ACriticalLookattheIonizationofProtoplanetaryDisks ... · 1)Introduction...
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1) Introduction Ionization is key in driving pathways towards gas phase chemical complexity and disk dynamics such as accretion driven by the magnetorotational instability (MRI; Balbus & Hawley 1991). Dominant sources of ionization (and stopping distances) in protoplanetary disks are: UV (~10 3 g cm 2 ), X rays (0.05 g cm 2 at 1 keV) and CRs (96 g cm 2 ; Umebayashi et al 1981) . Thus CRs are most able to ionize the densest gas in disks. However, as evidenced by the presence of our own Heliosphere, the degree to which CRs are present in the circumstellar environment is unclear. We have examined the effects of the absence of CRs due to wind modulation and magnetic modulation on socalled MRI deadzones (Cleeves, Adams & Bergin 2013; Paper I) as well as on the chemical nature of disks (Paper II, in preparation). 2) Disk Model ๏ Disk model computed with Torus (Harries 2000). Gas temperatures from Xray heating and dust collisional cooling (Glassgold & Najita 2001). ๏ Monte Carlo UV & Xray R.T. (Bethell & Bergin 2011 a,b). ๏ Disk chemical calculations of Fogel et al. 2011. ✴ Updated to include a small, 70 reaction deuterium network to make additional predictions for N2D + , H2D + , and HD2 + . 3) Cosmic Ray Exclusionary Mechanisms ๏ Physical mechanisms by which CRs can be reduced are: (1) Modulation by stellar winds, i.e., a “TTauriosphere,” (2) Magnetic mirroring by large scale stellar or environmental, e.g., “hourglass,” magnetic field structure (Li & Shu 1996, Padovani et al. 2011), see Fig. 1 ๏ We have investigated incident CR spectra under solarlike and “elevated” wind conditions, using spot coverage as a proxy for magnetic activity and thus wind modulation efficiency. ISM cosmic ray rates from Moskalenko et al. 2002 (grey line) and Webber et al. 1998 (orange line), see Fig 2. ✴ Magnetic effects reduce the net flux by ~2x, while winds can reduce by more than 15 orders of magnitude. ✴ Subsequent CR transport into the disk with the “Continuous Slowing Down Approximation” (Padovani et al. 2009). RESULTS 4) Molecular Ion Abundances ๏ Molecular ions such as HCO + , DCO + , and N2H + have been previously detected towards protoplanetary disks (e.g.,van Dishoeck et al. 2003, Qi et al. 2008, Öberg et al. 2010,2011). ✴ Ion abundances trace the ionizing flux as well as environmental factors such as freeze out of parent molecules such as CO. ๏ We have computed disk chemical models holding other properties fixed and varying only CR ionization. Other sources of ionization are UV, Xrays and (in progress) ionization by radionuclide (RN) decay. ๏ The largest change in abundance occurs between an ISM CR rate (ζ CR ≥ 10 17 s 1 ) and a solar modulated rate (ζ CR ~ (0.11) x 10 18 s 1 ). ✴ Molecules such as HCO + have partial contribution from Xrays, need optically thin isotopologues to determine midplane ionization. Deuterated molecular ions more sensitive to cold gas, but less abundant and thus more difficult to observe. 5) How to Search For aTTauriosphere: Observational Constraints from ALMA ๏ These scenarios can be differentiated by observing rotational transitions of molecular ions that trace the dense midplane gas. ๏ Because of the high Xray luminosity of T Tauri stars, it is important to observe optically thin tracers that peer through the ionized gas on the disk surface. ๏ We have calculated the emergent (nonLTE for deuterium free species) line intensity for ALMA observable transitions with our abundance models using Lime (Brinch & Hogerheijde 2010), and simulated ALMA observations using CASA for the most compact full ALMA configuration, 66 antennas, and sensitivites from 10 h on sky, Fig. 4. ๏ The most sensitive tracers of the midplane ionization are N2H+ (32), (43) and N2D+ (32). H2D+ is detectable for only the ISM (M02) ionization case. Usefulness of HCO+ and isotopologues are hindered by significant contribution from Xrays and CO freeze out. 6) Dead Zones ๏ In regions where the ion abundance drops below a critical value (e.g., PerezBecker & Chiang 2011), the disk cannot sustain MRI turbulence, i.e. be active. Paper I shows that under the influence of elevated CR (“TTauri like”) wind modulation, the disk would be largely dead to MRI, i.e. accretion cannot proceed in the midplane, Fig. 5. ๏ However, a minimal amount of either radionuclide ionization or hard ~7 keV Xray ionization (L XR > 10 31 erg/s), which can penetrate to the midplane, can potentially provide a baseline to sustain MRI turbulence and thus power accretion. 7) Radionuclide (RN) Ionization ๏ Without CRs, decay of shortlived RNs (now extinct) becomes the dominant contributor to the otherwise shielded midplane. The problem has been investigated by Umebayashi et al 1981, 2009, 2013, Finocchi & Gail 1997. In Cleeves, Bergin, Adams & Visser in prep, we are investigating the vertical ionization rate (Fig. 6) including the significant effects of energy/process dependent particle loss, previously untreated. A Critical Look at the Ionization of Protoplanetary Disks: Chemistry in the Presence of an Analogue Heliosphere L. Ilsedore Cleeves, Edwin A. Bergin, Fred C. Adams; University of Michigan Abstract Cosmic rays as ionizing agents have important astrochemical and physical consequences in the dense ISM. However, as evidenced by the existence of our own solar Heliosphere, the degree to which CRs are present in the circumstellar environment is unknown. This issue is especially important for low mass premain sequence stars with molecular disks, i.e. TTauri systems. In these disks the physical and chemical properties, such as ionization fraction, are key in setting the conditions for future planet formation. Within our own Solar System the solar wind shields the inner ∼100 AU from galactic cosmic rays with energies below a 300 MeV.TTauri stars with relatively high mass loss rates and magnetic activity can likewise power a ”TTauriosphere” that could substantially reduce the galactic CR flux incident on their protoplanetary disks. We find thatTTauri wind modulation of CRs can be so effective that ionization resulting from shortlived radionuclide decay could dominate the outer disk midplane ionization at typical ISM abundances, if the decay products do not freely escape. We examine implications for both the extent of MRI ”deadzones” as well as for ion chemistry in disks resulting from low CR fluxes. We also provide predictions on how to use observable molecular ions to infer the presence of extrasolar Heliospheres aroundTTauri stars. Model Parameters: •R out = 400 AU •M disk = 0.04 M sol •L ✷ = 1 L sol •M ✷ = 1 M sol •T ✷ = 4300 K •L XR = 10 29.5 erg s -1 REFERENCES: Balbus & Hawley 1991, ApJ, 376, 214. Bethell & Bergin 2011a, ApJ, 740, 7. Bethell & Bergin 2011b, ApJ, 739, 78. Brinch & Hogerheijde 2010, A&A, 523, 25. Cleeves, Adams & Bergin 2013, ApJ 772, 5. Fogel et al 2011, ApJ, 726, 29. Finocchi & Gail, A&A, 327, 825. Harries 2000, MNRAS, 315, 722. Li & Shu 1996, Apj, 472, 211. Moskalenko et al. 2009, ApJ, 565, 280. Öberg et al. 2010, ApJ, 720, 480. Öberg et al. 2011, ApJ 734, 98. Öberg et al. 2011, ApJ, 743, 152. Padovani et al. 2009, A&A, 501, 619. Padovani et al. 2011, A&A, 530, A109. Perez-Becker & Chiang 2011, 735, 8. Qi et al. 2008, ApJ, 681, 1396. Umebayashi et al 1981, PASJ, 33, 617. Umebayashi et al 2009 ApJ, 690, 69. Umebayashi et al 2013, ApJ, 764, 104. van Dishoeck et al. 2003, ApSS, 285, 691. Webber et al. 1998, ApJ, 506, 329. “Dragged” B Field T-Tauriosphere C.R. Mirroring P ISM CR Wind Pressure R (AU) log(ρ gas ) g cm 3 log(∫F XR dν) phot/cm 2 /s log(Temp.) K log(∫F UV dν) phot/cm 2 /s Z (AU) H 13 CO + N 2 H + HC 18 O + N 2 D + 0.15 0.10 0.05 0.00 100 200 300 -19.0 -17.5 -16.0 -14.5 -13.0 100 200 300 50 100 150 200 1.1 1.3 1.6 1.8 2.0 1.0 2.8 4.5 6.2 8.0 50 100 150 200 6.0 7.5 9.0 10.5 12.0 I) Wind Exclusion: a “T-Tauriosphere” II) Magnetic Modulation: Mirroring Fig. 1: Basic model framework for wind and magnetic field exclusion. 10 19 10 20 10 21 10 22 10 23 10 24 10 25 10 26 10 27 N(H 2 ) cm -2 10 -24 10 -23 10 -22 10 -21 10 -20 10 -19 10 -18 10 -17 10 -16 10 -15 10 -14 10 -13 ζ H 2 s -1 LIS - M02 LIS - W98 Solar Min Solar Max T.T. Min T.T. Max Fig. 2: CR ionization rate vs. column density for different degrees of wind modulation. T.T. Max Solar Min M02 (ISM) -200 -100 0 100 200 10 -11.6 10 -10.5 10 -9.4 10 -8.2 -200 -100 0 100 200 10 -14.5 10 -13.1 10 -11.7 10 -10.3 -200 -100 0 100 200 10 -19.4 10 -16.0 10 -12.6 10 -9.3 -200 -100 0 100 200 10 -15.5 10 -13.8 10 -12.0 10 -10.3 0 100 200 300 -200 -100 0 100 200 0 100 200 300 0 100200300 10 -16.5 10 -14.4 10 -12.4 10 -10.3 Abundance Relative to H 2 D 2 H + H 2 D + N 2 D + N 2 H + HCO + Fig. 4: Radial brightness profile of a selection of emission line models. All species shown at J = 3-2. Top portion of each panel includes: ISM, Solar and T-Tauri CR rates while the bottom portion is a zoom-in of only the modulated TT and Solar CR levels. Fig. 3: Chemical modeling results. Low CR to high CR flux from right to left. Abundances are given by n(X)/n(H 2 ). Fig. 6: RN ionization rates, ζ RN in s -1 H 2 -1 versus height from the midplane at the indicated radii. Top panels are 26 Al-derived; bottom panels also include 60 Fe 36 Cl decay. Left panels assume gas & dust are well mixed while the right include dust settling. Fig. 5: Dead zones under the influence of a reduced CR flux. Hatched regions: active according to the Magnetic Reynolds #, Re and Ambipolar Diffusion Criteria Am; orange line: active region defined by Re-only. Left to right, MRI dead regions as found under: 1) ISM, 2) T.T. Max and 3) T.T. Max plus uniform ζ RN = 7.3x10 -19 s -1 radionuclide ionization. M02 (ISM) T.T. Max T.T. Max + RN χ(e) Increasing CRs 0 100 200 300 0 50 100 150 Z (AU) 0 10 20 30 0 2 4 0 100 200 300 R (AU) 0 10 20 30 0 2 4 0 100 200 300 -10.7 -9.2 -7.7 -6.2 χ(e - ) 0 10 20 30 0 2 4
Transcript of ACriticalLookattheIonizationofProtoplanetaryDisks ... · 1)Introduction...
1) IntroductionIonization is key in driving pathways towards gas phase chemical complexity and disk dynamics such as accretion driven by the magnetorotational instability (MRI; Balbus & Hawley 1991).
Dominant sources of ionization (and stopping distances) in protoplanetary disks are: UV (~10-‐3 g cm-‐2), X-‐rays (0.05 g cm-‐2 at 1 keV) and CRs (96 g cm-‐2; Umebayashi et al 1981) . Thus CRs are most able to ionize the densest gas in disks. However, as evidenced by the presence of our own Heliosphere, the degree to which CRs are present in the circumstellar environment is unclear.
We have examined the effects of the absence of CRs due to wind modulation and magnetic modulation on so-‐called MRI dead-‐zones (Cleeves, Adams & Bergin 2013; Paper I) as well as on the chemical nature of disks (Paper II, in preparation).
2) Disk Model๏ Disk model computed with Torus
(Harries 2000). Gas temperatures from X-‐ray heating and dust collisional cooling (Glassgold & Najita 2001).
๏ Monte Carlo UV & X-‐ray R.T. (Bethell & Bergin 2011 a,b).
๏ Disk chemical calculations of Fogel et al. 2011.
✴ Updated to include a small, 70 reaction deuterium network to make additional predictions for N2D+, H2D+, and HD2+.
3) Cosmic Ray Exclusionary Mechanisms๏ Physical mechanisms by which CRs can be reduced are:
(1) Modulation by stellar winds, i.e., a “T-‐Tauriosphere,”
(2)Magnetic mirroring by large scale stellar or environmental, e.g., “hourglass,” magnetic field structure (Li & Shu 1996, Padovani et al. 2011), see Fig. 1
๏ We have investigated incident CR spectra under solar-‐like and “elevated” wind conditions,
using spot coverage as a proxy for magnetic activity and thus wind
modulation efficiency. ISM cosmic ray rates from
Moskalenko et al. 2002 (grey line) and Webber et al. 1998 (orange line), see Fig 2.
✴ Magnetic effects reduce the net flux by ~2x, while winds can reduce by more than 1-‐5 orders of magnitude.
✴ Subsequent CR transport into the disk with the “Continuous Slowing Down Approximation” (Padovani et al. 2009).
RESULTS4) Molecular Ion Abundances๏ Molecular ions such as HCO+, DCO+, and N2H+ have been previously
detected towards protoplanetary disks (e.g.,van Dishoeck et al. 2003, Qi et al. 2008, Öberg et al. 2010,2011).
✴ Ion abundances trace the ionizing flux as well as environmental factors such as freeze out of parent molecules such as CO.
๏ We have computed disk chemical models holding other properties fixed and varying only CR ionization. Other sources of ionization are UV, X-‐rays and (in progress) ionization by radionuclide (RN) decay.
๏ The largest change in abundance occurs between an ISM CR rate (ζCR ≥ 10-‐17 s-‐1) and a solar modulated rate (ζCR ~ (0.1-‐1) x 10-‐18 s-‐1).
✴ Molecules such as HCO+ have partial contribution from X-‐rays, need optically thin isotopologues to determine midplane ionization. Deuterated molecular ions more sensitive to cold gas, but less abundant and thus more difficult to observe.
5) How to Search For a T-‐Tauriosphere: Observational Constraints from ALMA๏ These scenarios can be differentiated by observing rotational transitions
of molecular ions that trace the dense midplane gas.
๏ Because of the high X-‐ray luminosity of T Tauri stars, it is important to observe optically thin tracers that peer through the ionized gas on the disk surface.
๏ We have calculated the emergent (non-‐LTE for deuterium free species) line intensity for ALMA observable transitions with our abundance models using Lime (Brinch & Hogerheijde 2010), and simulated ALMA observations using CASA for the most compact full ALMA configuration, 66 antennas, and sensitivites from 10h on sky, Fig. 4.
๏ The most sensitive tracers of the midplane ionization are N2H+ (3-‐2), (4-‐3) and N2D+ (3-‐2). H2D+ is detectable for only the ISM (M02) ionization case. Usefulness of HCO+ and isotopologues are hindered by significant contribution from X-‐rays and CO freeze out.
6) Dead Zones ๏ In regions where the ion abundance drops below a critical value (e.g.,
Perez-‐Becker & Chiang 2011), the disk cannot sustain MRI turbulence, i.e. be active. Paper I shows that under the influence of elevated CR (“T-‐Tauri like”) wind modulation, the disk would be largely dead to MRI, i.e. accretion cannot proceed in the midplane, Fig. 5.
๏ However, a minimal amount of either radionuclide ionization or hard ~7 keV X-‐ray ionization (LXR > 1031 erg/s), which can penetrate to the midplane, can potentially provide a baseline to sustain MRI turbulence and thus power accretion.
7) Radionuclide (RN) Ionization๏ Without CRs, decay of short-‐lived RNs (now extinct) becomes the
dominant contributor to the otherwise shielded midplane. The problem has been investigated by Umebayashi et al 1981, 2009, 2013, Finocchi & Gail 1997. In Cleeves, Bergin, Adams & Visser in prep, we are investigating the vertical ionization rate (Fig. 6) including the significant effects of energy/process dependent particle loss, previously untreated.
A Critical Look at the Ionization of Protoplanetary Disks: Chemistry in the Presence of an Analogue Heliosphere
L. Ilsedore Cleeves, Edwin A. Bergin, Fred C. Adams; University of MichiganAbstract
Cosmic rays as ionizing agents have important astrochemical and physical consequences in the dense ISM. However, as evidenced by the existence of our own solar Heliosphere, the degree to which CRs are present in the circumstellar environment is unknown. This issue is especially important for low mass pre-‐main sequence stars with molecular disks, i.e. T-‐Tauri systems. In these disks the physical and chemical properties, such as ionization fraction, are key in setting the conditions for future planet formation. Within our own Solar System the solar wind shields the inner ∼100 AU from galactic cosmic rays with energies below a 300 MeV. T-‐Tauri stars with relatively high mass loss rates and magnetic activity can likewise power a ”T Tauriosphere” that could substantially reduce the galactic CR flux incident on their protoplanetary disks. We find that T-‐Tauri wind modulation of CRs can be so effective that ionization resulting from short-‐lived radionuclide decay could dominate the outer disk midplane ionization at typical ISM abundances, if the decay products do not freely escape. We examine implications for both the extent of MRI ”dead-‐zones” as well as for ion chemistry in disks resulting from low CR fluxes. We also provide predictions on how to use observable molecular ions to infer the presence of extrasolar Heliospheres around T-‐Tauri stars.
Model Parameters:• Rout = 400 AU• Mdisk = 0.04 Msol• L✷ = 1 Lsol• M✷ = 1 Msol
• T✷ = 4300 K• LXR = 1029.5 erg s-1
REFERENCES:Balbus & Hawley 1991, ApJ, 376, 214.Bethell & Bergin 2011a, ApJ, 740, 7.Bethell & Bergin 2011b, ApJ, 739, 78.Brinch & Hogerheijde 2010, A&A, 523, 25.Cleeves, Adams & Bergin 2013, ApJ 772, 5.
Fogel et al 2011, ApJ, 726, 29.Finocchi & Gail, A&A, 327, 825.Harries 2000, MNRAS, 315, 722.Li & Shu 1996, Apj, 472, 211.Moskalenko et al. 2009, ApJ, 565, 280.Öberg et al. 2010, ApJ, 720, 480.Öberg et al. 2011, ApJ 734, 98.
Öberg et al. 2011, ApJ, 743, 152.Padovani et al. 2009, A&A, 501, 619.Padovani et al. 2011, A&A, 530, A109.Perez-Becker & Chiang 2011, 735, 8.Qi et al. 2008, ApJ, 681, 1396.Umebayashi et al 1981, PASJ, 33, 617.Umebayashi et al 2009 ApJ, 690, 69.
Umebayashi et al 2013, ApJ, 764, 104.van Dishoeck et al. 2003, ApSS, 285, 691.Webber et al. 1998, ApJ, 506, 329.
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I) WindExclusion: a
“T-Tauriosphere”
II) Magnetic Modulation: Mirroring
Fig. 1: Basic model framework for wind and magnetic field exclusion.
1019 1020 1021 1022 1023 1024 1025 1026 1027
N(H2) cm!2
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Fig. 2: CR ionization rate vs. column density for different degrees of wind modulation.
T.T. Max Solar Min M02 (ISM)
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Fig. 4: Radial brightness profile of a selection of emission line models. All species shown at J = 3-2. Top portion of each panel
includes: ISM, Solar and T-Tauri CR rates while the bottom portion is a zoom-in of only the modulated TT and Solar CR levels.
Fig. 3: Chemical modeling results. Low CR to high CR flux from right to left. Abundances are given by n(X)/n(H2).
Fig. 6: RN ionization rates, ζRN in s -1 H2-1
versus height from the midplane at the indicated
radii. Top panels are 26Al-derived; bottom panels
also include 60Fe 36Cl decay. Left
panels assume gas & dust are well mixed while the
right include dust settling.
Fig. 5: Dead zones under the influence of a reduced CR flux. Hatched regions: active according to the Magnetic Reynolds #, Re and Ambipolar Diffusion Criteria Am; orange line: active region defined by Re-only. Left to right, MRI dead regions as found under: 1) ISM, 2) T.T. Max and 3) T.T.
Max plus uniform ζRN = 7.3x10-19 s-1 radionuclide ionization.
M02 (ISM) T.T. Max T.T. Max + RN
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