deuteration in dm tau
TRANSCRIPT
Richard Teague, Dmitry Semenov, Stephane Guilloteau, Thomas Henning, Anne Dutrey & the CID Collaboration
Disk Diagnosticswith Deuteration
Hydrosta)c equilibrium leads to a strongly stra)fied structure.
Molecular line emission will probe the intermediate layer.
Henning & Semenov (2013)
RD is the ra)o of a deuterated isotopologue to the non-‐deuterated isotopologue.
In protoplanetary disks HCO+ is a readily observable molecule. It allows for comparisons with other epochs in the evolu)on of molecular gas.
We imaged with the Plateau de Bure Interferometer HCO+ J=(3-‐2), (1-‐0) and DCO+ J=(3-‐2) emission in DM Tau.
a hRp://www.iram.fr/IRAMFR/GILDAS/
Obtained a ~1.5’’ spaCal and ~0.2 km s-‐1 velocity resoluCon. Data was reduced with the standard MAPPINGa soXware.
Column densi)es of HCO+ and DCO+ were extracted using DISKFITa. Describes physical properCes of the disk with power laws.
Assumes LTE and fits the visibili)es in the uv-‐plane.
Column DensiCes RD = N(DCO+) / N(HCO+)
RD varies between 0.1 and 0.2. !
Computa)onal model used similar physical structure of D’Alessio et al. (1999). Chemical model run with ALCHEMICa which includes an
extended deuterium networkb and ortho/para chemistryc.
!a Semenov et al. (2010), b Albertsson et al. (2013), c Albertsson et al. (2014a)
Column densi)es were found to agree well with observaConally derived values. RD clearly evolves with Cme, and also fits qualita)vely well with observa)ons…
Column DensiCes RD = N(DCO+) / N(HCO+)
H2 H2 H2D+
H2 HCO+
DCO+ H3+ H2+ H2
CO
HD
H2
H2
ice
ice
CO
What controls the abundances of HCO+ and DCO+? An incredibly simplified chemical network for the molecular layer…
H2 H2 H2D+
H2 HCO+
DCO+ H3+ H2+ H2
CO
HD
H2
H2
ice
ice
CO
Cold region chemistry driven by ion -‐ neutral reac)ons. Requires ini)al ionizaCon of H2 to kick start chemistry.
H2 H2 H2D+
H2 HCO+
DCO+ H3+ H2+ H2
CO
HD
H2
H2
ice
ice
CO
…deuterium fracConaCon will occur in cold environments…(here we neglect the doubly and triply deuterated isotopologues.)
H2 H2 H2D+
H2 HCO+
DCO+ H3+ H2+ H2
CO
HD
H2
H2
ice
ice
CO
…due to the energy barrier which deters backwards reac)ons.
H2 H2 H2D+
H2 HCO+
DCO+ H3+ H2+ H2
CO
HD
H2
H2
ice
ice
CO
Assuming CO is present in the gas phase and has not frozen out…(or other forms of deple)on)
H2 H2 H2D+
H2 HCO+
DCO+ H3+ H2+ H2
CO
HD
H2
H2
ice
ice
CO
…fast ion-‐neutral reac)ons with CO transfer this deuterium ra)o to HCO+ and DCO+.
H2 H2 H2D+
H2 HCO+
DCO+ H3+ H2+ H2
CO
HD
H2
H2
ice
ice
CO
Ioniza)on, frac)ona)on efficiency and CO deple)on all affect the measured RD values.
100R, AU
11.5
12.0
12.5
13.0
13.5
14.0
14.5
Lo
g10[N
(X),
cm
-2]
HCO+
DCO+
Best-fitLX=1031 erg/s
100R, AU
11.5
12.0
12.5
13.0
13.5
14.0
14.5
Lo
g10[N
(X),
cm
-2] HCO+
DCO+
Best-fitLX=3x1028 erg/s
Reduced X-‐RaysIncreased X-‐RaysLX = 3x1028 erg s-‐1LX = 1031 erg s-‐1
Solid lines are the HCO+ column densi)es and the dashed DCO+
Comparisons between the best fit model and a model with an altered physical parameter.
X-‐rays are the dominant ionizaCon source where HCO+ is present. High ioniza)on can suppress frac)ona)on by increasing ortho-‐H2 abundances.
Solid lines are the HCO+ column densi)es and the dashed DCO+
100R, AU
11.5
12.0
12.5
13.0
13.5
14.0
14.5
Lo
g10[N
(X),
cm
-2] HCO+
DCO+
Best-fitno IS UV
100R, AU
11.5
12.0
12.5
13.0
13.5
14.0
14.5
Lo
g10[N
(X),
cm
-2]
HCO+
DCO+
Best-fitagr=1um
No Interstellar UV Increased Grain Sizesa = 1 um
Comparisons between the best fit model and a model with an altered physical parameter.
CO deple)on is sensi)ve to UV driven photodesorpCon of heavy ices and efficiency of CO freeze out onto grains.
Conclusions
• In DM Tau RD ~ 0.1 -‐ 0.2 between 50 and 550 au.Consistent with con)nued gaseous processing and values found in other disks.
!• X-‐rays are the dominant ionizaCon source in the molecular layer.
Different depths of HCO+ and DCO+ layers result in the two being affected by changes in X-‐ray luminosity differently .
!• DepleCon of CO strongly affects the local RD.
CO deple)on is sensi)ve to the freeze out efficiency and the level of UV driven photodesorp)on.