets can form. 1.1.2 Circumstellar Accretion On occasion, material will be perturbed in the circumstellar disk and accrete onto the parent star through various mechanisms possibly involving gravitiational 11
perturbations or disk instabilities (e.g., see works by Clarke et al. (1989); Bell et al. (1991); Bonnell & Bastien (1992); Armitage et al. (2001); D’Angelo & Spruit (2012)) from the inner portion of the circumstellar disk. Such accretion is likely controlled by magnetic flux tubes that connect the star to the disk. YSOs are known to be quite magnetically active, as is often indicated by pro- nounced CaII emission as well as observations of periodic transits of starspots (direct manifestations of magnetic activity), which are derived from the observed modula- tion of light curves (Stassun et al., 2006). Donati et al. (2007) have shown through three-dimensional modeling that the stellar magnetic fields are intimately linked with the magnetic fields of the circumstellar disk. During accretion, circumstellar material is funneled along magnetic field lines and deposited at high stellar latitudes, where it is shock-heated as it impacts the photosphere. The accretion footprint emits at ultraviolet and X-ray wavelengths. The accreting material above the footprint absorbs and thermalizes radiation given o↵ at the shock, causing it to re-radiate this energy at longer wavelengths. Since the material density is high near the accretion footprint, this plasma often radiates more as a blackbody than as an optically thin gas. The ultraviolet and optical emission thus comes in the form of a continuum overlaid on top of the stellar spectrum. This accounts for the ultraviolet excess observed in the spectra of many known accreting stars. In addition, the spectral features of accreting stars are often “veiled;” that is, the absorption features, here mostly in the optical wavelength regime, are e↵ectively filled in due to the presence of continuum emission. Hydrogen-balmer emission lines are observed in the spectra of T Tauri stars and are thought to result from absorption and re-emission of flux in the stellar atmosphere 12
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: the circumstellar disk, such as the
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 and 38: filtered light curves (which had la
Page 39 and 40: 3.1.4 Gleaning Other Information fr
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 71 and 72:
Count Rate (cts/s) Count Rate (cts/
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Count Rate (cts/s) Count Rate (cts/
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The highest measured X-ray mean cou
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during short-term variability, howe
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Change in Mean Hardness Ratio 1 0.5
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These results suggest that we can i
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Table 3. Model Fits for 2008-2009 C
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For the CXO observations of V1647 O
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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
Page 113 and 114:
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
Page 127 and 128:
Figure 29 shows the three CXO spect
Page 129 and 130:
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.
Page 149:
Venkat, V. & Anandarao, B. G. 2011,
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