On the Interstellar Extinction Curve toward HD 93222, A Sightline with an Exceedingly Narrow 2175 Angstrom Extinction Bump

TL;DR

This work analyzes the interstellar extinction toward HD 93222, revealing an exceptionally narrow 2175 Å bump with $\gamma \approx 0.76\,\mu{\rm m}^{-1}$ and a center near $2190$ Å. By deriving the extinction curve from UV to IR data and fitting with a silicate–graphite dust model augmented by nano-sized graphitic grains ($[\mathrm{C/H}]_{\rm nano} \approx 40$ ppm), the authors show that standard dust populations alone cannot reproduce the observed profile. The results imply that strong radiation fields and stellar winds in the Carina nebula fragment grains to nano sizes, enhancing the nano-graphitic component and narrowing the bump. This provides new constraints on the carrier of the 2175 Å feature and highlights the impact of the local interstellar environment on dust processing and extinction properties.

Abstract

The 2175 Angstrom extinction bump, the most prominent spectral feature superimposed on the interstellar extinction curve, is widely seen in the interstellar medium (ISM) of the Milky Way and external galaxies, both near and far. While its central wavelength is remarkably stable and independent with environment, its width shows considerable variation and environmental dependence. Here we examine the extinction curve for the line of sight toward HD 93222, a young star located in the Carina nebula. It is found that the 2175 Angstrom bump is extremely sharp, which is among the narrowest ever found in the Milky Way and external galaxies. We model the derived extinction curve and find that, to explain the extinction characteristics of HD 93222, in addition to the conventional silicate and graphite dust mixture, an additional population of nano-sized graphitic grains is required.

Figures (9)

• Figure 2: Left panel (a): Comparison of the observed, dust-obscured IUE spectrum ($F_\lambda^{\rm obs}$; green solid line) of HD 93222 with the "intrinsic", extinction-free spectrum represented by the Kurucz atmospheric model ($F_\lambda^{\rm int}$; black solid line). Note that, to facilitate comparison, the IUE spectrum is multiplied by a factor of 10. Middle panel (b): Comparison of the observed, dust-obscured IUE spectrum ($F_\lambda^{\rm obs}$; green solid line) with the best-fit model spectrum ($F_\lambda^{\text{int}} \exp \{-A_\lambda / 1.086\}$; red solid line). Right panel (c): The derived extinction curve expressed as $A_{\lambda}$/$A_V$ for the line of sight toward HD 93222. The red line is the FM88 parametrization at $\lambda^{-1} > 3.3\,\,{\rm \mu m}^{-1}$, which is the sum of a linear "background" (green line), a Drude bump of width $\gamma$ and central position of $x_0\equiv\lambda_0^{-1}$ (blue line), and a nonlinear far-UV rise (black dash-dotted line) at $\lambda^{-1} > 5.9\,\,{\rm \mu m}^{-1}$.
• Figure 3: The interstellar extinction curve from the far-UV to the far-IR for the line of sight toward HD 93222, with the FM88 parameterization for $\lambda^{-1}>3.3\,{\rm \mu m}^{-1}$, the CCM parameterization (with $R_V=4.76$) for $1.1\,{\rm \mu m}^{-1} < \lambda^{-1} < 3.3\,{\rm \mu m}^{-1}$, and the $R_V=3.1$ model curves of Weingartner & Draine (2001; red dashed line) and Wang, Li & Jiang (2015; blue dot-dashed line) for $\lambda > 0.9\,{\rm \mu m}$. The U, B, V, J, H, K photometric extinction data points are superimposed on the extinction curve as black squares.
• Figure 4: Comparison of the extinction curve derived in §\ref{['sec:extcurv']} for HD 93222 (red solid line) and the CCM representation for $R_V=4.76$ (blue dashed line), the optical total-to-selective extinction ratio of HD 93222. Also shown is the CCM $R_V=3.1$ curve (black dot-dashed line), which is commonly taken to represent the mean extinction curve of the Galactic diffuse ISM.
• Figure 5: The 2175$\,{\rm \AA}$ extinction bump width (FWHM; $\gamma$) plotted against $R_V^{-1}$ for the 328 Galactic interstellar extinction curves of Valencic et al. (2004) and Fitzpatrick & Massa (2007). The labelled sightlines (red circles) show those exhibiting extreme $\gamma$ and $R_V$ values.
• Figure 6: Left panel (a): Fitting the extinction curve of HD 93222 (black squares) with a mixture of silicate (blue dashed line) and graphite grains (magenta dashed line), each with an exponentially-cutoff power-law size distribution (red squares). Right panel (b): Same as (a) but with an additional population of nano graphite grains (cyan dashed line) which lock up a carbon abundance (relative to H) of $\left[{\rm C/H}\right]_{\rm nano}=40\,{\rm ppm}$. "VSG" refers to very small grains (i.e., nano graphite grains). The right panel represents our preferred model.