Lifetimes and Oscillator Strengths for Ultraviolet Transitions in Singly-Ionized Lead

TL;DR

This study measures lifetimes and oscillator strengths of ultraviolet transitions in Pb II, detects the $ ext{Pb II}\lambda1203$ line in the interstellar medium, and revises the oscillator strength for $ ext{Pb II}\lambda1433$, impacting interstellar lead abundance estimates.

Contribution

It provides new experimental lifetime measurements, first detection of the $ ext{Pb II}\lambda1203$ line in the ISM, and revised oscillator strengths that alter previous abundance calculations.

Findings

Experimental lifetimes for Pb II levels are reported.

First detection of Pb II $ ext{λ}1203$ in the interstellar medium.

Revised oscillator strength for Pb II $ ext{λ}1433$ increases interstellar Pb abundance estimates.

Abstract

We present the results of lifetime measurements made using beam-foil techniques on levels of astrophysical interest in Pb II producing lines at 1203.6 A ($6s6p^{2}$ $^{2}D_{3/2}$) and 1433.9 A ($6s^{2}6d$ $^{2}D_{3/2}$). We also report the first detection of the Pb II $\lambda1203$ line in the interstellar medium (ISM) from an analysis of archival spectra acquired by the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST). The oscillator strengths derived from our experimental lifetimes for Pb II $\lambda\lambda1203$, $1433$ are generally consistent with recent theoretical results, including our own relativistic calculations. Our analysis of high-resolution HST/STIS spectra helps to confirm the relative strengths of the Pb II $\lambda\lambda 1203$, $1433$ lines. However, the oscillator strength that we obtain for Pb II $\lambda1433$ ($0.321\pm0.034$) is significantly smaller than earlier theoretical values, which have been used to study the abundance of Pb in the ISM. Our revised oscillator strength for $\lambda1433$ yields an increase in the interstellar abundance of Pb of 0.43 dex over determinations based on the value given by Morton, indicating that the depletion of Pb onto interstellar dust grains is less severe than previously thought.