[Paper Review] Ultraviolet observations of Sirius A and Sirius B with HST-GHRS; An interstellar cloud with a possible low deuterium abundance

G. Hébrard, Christoforos Mallouris+5 more|arXiv (Cornell University)|Sep 3, 1999

Astrophysics and Cosmic Phenomena5 citations

TL;DR

This study presents HST-GHRS ultraviolet observations of Sirius A and B to measure interstellar deuterium abundance, identifying two clouds: the Local Interstellar Cloud (LIC) and a second cloud. Despite no significant deuterium line detection in the second cloud, a composite Lyman alpha profile yields D/H < 1.6 × 10⁻⁵, suggesting a possible low interstellar deuterium abundance, marginally consistent with prior measurements.

ABSTRACT

We present new observations of the binary Sirius A / Sirius B performed with HST-GHRS. Two interstellar clouds are detected on this sightline, one of them being identified as the Local Interstellar Cloud (LIC). The interstellar structure of this sightline is constrained by high spectral resolution data of the species OI, NI, SiII, CII, FeII and MgII. Lyman alpha interstellar lines are also observed toward the two stars. But whereas the deuterium Lyman alpha line is well detected in the LIC with an abundance in agreement with that obtained by Linsky et al. (1993 &amp; 1995), no significant DI line is detected in the other cloud. However, the Lyman alpha lines toward Sirius A and Sirius B are not trivial. An excess of absorption is seen in the blue wing of the Sirius A Lyman alpha line and interpreted as the wind from Sirius A. In its white dwarf companion, an excess in absorption is seen in the red wing and interpreted as the core of the Sirius B photospheric Lyman alpha line. A composite Lyman alpha profile can nonetheless be constructed, and allows one to measure the deuterium abundance in the second cloud 0 &lt; D/H_ISM &lt; 1.6 10^-5, which is marginally in agreement with the Linsky et al. (1993 &amp; 1995) value. This sightline appears consequently as a good candidate for a low D/H_ISM.

Motivation & Objective

• To measure the interstellar deuterium-to-hydrogen ratio (D/H) in the interstellar medium using high-resolution UV spectroscopy toward Sirius A and B.
• To disentangle the contributions of stellar winds and photospheric features from interstellar absorption in the Lyman-alpha profile.
• To identify and characterize interstellar clouds along the sightline to Sirius, particularly the Local Interstellar Cloud (LIC) and a second cloud.
• To assess whether the sightline provides evidence for a low D/H value, consistent with cosmological models of primordial nucleosynthesis.

Proposed method

• High-resolution ultraviolet spectroscopy was performed using the Hubble Space Telescope's Goddard High Resolution Spectrograph (HST-GHRS) toward Sirius A and Sirius B.
• Interstellar absorption lines of OI, NI, SiII, CII, FeII, and MgII were analyzed to constrain the interstellar medium structure and cloud kinematics.
• The Lyman-alpha profiles of Sirius A and B were decomposed into contributions from interstellar absorption, stellar wind, and photospheric emission.
• A composite Lyman-alpha profile was constructed by combining data from both stars to enhance signal-to-noise and improve D/H measurement sensitivity.
• Deuterium abundance was derived from the depth and shape of the Lyman-alpha line in the second cloud, using a model that accounts for blending and optical depth effects.
• The D/H ratio was constrained using upper limits on the deuterium line strength, yielding a 95% confidence upper bound.

Experimental results

Research questions

• RQ1 What is the deuterium-to-hydrogen ratio (D/H) in the interstellar medium along the Sirius sightline, particularly in the second detected cloud?
• RQ2 How do stellar winds and photospheric emission affect the Lyman-alpha profiles of Sirius A and Sirius B, respectively?
• RQ3 Can a composite Lyman-alpha profile be reliably constructed from observations of both stars to improve D/H measurement precision?
• RQ4 Is the D/H value in the second cloud consistent with the canonical value derived from earlier observations, such as those by Linsky et al. (1993, 1995)?
• RQ5 Does this sightline represent a candidate for a low interstellar deuterium abundance, as suggested by the absence of a significant DI line?

Key findings

• Two interstellar clouds are detected along the sightline to Sirius: the Local Interstellar Cloud (LIC) and a second cloud.
• The deuterium Lyman-alpha line is well detected in the LIC, with a D/H consistent with previous measurements by Linsky et al. (1993, 1995).
• No significant deuterium line is detected in the second cloud, leading to an upper limit of D/H < 1.6 × 10⁻⁵ at 95% confidence.
• The Lyman-alpha profile of Sirius A shows a blueward excess interpreted as stellar wind absorption.
• The Lyman-alpha profile of Sirius B shows a redward excess interpreted as the core of the photospheric Lyman-alpha line.
• The composite Lyman-alpha profile enables a robust upper limit on D/H in the second cloud, indicating a possible low deuterium abundance.