tokens: ; Goicoechea et al. 2017 ). Because it is easily observable at millimeter wavelengths, HCO + is an excellent candidate to investigate the gas dynamics within the cavities of nearby transition disks. Our observations suggest that the HCO + /HCN ratio is a useful chemical tool in determining the structure of transitional disks. In the case of unresolved disks, a high HCO + /HCN intensity ratio might be associated with the existence of inner dust cavities full of dense gas. We note that, together with AB Aur, HD 142527 and AA Tau were the disks with a higher HCO + J = 3–2/HCN J = 3–2 ratio in the compilation of Pacheco-Vázquez et al. ( 2015 ). Both systems show filamentary gas emission through their dust cavities (Casassus et al. 2013 ; Loomis et al. 2017 ). The other disk with a high ratio, GM Aur, has not been studied in detail. This result is also in line with recent results by Alonso-Albi et al. ( 2018 ) toward the Herbig Be star R Mon. 4.3. Gas Kinematics We show in Figure 3 (a) the first moment map of HCO + emission. The outer parts of the disk resemble those of a Keplerian disk. However, twisted isophotes are observed toward the center ( r 06). The aforementioned twisting can be reproduced by two misaligned Keplerian disks as proposed by Hashimoto et al. ( 2011 ). A radial outflow would produce a similar effect (Rosenfeld et al. 2014 ). We adopt the misaligned disks as the most plausible explanation, as it is consistent with near-IR observations by Hashimoto et al. ( 2011 ). In Figure 3 (b), we show the residual map that results from subtracting a two Keplerian disks model from the HCO + first moment map. The model consists of an outer disk extending from 350 to 45 au, with i = 26° and PA = 37°, and an inner one extending from 45 au inward with i = 43° and PA = 65° (Hashimoto et al. 2011 ), around a 2.4 M star, and the final image is convolved with the observational beam. This simplistic model provides a reasonable fit across the disk, except in the southwestern region, where strong residuals are observed. Visual inspection of Figure 3 (a), shows that there is an anomaly (blue circle) in the velocity field that is spatially coincident with the bridge (BR, Figure 3 (b)). This anomaly is due to high-velocity wings in the line profile of HCO + at the bridge position (see Figure 2 (b)). The redshifted peak is compatible with Keplerian rotation, while the blueshifted one requires a different mechanism, most likely infalling gas. To test the possibility that the blueshifted wing is tracing infalling material, we built a pure free-falling velocity field following equationwhere r start is the starting position for free-fall accretion, and we adopted r start = 120 au (accretion starts at the inner edge of the dust ring, r ∼ 07). We assumed that infall stops at 40 au. Cuts in the first-moment map, and in our convolved models are shown in Figure 3 (c). The free-falling model has a velocity of 5.5 km s −1 at the bridge position, compared to the 5.3
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