TestParticles.cs

using UnityEngine;
using System.Collections;

public class TestParticles : MonoBehaviour
{
    public ParticleAttractor[] particleAttractors;
    ParticleSystem.Particle[] particles = new ParticleSystem.Particle[1000];

    void Update()
    {

        int length = particleSystem.GetParticles(particles);
        for (int i = 0; i < length; i++)
        {
            for (int j = 0; j < particleAttractors.Length; j++)
            {
                Vector3 diff = particleAttractors[j].transform.position - particles[i].position;
                particles[i].velocity += diff.normalized * particleAttractors[j].strength * Time.deltaTime;
            }
        }
        particleSystem.SetParticles(particles, length);
    }
}