EX Lupi FROM QUIESCENCE TO OUTBURST: EXPLORING THE LTE APPROACH IN MODELING BLENDED H₂O AND OH MID-INFRARED EMISSION

Abstract

We present a comparison of archival Spitzer spectra of the strongly variable T Tauri EX Lupi, observed before and during its 2008 outburst. We analyze the mid-infrared emission from gas-phase molecules thought to originate in a circumstellar disk. In quiescence the emission shows a forest of H 2 O lines, highly excited OH lines, and the Q branches of the organics C 2 H 2 , HCN, and CO 2 , similar to the emission observed toward several T Tauri systems. The outburst emission shows instead remarkable changes: H 2 O and OH line fluxes increase, new OH, H 2 , and Hi transitions are detected, and organics are no longer seen. We adopt a simple model of a single-temperature slab of gas in local thermal equilibrium, a common approach for molecular analyses of Spitzer spectra, and derive the excitation temperature, column density, and emitting area of H 2 O and OH. We show how model results strongly depend on the selection of emission lines fitted, and that spectrally-resolved observations are essential for a correct interpretation of the molecular emission from disks, particularly in the case of water. Using H 2 O lines that can be approximated as thermalized to a single temperature, our results are consistent with a column density decrease in outburst while the emitting area of warm gas increases. A rotation diagram analysis suggests that the OH emission can be explained with two temperature components, which remarkably increase in column density in outburst. The relative change of H 2 O and OH emission suggests a key role for UV radiation in the disk surface chemistry.

The Depletion of Water During Dispersal of Planet-Forming Disk Regions

“…OH emission follows similar trends, but OH lines are tentatively detected up to larger disk radii (especially the cold lines at 30 μm). Variability in accretion luminosity may contribute to the order-of-magnitude scatter in line fluxes along these trends; however, a factor up to »10 change in line fluxes has been observed so far only in the extreme accretion outburst of EX Lupi, and not during the typically lower variability of other young stars included in this sample (Banzatti et al 2012(Banzatti et al , 2014. Similarly to H 2 O and OH, the CO v=1 − 0 line flux decreases with R co /Rsnow, but it is detected in disks well beyond the snow line radius.…”

“…As a basic test of this inside-out depletion scenario, we use a simple disk model composed of concentric rings of increasing radius, where each ring is a slab of gas described by one temperature and one column density as in Banzatti et al (2012). We adopt power-law profiles to set how the temperature and column density of each ring decrease with increasing disk radius, as T(R)=T 0 (R/R 0 ) −q and N(R)=N 0 (R/R 0 ) −p .…”

“…The constant ∼0.4 ratio with disk radius is also confirmed in this case by all three OH/H 2 O ratios as measured at different wavelengths (2.93 μm using the CRIRES data, 12.5-12.6 μm and 30-33 μm using the Spitzer data). Some disks show a higher OH/H 2 O ratio of ∼1.7 at 12.5-33 μm, perhaps linked to phases of stronger production of OH through increased UV irradiation (among these disks are, in fact, EX Lupi and DG Tau, where enhanced production of OH through UV photodissociation has been found by Banzatti et al 2012 and. A decreasing trend of OH/H 2 O may be present at log R co /R  -0.7 snow .…”

“…In the Spitzer-IRS spectra, to select pixels of continuum free from line emission, we use models of the strongest emitters at 10-35 μm, as identified in previous work (water, OH, HCN, C 2 H 2 , CO 2 , HI; see, e.g., Banzatti et al 2012). The following are the ranges adopted: the 12.52 μm line is measured on 12.50-12.537 μm, and its continuum fitted on 12.48-12.5 μm and 12.7-12.72 μm (at the two sides of the line); the 33 μm line on 32. the distribution of measured line fluxes after re-sampling the spectrum using the local noise, as in Banzatti et al (2012). Following this procedure, we find values consistent within 1σ…”

Chemical tracers of episodic accretion in low-mass protostars

“…For example, the 2008 burst in EX Lup sparked the production of crystalline silicates (Ábrahám et al 2009) and boosted the column densities of midinfrared H 2 O and OH lines (Banzatti et al 2012). In very lowluminosity embedded protostars, whose envelopes are too cold to produce pure CO 2 ice, the detection of double-peaked 15-µm absorption profiles is attributed to a higher luminosity at some unknown point in the past (Kim et al 2011(Kim et al , 2012; see also Poteet et al 2013).…”