Modeling the physical structure of the low-density pre-protostellar core Lynds 1498

Pre-protostellar cores likely represent the incipient stages of low-mass (≈1 M_⊙) star formation. Lynds 1498 is a pre-protostellar core (PPC) and was one of the initial objects toward which molecular depletion and differentiation was detected. Despite the considerable scrutiny of L1498, there has not been an extensive study of the density and temperature structure as derived from radiative transfer modeling of dust continuum observations. We present deep SCUBA observations of L1498 at 850 and 450 μm, high-resolution BEARS maps of the N₂H⁺ 1 → 0 transition, Caltech Submillimeter Observatory observations of the N₂H⁺ 3 → 2 transition, and Green Bank Telescope observations of the C ₃8 4 → 3 transition. We also present a comparison of derived properties between L1498 and nearby PPCs that have been observed at far-infrared and submillimeter wavelengths. The L1498 continuum emission is modeled using a one-dimensional radiative transfer code that self-consistently calculates the temperature distribution and calculates the spectral energy distribution and intensity profiles at 850 and 450 μm. We present a more realistic treatment of PPC heating that varies the strength of the interstellar radiation field (s_isrf) and includes attenuation of the ISRF due to dust grains at the outer radius of the core, A_V. The best-fit model consists of a Bonner-Ebert sphere with a central density of (1-3) × 10⁴ cm^-3, R₀ → 0.29 pc, 0.5 ≤ S _isrf ≤ 1, A_V ≈ 1 mag, and a nearly isothermal temperature profile of ≈10.5 K for OH8 opacities. C₃S emission shows a central depletion hole, while N₂H⁺ emission is centrally peaked. We derive a mean N₂H⁺ abundance of 4.0 × 10^-10 relative to H₂ that is consistent with chemical models for a dynamically young yet chemically evolved source. The observed depletions of C₃S and H₂CO, the modest N ₂H⁺ abundance, and a central density that is an order of magnitude lower than other modeled PPCs suggests that L1498 may be a forming PPC. Our derived temperature and density profile will improve modeling of molecular line observations that will explicate the core's kinematical and chemical state.